Running head: MENTAL TIME TRAVEL

	Mental Time Travel and the Evolution
	of the Human Mind
	Thomas Suddendorf and Michael C. Corballis
	University of Auckland, New Zealand

	Abstract  
We argue that the human ability to travel mentally in time constitutes a discontinuity 
between ourselves and other animals. Mental time travel comprises the mental 
reconstruction of personal events from the past (episodic memory) and the mental 
construction of possible events in the future. It is not an isolated module, but depends 
on the sophistication of other cognitive capacities, including self-awareness, meta-
representation, mental attribution, understanding the perception-knowledge 
relationship, and dissociation of imagined mental states from one's present mental 
state. These capacities are also important aspects of so-called "theory of mind", and 
they appear to mature in children at around age four. Furthermore, mental time travel 
is generative, involving the combination and recombination of familiar elements, and 
in this respect may have been a precursor to language. Current evidence, although 
indirect or based on anecdote rather than on systematic study, suggests that nonhuman 
animals, including the great apes, are confined to a "present" that is limited by their 
current drive states. In contrast, mental time travel by humans is relatively 
unconstrained, and allows a more rapid and flexible adaptation to complex, changing 
environments than is afforded by instincts or conventional learning. Past and future 
events loom large in much of human thinking, giving rise to cultural, religious, and 
scientific concepts about origins, destiny, and time itself.

	Introduction
	The question of whether there is a discontinuity between humans and other 
species is one that continues to haunt us. Despite Darwin's admonition "never to say 
higher or lower", most people continue to regard humans as at the top of the 
evolutionary tree. Perhaps this conceit is simply an example of a "false consensus 
bias" (Ross, Green, & House, 1977) created by Western scholars raised in the 
Christian tradition, which perpetuates an unbridgeable gap separating humans from 
other animals. Certainly, there are other religious traditions that emphasize continuity 
rather than discontinuity; Hinduism, for example, views animal and human minds as 
stages differing in merely quantitative fashion in the progression toward Nirvana.
	In some respects, modern scientific enquiry is also narrowing the gap. There is 
evidence that aspects of human thought that only a few years ago were assumed to be 
uniquely human, such as symbolic thought, the use and manufacture of tools, or self-
awareness, may also be present in the great apes (e.g., Gallup, 1983; Goodall, 1986; 
Greenfield & Savage-Rumbaugh, 1990). In order to sustain the belief in the divide 
between us and our nearest primate relatives, some researchers have resorted to 
increasingly restrictive definitions of qualities, such as language, that have been 
traditionally considered uniquely human (Gibson, 1990, 1993). Where it was once 
believed that only humans manufacture tools, for example, more recent evidence 
forced the more restrictive claim that only humans use tools to make tools (Beck, 
1980). Now even this can be disputed (e.g., Toth, Schick, Savage-Rumbaugh, Sevcik, 
& Rumbaugh, 1993; Westergaard & Suomi, 1994; Wynn & McGrew, 1989). There is 
also recent evidence that some great apes, in contrast to monkeys, may have at least 
some of the elements of a "theory of mind" (Premack, 1988; Premack & Woodruff, 
1978) that is manifest in a number of ways. These include the use of pedagogy in both 
the laboratory (Fouts, Fouts, & van Cantfort, 1989) and the field (Boesch, 1991), 
deception of conspecifics (Whiten & Byrne, 1991), displays of apparent empathy and 
compassion (Boesch, 1992), the ability to imitate (Byrne, 1994; Meador, Rumbaugh, 
Pate, & Bard, 1987), and the more general ability to imagine other possible worlds 
(Byrne & Whiten, 1992). On the basis of such evidence, one may even be tempted to 
relocate the "gap" so that it separates the great apes, rather than only humans, from the 
other animals (Savage-Rumbaugh, 1994b).
	Despite all this, there remains a strong case for a substantial gap between 
humans and the great apes, if only because of the profound effect that humans have 
had on the physical environment. As Passingham (1982) put it: "Our species is unique 
because, in only 35,000 years or so, we have revolutionized the face of the earth" (p. 
21). This rampant exploitation of the environment may be regarded as part of a more 
general human capacity for generativity--a capacity that also underlies propositional 
language, mathematics, and perhaps music and dance (Chomsky, 1988; Corballis, 
1992, 1994). The origins of this capacity, however, remain in doubt (Bloom, 1994).
	In this article we suggest one aspect of human thought that may conceivably 
claim some priority in the emergence of a uniquely human mode of thought. We refer 
to the ability to travel mentally in time--an ability that is itself characterized by 
generativity and combinatorial flexibility. The idea that mental time travel might be 
uniquely human was proposed by the German psychologist Wolfgang K”hler, whose 
pioneering work on the mentality of apes anticipated many of the more recent 
discoveries. Although he was able to show that chimpanzees were capable of using 
mental processes such as insight to solve problems, he was compelled to acknowledge 
an important limitation: "`The time in which the chimpanzee lives' is limited in past 
and future" (K”hler, 1917/1927, p. 272; our italics). In a recent review, Donald (1991) 
remarked similarly that the lives of apes "are lived entirely in the present" (p. 149), 
and the same idea has been expressed by Bischof (1978; 1985), Tulving (1983), 
Savage-Rumbaugh (1994a, 1994b), and Suddendorf (1994). We humans, by contrast, 
make persistent reference to events that are not limited to the present. Events as 
remote as the crucifixion of Christ exert a profound influence on large numbers of 
people. We even tackle questions about the extent of time itself by developing 
religious or scientific concepts such as genesis, judgment day, or the "big bang". 
Much of what we talk or write about refers to events that happened in the past, or 
could happen in the future, suggesting that language itself may be intimately related to 
time travel. Indeed it may not be too far-fetched to suppose that mental time travel lies 
at the heart of human consciousness. 
	Although our concern is with mental time travel in both directions, we begin 
by considering the ability to mentally reconstruct the past.
	Mental Travel into the Past
	Before people could concern themselves with history they must have been able 
to remember their personal past. It has often been suggested that there is a 
fundamental difference between animal and human memory (e.g., Bischof, 1985; 
Marshall, 1982; Tulving, 1983). So long as we regard memory as simply the ability to 
learn from past experience, however, the difference must be considered one of degree, 
at most, since other animals obviously possess memory in this sense. The case for a 
memory that is distinctly and uniquely human therefore depends on the proposition 
that there is more than one kind of memory, at least one of which is possessed only by 
humans.
	The idea that there is at least a dual memory system arose from work on 
amnesia. The famous subject H.M. has dense amnesia for events and knowledge 
dating from his temporal lobe surgery in 1953, and indeed for memories dating some 
years prior to that, yet his behaviour can still be influenced by past events without his 
being aware of it (see Ogden & Corkin, 1991, for a recent review). His amnesia seems 
to apply only to so-called explicit memories, or what Squire (1992) alternatively 
describes as declarative memories; these represent memories that can be declared, or 
brought into conscious awareness. Memories that seem to be unaffected in amnesia 
are those that we are not aware of, and include those implied by such phenomena as 
learned motor and cognitive skills, classical and operant conditioning, priming, 
habituation, and sensitization. Such memories have been called implicit or 
nondeclarative memories.  
	As the case of H.M. illustrates, declarative memories appear to be critically 
dependent on structures in the medial temporal lobe, including the hippocampus. 
Squire (1992) has summarized evidence that these structures appear to mediate similar 
memory systems in rats, monkeys, and humans, implying that the distinction between 
declarative and nondeclarative memory cannot provide the basis for a discontinuity 
between humans and other mammalian species. 
	However, Tulving (1972, 1983) has proposed a further division between 
semantic and episodic memory systems, and Squire (1992) suggests that this 
distinction lies within the declarative system. As formulated by Tulving, semantic 
memory has to do with general knowledge about the world, of the sort that is normally 
common to people of a given culture, whereas episodic memory represents the 
individual's personal experiences. Where semantic memories transcend space and 
time, episodic memories are linked to particular events in one's personal past that are 
spatially and temporally located. Tulving (1983, 1984) has further conjectured that, 
although semantic memory may be common to humans and other animals, episodic 
memory is uniquely human.  
	Not surprisingly, this conjecture has met with opposition. Following Roitblat 
(1982), Olton (1984) noted that animal behavior often seems to indicate the existence 
of a trace of an earlier event, as in a trial of a delayed conditioned discrimination task, 
or in foraging where an animal must remember not to go to the same flower twice to 
obtain nectar. According to Olton, such observations imply that the animal 
"represents" a past event, and therefore possesses episodic memory. But, as Dretske 
(1982) points out, an event A might produce a cognitive change B that affects 
behavior C at a later point in time, but this need not imply that B carries any 
information about A itself. That is, the mediator B might be causal rather than 
informational. 
	Tulving appears to have accepted that this is so, and in his latest formulation 
of the nature of episodic memory, he is clear that it holds the key to mental time 
travel:

	The owner of an episodic memory system is not only capable of remembering 
the temporal organization of otherwise unrelated events, but is also capable of mental 
time travel: Such a person can transport at will into the personal past, as well as into 
the future, a feat not possible for other kinds of memory (Tulving, 1993, p. 67).

Tulving also refers to evidence (Shimamura, Janowsky, & Squire, 1990) that episodic 
memory may depend on frontal-lobe structures rather than--or perhaps as well as--the 
medial temporal-lobe and diencephalic structures that appear to be critical to semantic 
memory. Given the prominent expansion of the frontal lobes in hominid evolution 
(Deacon, 1990), this might be taken as a further indication that episodic memory is 
unique to humans.  
Relation to other Mental Capacities
	There is evidence that episodic memory is not simply a memory system, but is 
critically dependent on other mental capacities, and it may even be these capacities, 
rather than the nature of the storage involved, that distinguishes humans from other 
species. The term "memory" is often associated with a fixed databank (e.g., a library), 
but this metaphor seems more appropriate for semantic knowledge than for episodic 
memory. Unlike retrieval of facts, retrieval of past episodes usually recodes, or 
updates, the information (Tulving, 1984)1. Freud (1895/1966) long ago observed that 
even memories that reveal themselves as images require a story grammar if they are to 
be distinguished from random hallucinations. The storyline, however, is often 
reconstructed on the basis of general knowledge (semantic memory) rather than on 
what actually happened (Bartlett, 1932), so that the memory trace itself may play a 
relatively small part. Thus, active reconstruction, rather than mere retrieval, appears to 
be essential to episodic memory, and this necessitates the involvement of certain 
cognitive faculties. We now consider some of them:
	The Role of the Self.  According to Tulving (1985), the different kinds of 
memory are linked to different levels of "knowing": Nondeclarative memory is 
anoetic (non-knowing), semantic memory is noetic (knowing), while episodic 
memory is autonoetic (self-knowing). This suggests that episodic memory is critically 
dependent on the concept of self. The relation between the two may actually be 
bidirectional: On the one hand, in providing autobiographical information about one's 
own past, episodic memories may be said to provide the basis for personal identity. 
On the other hand, one may also need an awareness of self in the present in order to be 
able to relate memory representations to experiences of one's self in the past (Howe & 
Courage, 1993). It is therefore necessary to dissociate a self-concept in the present 
from personal identity (or self-concept through time), the former being a prerequisite 
for mental time travel and the latter the consequence of mental time travel.  
	In human ontogeny, the development of self-awareness is commonly assessed 
in terms of children's ability to recognize themselves in a mirror (e.g., Amsterdam, 
1972). Some marker, such as red paint or a sticker, is placed on the child's forehead in 
the absence of his or her knowledge. A mirror is then placed in front of the child and 
the question is whether or not the child notices and responds to the marker. While this 
test may measure a basic sense of self, necessary for episodic memory, it clearly falls 
short of measuring the temporal aspect that underlies the personal identity of an adult. 
The latter may be characterized by, what according to Humphrey (1986) are, the most 
crucial of all questions: "Where have we come from? What are we? Where are we 
going?" These self-defining questions signify the existence of mental time travel, but 
the mirror test alone does not uncover their presence.  	The self-awareness that 
the test reveals may also be limited in other ways. According to Hart and Fegley 
(1994), it reveals objective but not subjective self-awareness; that is, children or 
chimpanzees who recognize themselves in a mirror may understand that the body they 
see is their own, but do not necessarily endow that body with their own subjective 
states. If Hart and Fegley's distinction is valid, then it is presumably subjective self-
awareness that is required in mental time travel into the past, since one must identify 
episodic memories with one's own experience, not merely with one's own body. There 
is evidence that subjective self-awareness is lost following prefrontal lobotomies 
(Freeman & Watts, 1942), which may explain the dependence of episodic memory on 
frontal-lobe function. 
	Mitchell (1994) makes a similar point.  He identifies three levels of self:

	1. the self as largely implicit, a point of view that experiences, acts and, at least 
in the case of mammals and birds, has emotions and feelings;
	2. the self as built on kinesthetic-visual matching, leading to [mirror self-
recognition], imitation, pretense, planning, self-conscious emotions, and imaginative 
experiences of fantasy and daydreams; and contributing to perspective taking and the 
beginnings of a theory of mind; and
	3. the self as built on symbols, language and artifacts, which provides external 
support for shared cultural beliefs, social norms, inner speech, dissociation, and 
evaluation by others as well as self evaluation. (p. 99)

These of course correspond at least roughly to Tulving's three categories of anoetic, 
noetic, and autonoetic thought, except that Mitchell's Level 2 seems to involve a more 
active concept of self than Tulving's notion of noetic thought. However Mitchell 
places autobiography in Level 3, implying that episodic memory belongs there rather 
than in Level 2.
	The critical aspect of episodic memory that raises it above Level 2 may in fact 
be dissociation, which Mitchell identifies with such phenomena as multiple-
personality disorders, hypnosis, self-deception, denial, or simply driving on a well-
known route while thinking about something else. We shall contend that episodic 
memory requires the dissociation of past from present, or more accurately, the 
dissociation of past from present self, and it is this critical feature that elevates it to 
Level 3.
	Temporal Components. One aspect of episodic memory that appears not to be 
encoded in the trace itself is the order of events.  Reviewing the evidence, Friedman 
(1993) recently concluded thus:

	In spite of the common intuition that chronology is a basic property of 
autobiographical memory, the research reviewed demonstrates that there is no single, 
natural temporal code in human memory.  Instead, a chronological past depends on a 
process of active, repeated construction. (p. 44)

Even the sense of "pastness" of an episode may not be inherent in the memory itself 
and may need to be added. One illustration of this is the phenomenon of d‚j… vu, in 
which we have the experience of reliving a past episode in the absence of an actual 
memory (Bowers & Hilgard, 1986). Conversely, "[h]aving--and even using--a 
memory representation of a prior event is not sufficient to ensure the subjective 
experience of remembering" (Jacoby, Kelley, & Dywan, 1989, p. 417).  These 
examples suggest that the sense of pastness may be doubly dissociated from actual 
memories.
	Meta-representation. The conferring of "pastness" on a remembered episode 
further implies the ability to form meta-representations of one's knowledge. Meta-
representation, according to Perner (1991), is representing a representation as a 
representation. That is, in addition to the primary representation (e.g., I am in a park), 
one has to understand that this representation is a memory. Other primary 
representations that comprise memories (e.g., I go shopping; I play ball) can then be 
constructed into a past episode (I was in a park, played ball, then went shopping). The 
ability to selectively choose representations and organize them into past episodes is a 
characteristic of human mental time travel that demands flexible access to one's own 
mind.
	Attribution. The conferring of pastness may also be regarded as an act of 
attribution. That is, in recollecting some past event we attribute it to the experience of 
an earlier self. Such attributions may well parallel our ability to attribute mental states, 
such as beliefs, desires, and emotions, to other people. Even memory states may be 
attributed to others as well as to ourselves; we usually assume that if we have shared 
an experience with another person, then that person will remember it too. A good deal 
of human conversation consists of mutual time travels down memory lane. Shared 
memories are the glue for the enlarged and complex social nets that characterize our 
species, and that go well beyond mere kinship.
	Understanding the Relation Between Perception and Knowledge. As pointed 
out above, in addition to knowing something about a past event, one has to meta-
represent this knowledge and attribute it to the experience of an earlier self in order to 
mentally travel into the past. Re-experiencing the event, that is, representing how this 
information became known, demands some understanding of the contingency between 
perception and the formation of knowledge (Perner, 1991; Perner & Ruffman, 1995). 
If one does not know that knowledge is the result of experience and that experience 
depends on the different channels of sensation and perception, one can scarcely 
reconstruct a particular experience from current information. Knowledge about the 
taste, colour, shape, temperature, etc. of an object can only have entered one's system 
in specific ways. The awareness that one knows something because it has been 
experienced (autonoetic, or self-knowing, consciousness) and the subsequent ability to 
mentally re-experience it, require an understanding about how experience is formed.

	We have identified several basic cognitive capacities that seem to be required 
for a fully fledged episodic memory system. Mental travel into the past demands some 
level of self-awareness, an imagination capable of reconstructing the order of events, 
an understanding of the perception-knowledge contingency, an ability to meta-
represent one's knowledge, to dissociate from one's current mental states and to 
attribute past mental states to one's earlier self. Some of these capacities seem to 
overlap and they seem so basic and natural to us that we find it hard to conceive of a 
mind without them. Yet, as we will see in the next section, only by about the age of 
four are they properly installed in the human brain.
Human Ontogeny of Mental Travel into the Past
	Nondeclarative memory can be shown to exist right after birth. Visual 
habituation (Friedman, 1972) and auditory recognition (DeCasper & Fifer, 1980) 
confirm that information is stored right from the start. In fact, familiarity effects can 
be observed even prior to birth (DeCasper & Spence, 1986). At the age of three 
months, experience with a particular stimulus has effects for up to a week (Rovee-
Collier, Sullivan, Enright, Lucas, & Fagan, 1980). Recent studies, such as that of 
Bauer, Hertsgaard, and Dow (1995) using elicited imitation, showed that experiences 
of one-year-olds can influence responses a year later. 
	Recall, in its widest sense, can first be observed at about seven months when 
infants will begin to look for objects that moved out of sight (Ashmead & Perlmutter, 
1980). The development of "object-permanence" (Piaget, 1954), i.e., the 
understanding that objects continue to exist independently of our perception, is 
however far from complete by this age. By ten months, infants can correctly locate an 
object hidden under one of two identical cloths if there is a delay of up to eight 
seconds between witnessing the placement of the object and being allowed to choose 
(Diamond, 1985). At longer delays, performance deteriorates to a chance level. By 16-
18 months, the critical delay period has expanded to 20 seconds (Daechler, Bukatko, 
Benson, & Myers 1976). 
	Declarative memory is evident when by two to three years of age children 
begin to reproduce details about past events (e.g., Fivush, Gray, & Fromhoff, 1987) 
and this knowledge can be retained for a year and a half (Fivush & Hamond, 1990). 
However, Perner and Ruffman (1995) cite evidence that these memory reports differ 
substantially from those of older children. They require a lot more cuing and the 
questions asked by adults strongly influence the structure of the recall. Perner and 
Ruffman (1995) argue that these young children know (semantic memory) rather than 
remember (episodic memory) what has happened. We will come back to their 
argument shortly. Others, for example Nelson (1992), argue that these early memories 
are episodic, but that they do not become truly autobiographic until age four. Only 
then can they later be recalled and become part of one's life story. Here, the 
phenomenon of childhood amnesia comes into play.
	Childhood amnesia, or the inability of adults to remember their early 
childhood, begins to fall away at about age three to four (Loftus, 1993; Pillemer & 
White, 1989; Sheingold & Tenny, 1982). If adults can have an episodic memory 
(recollective experience) of events from that age, then it follows that episodic memory 
must exist by that age. The question that remains is therefore whether episodic 
memory exists prior to the fading of childhood amnesia. The fact that younger 
children can report knowledge about events when prompted may reflect only semantic 
rather than episodic memory, just as the early use of the words "remember" and 
"forget" appears to be misleading. Lyon and Flavell (1994) showed recently that four- 
but not three-year-olds understand the sense of pastness implied by those words. The 
younger children use these terms merely to describe current success (remember) or 
failure (forget).  
	But what about the development of those capacities that we argued to be 
essential for a fully-fledged episodic memory system?  Do their maturations converge 
at the age of three to four? If so, then mental travel into the past, as outlined by us, can 
only emerge at this age. Let us now have a closer look at the development of the 
preconditions for episodic memory.
	Self-awareness. At the age of 18-24 months children pass the mirror-
recognition test (Amsterdam, 1972). We may assume that at this stage the prerequisite 
for episodic memory is achieved. However, as stated above, the self-concept implied 
by the mirror test need not extend to the more general adult sense of personal identity 
that extends through time. The emergence of the latter might be tested by introducing 
a delayed condition to the mirror test. Povinelli (in press) has reported intriguing 
preliminary results bearing on this issue. In studies with his colleagues Landau and 
Perillonx he marked two, three- and four-year-old children by putting stickers on their 
foreheads. When he showed the children a video of this action two minutes later, 75% 
of the four-year-olds reached up immediately to remove the sticker, while none of the 
two year-olds and only 25% of the three-year olds did so. All of the two and three 
year-olds immediately removed the sticker when the video was replaced by a mirror, 
providing direct feedback, confirming the earlier evidence that even two-year-olds 
pass the mirror test (e.g., Amsterdam, 1972). So, while the mirror test demonstrates 
the onset of a self-concept at around age two, the temporal dimension appears not to 
emerge until age three to four.    
	Temporal reconstruction. Mental time travel also implies that the order of 
events in time can be reconstructed, and Friedman (1991, 1992) has shown that four-
year-olds are capable of making correct earlier vs. later judgements about past events. 
Between four and about eight years children acquire an explicit knowledge about the 
culturally dependent time scales (e.g., days, weeks, months etc.) that assist the 
structuring of one's own past experiences. The basic reconstructive capacity, however, 
might be in place in children even younger than four.
	Meta-representation. It has been argued that meta-representation first 
manifests in form of pretense (Leslie, 1987). Whether the representations of the 
pretended and real world are hierarchically organized is still debated (see Jarrold, 
Carruthers, Smith, & Boucher, 1994, for a critical appraisal). Pretend play develops in 
the second year and therefore clearly precedes the proposed time frame for the 
emergence of mental travel into the past. However, complex social pretend play as 
well as individual pantomime develop later at around age three and a half, and only 
then, it has been argued, may pretend play be based on meta-representation (Jarrold et 
al., 1994; Suddendorf & Fletcher-Flinn, 1996). This would be consistent with the 
emergence of meta-representation in other domains such as in mental attribution.
	Mental Attribution. The utilization of representational skills for the attribution 
of mental states, develops progressively between age two and four, from attributing 
desires and intention to knowledge and belief and, finally, false beliefs (Gopnik, 1993; 
Wellman, 1991; Whiten, 1991; Wimmer & Perner, 1983). At first, the new attribution 
of mentality is characterized by overgeneralization, or what has been called animism. 
By three and a half to four years, when they finally pass appearance-reality and false 
belief tasks (Astington, Harris, & Olson, 1988; Flavell, 1993; Gopnik & Astington, 
1988; Wimmer & Perner, 1983), are children said to have a `theory of mind', a truly 
representational view of the world, including the meta-understanding that 
representations can be wrong, can be changed, and depend on informational access 
(e.g., Perner, 1991). Meta-representational dissociation from primary mental states 
becomes evident.
	 For our purposes it is important to note that this development is not restricted 
to the attribution of mental states to others, but appears to include the attribution of 
past mental states (to a past self and others). In a classical false-belief paradigm, for 
example, three-year-olds fail to understand that their current knowledge that there are 
pencils and not smarties in the candy box is not available to others; that is, they 
wrongly predict that another child also believes the box to contain pencils. They also 
fail to understand that, before they were shown to the contrary, they once believed the 
box to contain smarties (Gopnik & Astington, 1988). This is also true of intentions, 
desires and beliefs. Gopnik and Slaughter (1991) showed that three-year-olds, 
although able to recall past mental states of pretence, imagination and perception, 
have severe difficulties remembering past mental states of desire, intention and belief.   
	In regard to past knowledge, Gopnik and her colleagues (Gopnik & Graf, 
1988; O'Neill & Gopnik, 1991) demonstrated young children's difficulty in recalling 
the source of their current knowledge, even though the learning event may have 
occurred only minutes ago. The children can report the content of learning before they 
become able to recall the learning event itself. Taylor, Esbensen, and Bennett (1994) 
found that even older children (4 and 5) have problems with source memory for 
recently acquired skills and color names. Those children who claim to have known the 
names yesterday, while in fact they learned them today, also tend to claim that they 
have always known them.  
	In sum, two- and three-year-olds have problems representing their own (and 
others') former mental states of desire, intention, knowledge and belief. This severely 
limits their potential ability for mental travel into the past. Gopnik and Slaughter 
(1991) acknowledge this point when they write that their findings (see above) "may 
have implications for the development of fully-fledged, autobiographic, episodic 
memory. One characteristic of such memory is that we not only know that past events 
took place, but we also know that we experienced and represented them in a particular 
way" (p.109). It poses severe limits for the ability to reconstruct the narrative of past 
events, if one cannot represent what one (and others involved) wanted, intended, 
knew, and believed, and how these mental states changed.
	Perhaps mental attribution and mental travel in time develop quite similarly. 
Here is a suggestion. One view on how children develop mental state attribution is via 
simulation (Gordon, 1986; Harris, 1991; Humphrey, 1986; Johnson, 1988). At first, 
the child's own state may interfere with the pretense, but by the age of four children, 
by now consummate actors, can detach from their own states to assume the states of 
others. At this point, then, there is dissociation. A similar pattern may characterize the 
development of episodic memory. That is, young children may have difficulty 
simulating their own past experiences because they cannot escape their present one. 
Interestingly, Kinsbourne (1989) has attributed the memory failures shown by patients 
with Korsakoff syndrome to the same difficulty, and not to the loss of memory per se. 
By the age of four, however, the child can escape the present and simulate the past 
without interference. The simulation account for the development of mental 
attribution is, however, not undisputed (see "Mental Simulation", 1992, for a thorough 
discussion). It is therefore premature to assume the validity of the proposed parallel 
development of mental time travel and mindreading via simulation.   
	Be this as it may, the development of the final precondition adds further 
empirical evidence to the argument for the late (around age four) emergence of 
episodic memory.
	Understanding the Relationship between Perception and Knowledge. The 
research on source memory (see above) already indicates that children younger than 
four may not understand much about the relationship between perception and 
knowledge. Wimmer, Hogrefe, and Perner (1988) studied this understanding 
explicitly and found indeed that four-year-olds, but not three-year olds, correctly 
answered questions regarding informational access (e.g., seeing) and current 
knowledge.  
	Perner (Perner, 1991; Perner & Ruffman, 1995) saw the connection to episodic 
memory and sought empirical support for the claim. He appealed to Tulving's (1985) 
finding that subjects tend to report items in a free recall condition to be 
"remembered", while items in cued recall conditions are deemed "known". Perner and 
Ruffman (1995) cite several other studies (e.g., Gardiner & Java, 1990) in support of 
the claim that the adult judgement of whether items are remembered or known is a 
valid measure. The argument, then, is that in recognition one can use semantic cues to 
retrieve the items (which results in knowing), while in free recall retrieval depends 
largely on internal episodic traces, especially the awareness of having experienced, i.e. 
perceived, the item (which results in remembering). The target group, preschool 
children, of course cannot be asked to make a valid judgement about this (see the 
results of Lyon and Flavell, 1994, above). But, if the reasoning is correct, one would 
expect to find a correlation between free recall and children's performance on tests 
that measure their understanding of the relationship between informational access and 
knowing. Those who pass these tests should do much better on free recall tasks than 
should those who fail, while no significant difference would be expected in their 
performance on recognition tasks.
	This hypothesis has been tested in four experiments instigated by Perner 
(1991) and Perner and Ruffman (1995). And, indeed, a strong and significant 
correlation (r >.4) between free recall and various measures of an understanding of 
how perception leads to knowledge has been found. This correlation remained 
significant even after correlations with cued recall and intelligence (i.e. scores on the 
BPVS) were partialed out. Thus, the results strongly support the idea that 
understanding the perception-knowledge relationship is essential for episodic memory 
(performance in free recall) because it entails the ability to represent the experiential 
origin of one's knowledge (a so-called episodic trace). This understanding, according 
to Perner's results, develops gradually between the ages three and six. Prior to this, a 
child can only know something about past events but cannot reexperience the event in 
the way required for true episodic memory.
	Perner and Ruffman (1995) also conclude that their findings provide an 
explanation for the phenomenon of childhood amnesia in that the development of true 
episodic memory is causing it to fade, somewhere between age three and four. We 
believe this claim is supported by our analysis of the development of the other 
cognitive capacities which we hold to be important for mental travel into the past. 
While the earliest form of meta-representation may develop in the second year, only 
by three and a half to four years of age can children properly use this ability to 
attribute past mental states (such as desires, intentions, knowledge, belief and false 
beliefs) to their past selves. Only then can their personal past experiences be properly 
reconstructed. This, in turn, is necessary to the formation of a record of one's history: 
the foundation of a personal identity.  
	Howe and Courage (1993) have proposed a relationship between the cognitive 
sense of self and childhood amnesia. While first empirical evidence for a sense of self 
develops at 18 to 24 months, it may rather be Povinelli's (in press) delayed paradigm 
that accurately measures the emergence of an identity through time. Taken together 
with the results of Perner's studies, we believe that we have an explanation both for 
childhood amnesia and the subsequent emergence of episodic memory at about the 
age of three and a half. 
Support for the Model from a Clinical Population: 
The Case of Autism 
	Finally, we are going to have a look at a disorder that, we believe, supports our 
argument and nicely wraps up most of the points being made so far. If our argument is 
correct, then clinical populations that lack one or more of the proposed requirements 
should consequently be impaired in their mental time travel ability. On the other hand, 
if clinical populations exist who, despite lacking these proposed prerequisites, show 
proper mental time travel, then this would clearly contradict our argument.
  	At least one disorder has been claimed to be based on a lack of `theory of 
mind': autism (e.g., Baron-Cohen, 1995; Baron-Cohen, Leslie, & Frith, 1985). 
Deficits have been shown in autistic children's ability to meta-represent (e.g., Baron-
Cohen, 1989; Frith, 1989), to understand the perception-knowledge relationship (e.g., 
Baron-Cohen & Goodhart, 1994; Leslie & Frith, 1988), to distinguish appearance 
from reality (Baron-Cohen, 1989) and to attribute mental states to others and 
themselves (e.g., Baron-Cohen, 1995, Perner, Frith, Leslie, & Leekam, 1989). It has to 
be noted that a small minority of autistic people do overcome these deficits to some 
extend. In fact, various degrees of autism (e.g., regarding IQ and verbal ability) make 
this clinical group very heterogenous. Our hypothesis, therefore, predicts that most 
(i.e., those without the proposed requirements) autistic children are impaired in the 
ability to travel mentally in time.
	Although people with autism can have a good and sometimes even 
extraordinary ability for rote memory (e.g., associative and cued memory, Boucher & 
Warrington, 1976), episodic memory seems to be impaired (Boucher & Lewis, 1989; 
Powell & Jordan, 1993). Powell and Jordan (1993) speak of a lack of `personal 
episodic memory' where events can be recalled but individuals are unable to 
"remember themselves performing actions, participating in events or possessing 
knowledge and strategies" (p. 362). They further argue that an `experiencing self', 
much like the one invoked by Perner and Ruffman (1995), is needed to code episodes 
as part of a personal dimension. In accordance with Perner and Ruffman's (1995) 
findings, then, it has to be noted that Boucher and Lewis (1989) as well as Tager-
Flusberg (1991) found autistic children to be impaired in free, but not in cued, recall. 
Tager-Flusberg acknowledges that, as suggested by Perner for young children, lack of 
experiential awareness may be responsible for autistic children's impaired episodic 
memory and consequent deficits in free recall. 
	According to Powell and Jordan (1992) a "continuing sense of self `from the 
inside'" (p. 362) rather than a mere sense of self as seen `from the outside' is needed 
for this kind of memory. `Sense of self from the outside' clearly reminds us of what is 
measured by the mirror-recognition task and the distinction from self `from the inside' 
strikingly resembles that of Hart and Fegley's (1994) between subjective and objective 
self-awareness. Without this subjective or `inside' sense of self and the accompanying 
`theory of mind', children with autism appear to be, as was predicted, unable to 
mentally transport themselves into their past, re-experience the events and see the 
causal relation between past and present self. We would therefore predict that while 
autistic children can recognize themselves in a mirror (Dawson & McKissick, 1984) 
they will fail the delayed video version of the task. Further research is needed to 
address this issue and to determine and investigate only those individuals who lack 
our proposed prerequisites.
  	We argued above that one crucial underpinning of `theory of mind' and mental 
time travel might be the ability to dissociate from one's current state. This ability also 
appears to be impaired in most autistic children.  "`Executive function' is an umbrella 
term for the mental operations which enable an individual to disengage from the 
immediate context in order to guide behaviour by reference to mental models and 
future goals" (Hughes, Russel, & Robbins, 1994, p. 477), and evidence for executive 
dysfunction in autistic children has accumulated in recent years (e.g., Hughes & 
Russel, 1993; Hughes, Russel, & Robbins, 1994; Ozonoff, Pennington, & Rogers, 
1991). While the relationship between executive functions and `theory of mind' is still 
debated (e.g., Baron-Cohen, 1995; Russel, Jarrold, & Potel, 1994), autistic children's 
inability to disengage or dissociate from the present and to form strategic plans 
demonstrates impairment of mental travel into the future. Harris (1991) noted the lack 
of planning that is entailed by typical characteristics of autism: Lack of flexibility and 
the tendency to engage in stereotyped and routinized actions. Thus, while further 
research is needed, current knowledge about autism supports our here presented 
theory.
	Having substantiated our model and established that the proposed mental 
capacities required for mental travel into the past develop only at about age three and 
a half, we shall now review the evidence on whether they develop in animals at all. 
Evidence for the Existence of the Required Capacities in Animals
	The vast literature on animal memory (see Kendrik, Rilling, & Denny, 1986, 
for a review) demonstrates clearly that we are not the only species benefiting from 
past experience. However, the question of whether other animals mentally reconstruct 
the past, have recollective experience, or, in other words, travel mentally into the past, 
cannot be answered by these data. Nonetheless, research on the related capacities may 
shed light on this question.
	Self-awareness. The role of the self may not be sufficient to deny all other 
animals the capacity for episodic memory, since there is evidence that the concept of 
self is not restricted to humans. Chimpanzees (Gallup, 1970), gorillas (Patterson, 
1991), and orangutans (Suarez & Gallup, 1981) appear to demonstrate self-
recognition in a mirror (see also Parker, Mitchell, and Boccia, 1994, for a recent 
review). Monkeys and even elephants and parrots can learn how a mirror works (e.g., 
correctly using mirrored information about approaching objects), but unlike the great 
apes they cannot locate markings viewed in a mirror if these are on their own bodies 
(Anderson, 1986; Gallup, 1994; Pepperberg, Garcia, Jackson, & Marconi, 1995; 
Povinelli, 1989). This area of research is somewhat controversial, however, since 
there is also evidence that some chimpanzees do not show self-recognition on the 
mirror test, even after extended exposure to their own mirror images (Swartz & Evans, 
1991), and the data on gorillas are also somewhat equivocal (e.g., Povinelli, 1993). 
Even so, this work at least raises the possibility that the great apes are capable of a 
concept of self, and therefore possess one of the prerequisites for episodic memory. 
Whether they can pass Povinelli's delayed version remains to be seen. We would 
expect them not to pass this test, for reasons that will become clear in the following 
sections.
	Temporal order. Great apes have provided some evidence for the ability to 
imagine other possible worlds (see Byrne & Whiten, 1992). Furthermore, even 
monkeys and pigeons have been shown to learn serial orders (e.g., Terrace & 
McGonigle, 1994). Whether great apes can use their imagination to reconstruct the 
order of past events, however, remains questionable.  For what it is worth, it can be 
noted that none of the ape-language studies have resulted in apes acquiring tense or 
timescales. 
	Meta-representation. There is some evidence that great apes may be capable of 
some degree of meta-representation. There are some records of seemingly pretend 
play with imaginary objects (e.g., Hayes, 1951; Savage-Rumbaugh, 1986; Savage-
Rumbaugh & McDonald, 1988). Furthermore, Whiten and Byrne (1991) argue that 
tool manufacture and insightful spontaneous problem solving (e.g. K”hler, 
1917/1927) by great apes also indicates their meta-representational ability. It remains 
debatable whether these observations indicate representation of a higher order, but if 
we accept similar behavioural evidence for children (e.g., Leslie, 1987), then we 
should also grant it to apes. It is interesting to note that, just as with mirror self-
recognition, only the great apes show these behaviors, while monkeys do not.
  	Mental Attribution and the Perception-Knowledge Relationship. A similar 
discrepancy can be observed in regard to knowledge representation and mental 
attribution. On the basis of extensive observations in the wild, Cheney and Seyfarth 
(1990) have inferred that monkeys are not able to recognize and internally represent 
their own knowledge. Just as people with "blindsight" are not consciously aware that 
they have vision, so monkeys do not seem to know what they know, or even that they 
know (Gallup, 1983; Humphrey, 1986). If this were the case we could hardly expect 
them to know how they got to know what they currently know; that is, they could not 
have Perner and Ruffman's "experiential awareness" and thus episodic memory. Great 
apes, on the other hand, have provided at least suggestive evidence that they may have 
at least some elements of a `theory of mind'.
	In fact, the whole enterprise of studying `theory of mind' development was 
triggered by Premack and Woodruff's (1978) experiments suggesting that 
chimpanzees attribute intention. This has received at least some support. Records of 
apparent compassion (e.g., Goodall, 1986), perhaps even empathy (Boesch, 1992), 
cooperation (e.g., de Waal, 1982, 1989; Menzel, 1974), imitation (Byrne, 1994; 
Meador et al., 1987), role-taking (Povinelli, Nelson, & Boysen, 1992; Povinelli, 
Parks, & Novak, 1992) and tactical deception (e.g., Byrne & Whiten, 1990, 1992; 
Whiten & Byrne, 1988) can be cited in support of the claim that great apes may have 
at least some understanding of motivational mental states. There is virtually no 
evidence for these qualities in monkeys.
	Evidence for the attribution of informational states, such as knowledge and 
belief, is less extensive. There are at least two recorded incidences of teaching 
(Boesch, 1992; Fouts et al., 1989). Although several ingenious attempts to 
experimentally prove that chimpanzees attribute informational states (Povinelli, 
Nelson, & Boysen, 1990; Premack, 1988) have been published, none has provided 
unequivocal evidence (Heyes, 1993; Gagliardi, Kirkpatrick-Steger, Thomas, Allen, & 
Blumberg, 1995). The only published attempts to show that apes may represent false 
beliefs, and thus have a fully-fledged `theory of mind', were unsuccessful (Premack, 
1988; Premack & Dasser, 1991).
	Heyes (e.g., 1993) has recently argued that all of the ape behavior that has 
been cited as evidence for `theory of mind' can be explained by learning processes, 
without the need to postulate the attribution of mental states. Accepting her position 
would mean that there is no reason to believe that even apes have the capacity for 
mental travel into the past. But even if we suppose that Heyes wielded Occam's razor 
a little too vigorously, and that chimpanzees can draw some inferences about the 
mental states of others (a view favoured by the authors), there may still be a 
significant gap between chimpanzees and humans (Premack, 1988). Apes may have 
developed only to the level of attributing motivational states. This, in the light of the 
importance of understanding past knowledge and belief, would render proper episodic 
memory impossible. If apes `only' fail to understand false beliefs, then they would still 
be short of comprehending the full extend of the perception-knowledge relationship. 
According to Perner and Ruffman's (1995) analysis, this shortcoming alone would 
make episodic memory impossible. Furthermore, if our proposed model is correct, 
then dissociation - the ability to simultaneously entertain different, even opposing 
mental states - is required for both, mental time travel and attribution of false beliefs. 
We have no reason to believe that chimpanzees, or any other animal, has mastered this 
mental feat.
	Finally, while we acknowledge the risk of arguing from ontogeny to 
phylogeny, the timing of the onset of episodic memory in humans may put it out of 
reach for chimpanzees. Premack (1988) and Parker and Gibson (1979) have proposed 
as a rule of thumb that what a child of three and a half cannot do also cannot be done 
by a chimpanzee. This appears to be true, for example, of language development (e.g., 
Bickerton, 1986; Pinker, 1994). This need not imply that chimpanzees are simply 
developmentally arrested children (cf., Povinelli, 1993); species-specific differences 
in mental capacities surely exist, and may be qualitative as well as quantitative. 
Nevertheless, if we are to ask whether chimpanzees have the ability to mentally travel 
in time, it seems reasonable to ask whether they can master the steps that humans have 
to master in the process of acquiring that ability. On present evidence we have to 
answer this question in the negative.
	Mental Travel into the Future
	In view of the generative aspect of episodic memory, it seems reasonable to 
suppose that basically the same mechanisms might be involved in imagining the 
future as in constructing the past. Time travel into the future is in a sense an 
extrapolation from time travel into the past, similarly involving the ability to escape 
the influence of the current mental state. The same mental platform might be used to 
entertain scenarios in different modes (such as what was, would, could, should, might 
or will be).
	It is important to distinguish mental time travel into the future from 
anticipatory behavior. This is a distinction that in some respects parallels that between 
episodic and other memory systems, which may reflect the influence of the past 
without necessarily involving mental time travel into the past. Similarly, many 
behaviors involve anticipation of future events in some way, but need not involve the 
actual simulation or imagining of future events. The link with memory runs even 
closer; learning and memory are themselves as much oriented to the future as to the 
past, because they increase the organism's chances of future survival.
	Insight-free instincts, such as hibernation, provide a further mechanism for 
dealing with recurring environmental changes, but again there is no need for the 
organism to actually imagine the future. Hibernators prepare for winter even if they 
have not experienced that season before. True anticipation of the future, involving the 
imagining of different scenarios, is what we might consider intelligent rather than 
instinctual. The distinction may sometimes be elusive, however, and Gibson (1990) 
suggested that instinct and intelligence should be regarded, not as polar opposites, but 
as the two ends of a continuum, which she calls "mental constructional ability." 
	Be that as it may, the insightful behavior shown by K”hler's (1917/1927) apes 
implies constructive thought with an eye to the future solution of a problem, and 
seems clearly more intelligent than instinctive. Even more strikingly, D”hl (1970) 
showed that the chimpanzee Julia was able to look several steps ahead in a sequential 
problem-solving task. She had to choose between two keys in a transparent box which 
opened further boxes with keys, until she arrived at a final box that contained either 
nothing or a food reward. Only by choosing the right keys at each point was she able 
to obtain the reward. Julia learned to act, not by chance, but by determining the route 
leading to reward before she chose the initial key. Since each trial involved a different 
sequence, this learning could not be accomplished by simple chaining. Julia was able 
to look as many as five steps ahead in pursuit of the final goal, an anticipatory skill 
that some chess players might envy.
	Chimpanzee tool cultures also suggest flexible forethought. For example, the 
chimpanzees at Gombe manufactured pointed tools from sticks at one place to use 
them later for termite fishing at another place that was out of sight (Goodall, 1986). 
Since the stick is trimmed to give it a pointed end, Whiten and Byrne (1991) argue 
that besides seeing the stick as a stick, the animal must also generate a meta-
representation of it as a termite probe.
	But despite this evidence for chimpanzees' capacity to imagine the future, 
K”hler (1917/1927) earlier suggested an important restriction: The anticipations do 
not go beyond the context of the present. Sultan's construction of the future, which 
enabled him to solve the problem and get the bananas, was bound by the context of 
his present hunger. The same is true of the more recent examples: Julia's performance 
was driven by her present desire for food reward, and the Gombe chimpanzees' 
manufacture of sticks by their appetite for termites. K”hler viewed such anticipations 
as essentially belonging to the present. The same theme has appeared in the writings 
of other authors. For example, Donald (1991) recently wrote that ape's behavior, 
"complex as it is, seems unreflective, concrete, and situation-bound" (p. 199). 
Conversely, Stebbins (1982) and Eccles (1989) refer to "time-binding," meaning 
simultaneous access to past and future, as uniquely human.  
The Bischof-K”hler Hypothesis
	Bischof (1978, 1985) and Bischof-K”hler (1985), based on K”hler's writings, 
suggest a more explicit limit on the extent to which animals can represent the future. 
Their hypothesis is that animals other than humans cannot anticipate future needs or 
drive states, and are therefore bound to a present that is defined by their current 
motivational state.  We shall refer to this as the Bischof-K”hler hypothesis, noting that 
the name acknowledges all three of its proponents, namely, Wolfgang K”hler, Norbert 
Bischof, and Doris Bischof-K”hler.
	The hypothesis still retains a measure of ambiguity, since there is no clear 
definition of drive or need. It relies instead on commonsense notions.  Bischof (1985) 
illustrates with the example of a homeostatic drive, thirst. When an animal is thirsty, it 
tries to find drink: Perception is focused on key stimuli, memory is searched, perhaps 
a plan of action is worked out. To begin these procedures, however, the animal must 
in fact be thirsty. Humans, by contrast, plan the future regardless of present need; a 
full-bellied lion is no threat to nearby zebras, but a full-bellied human may be. We 
humans anticipate future needs in multifarious ways, as when we buy food or other 
provisions, install burglar alarms, or manufacture or purchase tools. Business is to a 
great extent dependent on anticipation of our own and others' future needs.
	The Bischof-K”hler hypothesis is consistent with the idea, developed earlier, 
that nonhuman species may be unable to dissociate another mental state from their 
present one. Future need anticipation therefore might be only a special case of 
animals' general problem with simultaneously representing conflicting mental states. 
Like three-year old children, they may be unable to imagine an earlier belief (or state 
of knowledge, or drive, etc.) that is different from a present one, or to understand that 
another individual holds a belief different from their own. This may apply to future 
states as well as to past ones. That is, a satiated animal may be unable to understand 
that it may later be hungry, and therefore unable to take steps to ensure that this future 
hunger will be satisfied.
	Griffin (1978) pointed 17 years ago to the importance of studying animals' 
sense of a remote future, or in terms of the Bischof-K”hler hypothesis, to a future 
beyond the present drive state, but to date little has been published on the topic. The 
evidence that exists is anecdotal. Goodall (1986), for example, records the case of a 
chimpanzee, Satan, who followed a female in estrus, then slept close beside her. This 
suggests an activity designed for sexual gratification the next morning. Even if Satan 
planned this, one can still argue that he was acting according to his present sexual 
drive; that is, his plan did not extend into the "future" in K”hler's sense.  
	Bischof (1985) suggests that, in the course of evolution, there was a 
progressively increasing gap between drive and action. Great apes display quite 
extensive gaps; they can postpone the immediate enactment of their current drive, and 
make plans to receive gratification at a later point in time. De Waal (1982), for 
example, reported an incidence in the Arnhem Zoo in which the researchers hid 
grapefruit in the chimpanzee enclosure by burying them in the sand. The chimpanzees 
searched enthusiastically but apparently unsuccessfully, although several, including 
Dandy, passed over the spot. Later in the afternoon, unnoticed by the others, Dandy 
went straight to the spot, dug up the grapefruit, and enjoyed them without competition 
from the others. Similar examples of tactical deception have been recorded by Byrne 
and Whiten (1990). Such cases may demonstrate an impressive delay of gratification 
to achieve greater gain, but they do not necessarily reflect mental travel beyond the 
present drive state.
	Chimpanzees carry stones over long distances to open nuts at a place where no 
suitable stones can be found (Boesch & Boesch, 1984), but even this fairly extreme 
example of forethought may still be controlled by a single drive state. "What is 
imagined is the resonance of current needs in a future environment" (translated from 
Bischof, 1985, p. 541).
	Another anecdote that suggests an awareness of the future was recounted by 
Byrne and Byrne (1988). A group of chimpanzees surrounded a cave in which a 
leopard and its infant had hidden, and amid much excitement, and after several 
unsuccessful attempts, one old male lunged into the cave and emerged with a very 
small leopard cub. The group inspected the cub, bit it, and eventually killed it. 
However they did not eat it, and some of them (not the killers) groomed its body. One 
interpretation of this behavior is that the chimpanzees had acted to eliminate a future 
predator. But is this what they had in mind when they began their siege? We do not 
really know.
	An anecdote recounted by de Waal (1982) is perhaps more compelling:

	It is November and the days are becoming colder.  On this particular morning 
Franje collects all the straw from her cage (subgoal) and takes it with her under her 
arm so that she can make a nice warm nest for herself outside (goal).  Franje does not 
do this in reaction to the cold, but before she can have actually felt how cold it is 
outside. (p. 192)

However no further details are provided, and taken by itself it scarcely provides a 
convincing refutation of the Bischof-K”hler hypothesis.
	The widespread use of anecdotes in the 19th century led to wildly exaggerated 
accounts of the mental capacities of nonhuman animals--Lindsay (1880), for example, 
concluded that animals engage in criminal activities and commit suicide. There was 
also the infamous case of Clever Hans, the horse that appeared to be able to perform 
prodigious feats of arithmetic by tapping out the answers to questions put to him by 
his owner. It transpired that the owner, unknownst even to himself, was giving subtle 
signals to the horse that indicated when to stop tapping (Pfungst, 1911/1965). Claims 
about animal intelligence came to be mistrusted, and a more sceptical attitude was 
enshrined in Lloyd Morgan's canon and the principle of parsimony. However the 
pendulum may have swung too far, making it virtually impossible even to obtain 
evidence of future time travel. We may now be entering a phase of more balanced 
enquiry. The anecdotal method has been successfully introduced for studying primate 
deception (Whiten & Byrne, 1988; Byrne & Whiten, 1985, 1990, 1992), and a similar 
survey of anecdotes relating to mental time travel into the future has also been 
instigated (Suddendorf, 1994). This has yet to reveal convincing evidence of mental 
travel into the future by nonhuman primates. 
	Of the 73 leading primatologists, comparative psychologists and 
representatives of the ape-language projects initially surveyed, only five contributed 
observations they thought might contradict the Bischof-K”hler hypothesis. None of 
these observations described clear cases of future-need anticipation such as refinment 
or continued carrying of tools after need satisfaction or, in the case of the ape-
language studies, the acquisition and appropriate use of words referring to the remote 
future. Only two respondents, Tutin (see below) and Savage-Rumbaugh, stated that 
they believed apes to be capable of anticipating the future beyond the current state of 
needs/drives. Savage-Rumbaugh, however, appears to have changed her view, since 
she has recently stressed the importance of the encoupling of current and future needs 
in hominid evolution (Savage-Rumbaugh, 1994a). 
	While this survey confirms that the Bischof-K”hler hypothesis is consistent 
with our current data, it still remains difficult to distinguish mental time travel from 
instinctive behavior that may give the appearance of forethought. As de Waal (1982) 
has pointed out, for example, adolescent humans often provoke and challenge their 
parents in displays of independence, but are generally unaware of the true motive for 
their actions, which are based on instinct rather than explicit mental constructions of 
the future. De Waal suggests a similar explanation for the apparent strategic 
intelligence displayed by an ex-alpha male, called Yeroen, at the Arnhem chimpanzee 
colony. After losing his alpha status to Luit, Yeroen formed an alliance with a third 
male Nikkie, a strategy that eventually brought him back to power. The strategy was 
at first unsuccessful, and took months to pay off. Although noting that alternative 
explanations are possible, de Waal suggests that the strategy may not have been 
formulated with the future goal in mind. Even so, such anecdotes clearly raise the 
possibility that chimpanzees have a greater capacity for forward planning than we are 
yet willing to grant them.
	Similar arguments may apply to the acquisition of mental maps for future use. 
Chimpanzees and gorillas seem to acquire an extensive knowledge of territory, 
allowing them later to take the shortest route to trees when they fruit, or to stones for 
opening nuts (C.E.G. Tutin, record #14 in Suddendorf, 1994; Boesch & Boesch, 
1984). Whether this knowledge is acquired intentionally, having in mind its 
usefulness for future needs, is questionable. Spatial knowledge seems to be acquired 
implicitly rather than explicitly, and may be a general adaptive mechanism that 
requires no explicit reference to the future.
	Evolutionary Considerations
	There must be some question as to why it might be adaptive to travel mentally 
into the past when phylogenetically older forms of memory already allow for learning 
from a single event. Part of the answer may lie in the nature of the information 
extracted. Sherry and Schacter (1987) argue that the older form of memory 
(procedural) is essentially concerned with extracting invariances from stimulus events, 
as in pattern recognition, whereas the newer form is concerned with preserving the 
individuality of events. Since these characteristics are mutually incompatible, the later 
form of memory evolved as a separate system. While this distinction may capture the 
difference between nondeclarative and declarative memories, however, it does not 
seem to capture that between the two varieties of declarative memory, namely, 
semantic and episodic. Semantic memories themselves may vary considerably in 
individuality; knowing that Canberra is the capital of Australia, for example, is more 
specific than knowing what a capital is. Even more individual, however, is 
remembering precisely when and how we learned that Canberra is the capital of 
Australia. (Some of our readers may have learned it just then).
	The ability to travel mentally back in time may confer the added advantage of 
allowing events to be repeated, mentally if not physically, so that we can reflect on 
them, draw more general or abstract conclusions from them, and so on. In that sense, 
episodic memory may contribute to the elaboration of semantic memory. On these 
grounds, some have argued that episodic memory may have preceded semantic 
memory in hominid evolution (e.g., Donald, 1991; Seamon, 1984). However since 
other species seem to be capable of at least a primitive form of semantic memory, we 
agree with Tulving (1983, 1984, 1985) that episodic memory emerged later, but then 
allowed the semantic memory system to develop more fully. Kinsbourne and Wood 
(1975) have shown that the absence of episodic memory slows the acquisition of new 
knowledge.
	This relationship is also observed in human development, and we suggest that 
again semantic memory preceeds episodic memory. The Taylor et al. studies (1994, 
see above) show that children between four and five years of age begin to remember 
learning events, and in so doing gradually overcome so-called source amnesia. This 
age period can therefore be viewed as containing the onset of semantic learning based 
on episodic memory. That is, only by this age can children travel mentally back to the 
source of their knowledge and, for example, assess the accuracy and reliability of the 
source or whether there might other things to be learned from the event. With mental 
access to the learning event children can truly become generative in Corballis' (1991) 
sense, because knowledge can be flexibly transferred across different domains. This is 
supported by the recent finding that when false-belief tasks are passed, and thus 
dissociation is evident, children generate significantly more, and more diverse, 
answers to simple problems (Suddendorf & Fletcher-Flinn, 1996).
	Although the ability to build up semantic memory increases the fitness of the 
organism, we doubt that this fully explains the evolution of mental time travel. Rather, 
the precursors of mental time travel, such as the ability to attribute mental states to 
others, may have evolved as a result of the pressures of an increasingly complex social 
structure. This underlies the theory of so-called "Machiavellian intelligence" (Byrne & 
Whiten, 1988; Humphrey, 1976, 1986; Jolly, 1966); at some point in primate 
evolution, there was a selective pressure for the ability to read the minds of other 
individuals, since this allowed for better planning, cooperation, imitation, and 
teaching--and, no doubt, deception. Humphrey (1986) argues that the human desire 
for varied experience emerged because it allowed individuals to understand others; in 
a sense, psychology was born. Self-knowledge might then have been an exaptation 
derived from the ability to know others. As support for this, it has been noted that 
chimpanzees reared in social isolation seem unable to recognize themselves in a 
mirror (Gallup, McClure, Hill, & Bundy, 1971).
	These considerations suggest that the real importance of mental time travel 
applies to travel into the future rather than into the past; that is, we predominantly 
stand in the present facing the future rather than looking back at the past. This 
assertion is supported by the finding that "children can judge the forward order of 
parts of the day, days of the week, and months of the year at earlier ages than they can 
mentally move backwards through the sequences" (Friedman, 1992, p.173). This may 
help explain why we are in fact such poor witnesses. That is, the constructive element 
in episodic recall is adaptive in that it underlies our ability to imagine possible 
scenarios rather than actual ones, but it may be rather maladaptive with respect to 
reconstruction of the actual past. If it were important to remember the past in faithful 
detail, then we might have expected a more efficient system to have evolved. Instead, 
we bolster our faulty memories with external storage systems, such as drawings, 
books, tapes, films, and computer disks, leaving our minds ever freer to create 
scenarios for the future (not to mention fantasies about the past).
	The ability to represent possible future events has clear advantages over the 
older systems for generating anticipatory behavior, namely, instinct and learning. The 
flexibility of the newer system allows one to consider different options, whereas 
inherited instincts or insight-free acquisition of response patterns are effectively fixed 
by the motivational state of the organism and by environmental contingencies. 
Through the combining of different options, we can generate scenarios that are highly 
specific, and that are novel; we can plan to do things we have never done before. The 
future exerts so obvious an influence over our thoughts and actions, and indeed over 
the shaping of society itself, that it needs no further elaboration here.
	In recent years motivation theorists have come to appreciate that human 
behavior is not governed merely by internal drives, habits, and external stimuli, but 
depends very largely on anticipatory cognition. Bandura (1991) writes that "even in 
the so-called biological motivators, human behavior is extensively activated and 
regulated by anticipatory and generative cognitive mechanisms rather than simply 
impelled by biological urges" (p. 70). This is not to say that humans have overcome 
their biological needs; rather, they have the capacity to integrate the enactment of 
present and future drives in a complex set of action plans directed at a variety of goals. 
Only through considering the cognitive component, and the importance of mental time 
travel, can we begin to explain the evolution of human volition, including such 
biologically paradoxical phenomena as celibacy or hunger strikes.
	The self-regulation required for the management of our motivation appears to 
begin with the emergence of mental time travel, that is at around age 4 (Perner, 1991). 
Because of the limited scope of this paper we refer the reader to the work of Kuhl 
(e.g., Kuhl & Kraska 1989) for an excellent analysis of the development of meta-
volition and to Frankfurt (1988) for a philosophical discussion of the logic behind this 
issue. That meta-motivation is vital for human culture, however, should be clear 
without further elaboration. 
When Did Mental Time Travel Evolve?
	We suggested earlier that a critical ingredient of mental time travel is 
dissociation, or the ability to maintain different mental states simultaneously. Savage-
Rumbaugh (1994a) has proposed the intriguing hypothesis that this may have arisen 
as a consequence of bipedal locomotion and the ensuing problem of transporting 
infants. The precursors of the hominoids moved primarily by brachiation--swinging 
from branch to branch--as gibbons and siamangs do today. Infants were transported 
simply by clinging, and the mother could assist by simply raising her hindlegs to 
provide extra support. With deforestation during the Miocene, it was necessary to 
develop alternative methods of locomotion across the savanna, between forest 
patches. Chimpanzees and gorillas solved this problem by knuckle-walking, which 
allowed the infant to cling to the mother's back2.
	However the hominids, for whatever reason, adopted a bipedal mode of 
locomotion, which posed a problem in the transportation of infants. It was no longer 
sufficient to assume that the infant would simply cling, and greater demands were 
placed on the mother--and perhaps the father as well--to ensure that the infant was 
supported and monitored. Infants would be put down while sleeping, but it would be 
important to remember them, and pick them up before moving on. Human infants are 
held in front of the parent, allowing a more direct monitoring of their expressions, 
direction of gaze, and attentional fixations. In short, it may have proven adaptive for 
the parent to be able to take the perspective of the infant, mentally as well as 
physically.
	The requirement to monitor the presence and needs of an infant may have led 
to an expansion of the ability to keep several things in mind. Savage-Rumbaugh 
suggests that this also enabled hominids to carry tools and weapons that were related 
not to current needs, but to contingencies that might arise, such as unexpected attacks, 
or terrains unlike those encountered before. It may well have been such considerations 
that eventually permitted the migration of Homo erectus from Africa to various parts 
of the Old World. This is generally considered to have begun from about 1.6 million 
years ago, although recent dating of fossils from Java suggest that some migration 
may have occurred some 1.8 million years ago, before any known evidence of bifacial 
tools (Swisher, Curtis, Jacob, Getty, Suprijo, & Widiasmoro, 1994). These migrations 
took H. erectus into diverse environments with differing climates, suggesting a facility 
for rapid adaptation. Instead of slow morphological adaptations, such as changes in 
size or the growth of fur, these early hominids must have been able to construct 
ecological niches in conditions that originally could not have met human 
requirements. So began the human propensity to shape virtually any terrestrial 
environment to our own ends.
	It is sometimes suggested that the stone tools of the so-called Oldowan culture, 
dating from some 1.6 to 2.4 million years ago, provide the earliest evidence for 
deliberate planning for the future. These tools are generally associated with Homo 
habilis, regarded as the first hominid to show an increase in cranial size beyond that of 
an ape. While there has been considerable emphasis in the past on the importance of 
tools in early hominid evolution, recent evidence has suggested a reappraisal. For 
example, the creation of simple Oldowan tools appears to be within the competence of 
modern chimpanzees (Toth et al., 1993; Wynn & McGrew, 1989), and it has been 
claimed that the tool culture of Tai chimpanzees, although not involving the making 
of stone tools, represents a comparable stage of development (Boesch & Boesch, 
1984). Moreover, while the production of an Oldowan tool may require some advance 
mental picture of the finished product and the use to which it will be put, it is not 
convincing evidence for mental time travel according to the Bischof-K”hler 
hypothesis. Like Tai chimpanzees, H. habilis may have manufactured primitive stone 
tools simply to satisfy a current need.
 	The more sophisticated Acheulian culture associated with H. erectus around 
1.6 million years ago may provide more convincing evidence. For example, the 
bifacial hand-ax involved symmetrical removal of flakes from a stone core to produce 
a tool that was sharper and more pointed than the primitive Oldowan scrapers. This 
more costly and time consuming procedure suggests that these tools were not intended 
for one-time use only, but were kept for future use. This implies that the manufacturer 
was able to anticipate future needs, possibly extending beyond the present drive state 
(Suddendorf, 1994); as Savage-Rumbaugh (1994a) puts it, the Acheulian hand ax 
provides the first evidence of the "uncoupling" of present and future needs. We might 
regard this as representing an intermediate stage of mental time travel, perhaps 
roughly that of a four-year-old human child, in which the simulation of past and future 
episodes was possible, but there was little development of abstract semantic concepts 
and theories about the future.
	With Homo sapiens neandertalensis, between 100,000 and 35,000 years ago, 
we find the first evidence for burial and associated rituals. This perhaps signals a final 
step in the freeing of mental time travel, to the point that it outstrips bodily time 
travel, giving rise to that singularly unwelcome concept--death. Consequently, 
personal identity through time must have existed in Neanderthals. Here too we see 
evidence of the generative nature of time travel, in which scenarios are created for the 
possibility of life after death. Religion was born.
	As we have already intimated, the emergence of mental time travel may have 
been dependent on increased encephalization, beginning with H. habilis and reaching 
its peak some 300,000 years ago with Homo sapiens. Not every part of the brain 
enlarged at the same rate, however. The limbic system, a prominent structure in most 
mammalian brains, significantly reduced in relative size. Given its role in basic 
motivation (drives, needs, and emotions), this might be taken as evidence that other 
parts of the brain became increasingly important in driving behavior. This is not to say 
that emotions are no longer an important part of the human condition - the limbic 
system did increase in absolute size. With mental time travel, "cognition" challenged 
"impulse" for the driver's seat, as it were. The neurological correlate appears to be the 
disproportionate development of the prefrontal lobe, which is reciprocally connected 
to the limbic region and to sensory association areas (Fuster, 1989). The prefrontal 
cortex plays a vital role in subjective self-awareness (Freeman & Watts, 1942), 
temporal organization of action (Fuster, 1989; Ingvar, 1985), and episodic memory 
(Shimamura et al., 1990). Lesions to the prefrontal area may also lead to impaired 
goal-directed behavior, lack of ambition, apathy, unawareness of behavioral 
consequences, or what Ingvar (1985) refers to as a "lack of future". 
Relation to Language
	One characteristic of mental time travel that distinguishes it from instinct and 
associative learning is its flexibility. That is, given a basic vocabulary of actors, 
objects, and events, we can reconstruct unique episodes in the past, and create 
scenarios to deal with unique contingencies in the future. This ability to generate an 
infinite variety of combinations from a finite vocabulary is also what characterizes 
human language, and sets it apart from the communication systems of all other species 
(Chomsky, 1988). Generativity may not be unique to language, but may be an aspect 
of thought that arose as a means of rapid adaptation to complex physical and social 
environments. 
	Again it may have been the emergence of multiple monitoring that led to the 
development of language from a relatively crude associative device that may be 
within the competence of both chimpanzee and two-year-old child, to the 
sophisticated generative, recursive system that every human over the age of about four 
seems effortlessly to have acquired. The ability to create a sentence with an embedded 
clause, such as this one, requires that one keep track of the overall structure while the 
embedded clause is generated. That is, even at the level of word production, multiple 
monitoring (and short-term memory) is required. But one must also keep track of 
meaning--what it is one is trying to say.
	True language may also require a dissociation between one's own thoughts and 
the thoughts of those to whom we speak. Premack and Premack (1994) have 
emphasized that human language requires a theory of mind; through language, we aim 
not merely to change the behaviors of others, but to change their beliefs. This of 
course requires that we have a theory of what others believe; that is, a theory of others' 
minds. We attribute mental states to the people we talk to, but dissociate those states 
from our own. We speak differently to an ignorant audience than to a knowledgeable 
one, to an angry person than to a happy one. We have argued in this article that this 
ability to dissociate is also involved in mental time travel.
	Recursion itself depends on dissociation. For example, social behavior may be 
governed by the knowledge that individual A knows something, or that A knows that 
B knows something. Mental time travel may involve similar propositions: I am not 
hungry now, but I know that I will be hungry soon; I am here today, but last week I 
was in Wellington and went to the opera. These kinds of propositions are 
characteristic of the sorts of things that we use language to express. Premack and 
Premack (1994) suggest that the attributions involved in language may involve as 
many as four levels of meta-representation: "A speaker believes that his listener 
believes that he will tell the truth; further that the listener believes he believes that the 
listener believes that he will tell the truth" (p. 105). 
	These considerations need not imply that mental time travel is dependent on 
language. Intuitively, at least, we seem to be able to create or recreate scenarios that 
rely on imagery rather than on language, and indeed it is not always easy to express in 
words something that we have seen. This suggests that language and mental time 
travel both exploit more general attributional, dissociative, and generative abilities. 
Even so, language is in many ways ideally crafted to recount episodes and sequence 
them into narratives (Corballis, 1994; Pinker & Bloom, 1990). Episodes are often 
about who did what to whom, and when, and where, and why, and what happened 
next. Although mental time travel and language may well have co-evolved to some 
extent, we suggest that the true priority lay with mental time travel; that is, the ability 
to generate mental experience probably preceded the ability to communicate it.  
	It is worth noting, however, that recent research on counting in chimpanzees 
(e.g., Boysen & Berntson, 1995) demonstrated how symbolic systems can foster the 
detachment from immediate impulses. Selection of one of two arrays with different 
amounts of candy, resulted in the other, nonselected, array being received. Thus, 
choosing the smaller array results in more obtained candy. The chimpanzees seemed 
not to comprehend this simple, yet counterintuitive, rule and tended to choose the 
larger array. However, when the actual candy was replaced by Arabic numerals, the 
chimpanzees reliably selected the smaller number to obtain the larger reward.  
Apparently, the symbolic system helped the chimpanzees to override their natural 
impulse, or evaluative disposition (to select the larger amount of candy), and created 
the space for applying what cognitvely was well understood. These results suggest 
that symbolic representation might have paved the way for effective meta-motivation, 
that is, the practical application of forethought to behavior. Language clearly was 
important for the evolution of the fully-fledged mental time travel capacity.
	Conclusions
	We have argued that the ability to travel mentally in time constitutes a 
discontinuity between humans and other animals. Current empirical data and 
theoretical analyses from a wide range of research fields has been brought together in 
support for our argument. We recognize, however, that the ideas we have developed in 
this article might at times be no more than "just so" stories, in which it is assumed that 
things simply had to be the way they are. Moreover, we have relied fairly extensively 
on comparisons between apes and humans, on the grounds that chimpanzees, in 
particular, are closest to humans in genetic makeup (e.g., Miyamoto, Slightom, & 
Goodman, 1987). However the most recent common ancestor of humans and 
chimpanzees probably existed some 5-7 million years ago, so considerable divergence 
can be expected. Our hominid ancestors lived in very different environments, and 
were subjected to very different selective pressures. Inevitably, then, there is a good 
deal of pure speculation in any attempt to bridge the gap between ape and human, and 
there may be important respects in which comparisons with other species may be 
more relevant. 
	However, it is important to conduct meta-analyses that integrate up-to-date 
data from diverse and fast paced fields. This is particularly critical if the analysis can 
shed new light on the data by providing a novel perspective. We believe that the 
obviously important, yet largely overlooked, human ability to travel mentally in time, 
constitutes such a perspective. Our analysis challenges experimenters to provide 
evidence for mental time travel in other species and to study its development in 
children (a more promising area of research). Anecdotes too should be subjected to 
careful scrutiny to ensure that they meet appropriate criteria. Demonstrations of 
putative time travel must not merely reflect habits, or instinctive behaviours, or 
behaviors based on semantic knowledge or generalized rules. The essence of mental 
time travel lies in its particularity, and this in turn implies the ability to generate 
unique representations from combinations of elements. We believe that the 
importance of mental time travel as a prime mover in human cognitive evolution has 
not been adequately recognized. It may hold the key to the evolution of such 
characteristically human phenomena as agriculture, morality, philosophy, science, 
technology, and trade.


Author Note

	Thomas Suddendorf, Department of Psychology; Michael C. Corballis, 
Department of Psychology. 
	This monograph draws in part on the first author's Masters thesis "Discovery 
of the Fourth Dimension" (1994). We thank Richard W. Byrne for comments on an 
earlier draft of the manuscript. 
	Correspondence concerning this article should be addressed to Thomas 
Suddendorf, Department of Psychology, University of Auckland, Private Bag 92019, 
Auckland, New Zealand. Electronic mail may be sent to: 
t.suddendorf@auckland.ac.nz


NOTES

	References

	Amsterdam, B.K. (1972). Mirror self-image reactions before age two. 
Developmental Psychobiology, 5, 297-305.
	Anderson, J.R. (1986). Mirror mediated finding of hidden food by monkeys 
(Macaca tonkeana and M. fascicularis). Journal of Comparative Psychology, 100, 
237-242.
	Ashmead, D.H., & Perlmutter, M. (1980). Infant memory in everyday life. In 
M. Perlmutter (Ed.), New directions for child development (Vol. 10). San Francisco: 
Jossey-Bass.
	Astington, J.W., Harris, P.L., & Olson, D.R. (Eds.). (1988). Developing 
theories of mind. London: Cambridge University Press.
	Bandura, A. (1991). Self regulation of motivation through anticipatory and 
self-reactive mechanism. In R.A. Dienstbier (Ed.), Perspectives on motivation: 
Nebraska symposium on motivation (pp. 69-164). Lincoln: Nebraska University 
Press.
	Baron-Cohen, S. (1989). Are autistic children behaviorists? An examination of 
their mental-physical and appearance-reality distinctions. Journal of Autism and 
Developmental Disorders, 19, 579-600.
	Baron-Cohen, S. (1995). Mindblindness. Cambridge, MA.: MIT Press.
	Baron-Cohen, S., & Goodhart, F. (1994). The "seeing leads to knowing" 
deficit in autism: The Pratt and Bryant probe. British Journal of Developmental 
Psychology, 12, 397-402.
	Baron-Cohen, S., Leslie, A., & Frith, U. (1985). Does the 
autistic child have a `theory of mind'? Cognition, 21, 37-46.
	Bartlett, F.C. (1932). Remembering. London: Cambridge University Press.
	Bauer, P.J., Hertsgaard, L.A., & Dow, G.A. (1995). After 8 months have 
passed: longterm recall of events by 1-to 2-year-old children. Memory, 2, 353-382.
	Beck, B.B. (1980). Animal tool behavior: The use and manufacture of tools by 
animals. New York: Garland STPM Press.
	Bickerton, D. (1986). More than nature needs?  A reply to Premack. 
Cognition, 23, 73-79.
	Bischof, N. (1978). On the phylogeny of human morality. In G. Stent (Ed.), 
Morality as a biological phenomenon (pp. 53-74). Berlin: Abakon.
	Bischof, N. (1985). Das R„tzel ™dipus [The Oedipus riddle]. Munich: Piper.
	Bischof-K”hler, D. (1985). Zur Phylogenese menschlicher Motivation [On the 
phylogeny of human motivation]. In L.H. Eckensberger & E.D. Lantermann (Eds.), 
Emotion und Reflexivit„t (pp. 3-47). Vienna: Urban & Schwarzenberg.
	Bloom, P. (1994). Generativity within language and other cognitive domains. 
Cognition, 51, 177-189.
	Boesch, C. (1991). Teaching wild chimpanzees. Animal Behavior, 41, 530-
532. 
	Boesch, C. (1992). New elements of a theory of mind in wild chimpanzees. 
Behavioral and Brain Sciences, 15, 149-150.
	Boesch, C., & Boesch, H. (1984). Mental map in wild chimpanzees: An 
analysis of hammer transports for nut cracking. Primates, 25, 160-170.
	Boucher, J., & Lewis, V. (1989). Memory impairments and communications in 
relatively able autistic children. Journal of Child Psychology and Psychiatry, 30, 99-
122.
	Boucher, J., & Warrington, E.K. (1976). Memory deficits in early infantile 
autism: Some similarities to the amnesic syndrome. British Journal of Psychology, 67, 
73-87.
	Boysen, S.T., & Berntson, G.G. (1995). Responses to quantity: perceptual 
versus cognitive mechanisms in chimpanzees (Pan Troglodytes). Journal of 
Experimental Psychology: Animal Behavior Processes, 21, 82-86.
	Bowers, K.S., & Hilgard, E. (1986). Some complexities in understanding 
memory. In H.M. Pettinadi (Ed.), Hypnosis and memory (pp. 3-18).  New York: 
Guilford Press.
	Byrne, R.W. (1994). The evolution of intelligence. In P.J.B. Slater & T.R. 
Halliday (Eds.), Behavior and evolution (pp. 223-265). London: Cambridge 
University Press.
	Byrne, R.W., & Byrne, J.M. (1988). Leopard killers of Mahale. Natural 
History, 3, 22-26.
	Byrne, R.W., & Whiten, A. (1985). Tactical deception of familiar individuals 
in baboons (Papio ursinus). Animal Behavior, 33, 669-673.
	Byrne, R.W., & Whiten, A. (Eds.) (1988). Machiavellian intelligence. Oxford: 
Clarendon Press.
	Byrne, R.W., & Whiten, A. (1990). Tactical deception in primates: The 1990 
database. Primate Report, 27, 1-101.
	Byrne, R.W., & Whiten, A. (1992). Cognitive evolution in primates. Man, 27, 
609-627.
	Cheney, D.L., & Seyfarth, R.M. (1990). How monkeys see the world. 
Chicago: University of Chicago Press.
	Chomsky, N. (1988). Language and the problem of knowledge: The Managua 
Lectures. Cambridge, MA: MIT Press.
	Corballis, M.C. (1991). The lopsided ape.  New York: Oxford University 
Press.
	Corballis, M.C. (1992). On the evolution of language and generativity. 
Cognition, 44, 197-226.
	Corballis, M.C. (1994). The generation of generativity: A response to Bloom. 
Cognition, 51, 191-198.
	Daechler, M., Bukatko, D., Benson, K., & Myers, N. (1976). The effects of 
size and color cues on the delayed response of very young children. Bulletin of the 
Psychonomic Society, 7, 65-68.
	Dawson, G., & McKissick, F.C. (1984). Self-recognition in autistic children. 
Journal of Autism and Developmental Disorders, 14, 383-394.
	de Waal, F. (1982). Chimpanzee politics. London: Jonathan Cape.
	de Waal, F. (1989). Peacemaking among primates. Cambridge, MA: Harvard 
University Press.
	Deacon, T.W. (1990). Brain-language coevolution. In J.A. Hawkins & M. 
Gelman (Eds.), The evolution of human languages. SFI Studies in the Sciences of 
Complexity, Proceedings (Vol. 10). Reading, MA: Addison-Wesley.
	DeCasper, A.J., & Fifer, W.P. (1980). Of human bonding: Newborns prefer 
their mother's voices. Science, 208, 1174-1176.
	DeCasper, A.J., & Spence, M.J. (1986). Prenatal maternal speech influences 
newborns' perception of speech sounds. Infant Behavior and Development, 9, 133-
150.
	Diamond, A. (1985). Development of the ability to use recall to guide action, 
as indicated by infants' performance on AB. Child Development, 56, 868-883.
	D”hl, F. (1970). Zielorientiertes Verhalten beim Schimpansen [Goal-directed 
behavior in chimpanzees]. Naturwissenschaft und Medizin, 34, 43-57.
	Donald, M. (1991). Origins of the modern mind. London: Harvard University 
Press.
	Dretske, T. (1982). The informational character of representations. Behavioral 
and Brain Sciences, 5, 376-377.
	Eccles, J.C. (1989). Evolution of the brain: creation of the self. London: 
Routledge.
	Flavell, J.H. (1993). The development of children's understanding of false 
belief and the appearance-reality distinction. International Journal of Psychology, 28, 
595-604.
	Fivush, R., Gray, J.T., & Fromhoff, F.A. (1987). Two-year-olds talk about the 
past. Cognitive Development, 2, 396-409.
	Fivush, R. & Hammond, N.R. (1990). Autobiographical memory across the 
preschool years: toward reconceptualizing childhood amnesia. In R. Fivush & J.A. 
Hudson (Eds.), Knowing and remembering in young children (pp. 223-248). New 
York: Cambridge University Press.
	Fouts, R.S., Fouts, D.H. & van Cantfort, T. (1989). The infant Loulis learns 
signs from cross fostered chimpanzees. In R.A. Gardner, B.T. Gardner & T.E. van 
Cantfort (Eds.), Teaching sign language to chimpanzees (pp. 280-292). New York: 
State University of New York Press.
	Frankfurt, H.G. (1988). The importance of what we care about. New York: 
Cambridge University Press.
	Freeman, W. & Watts, J. (1942). Psychosurgery: Intelligence, emotion, and 
social behavior following prefrontal lobotomy for mental disorder. Springfield, IL: 
Charles C. Thomas.
	Freud, S. (1966). Project for a scientific psychology. In J. Strachey (Ed. and 
Trans.), The standard edition of the complete works of Sigmund Freud (Vol. 1). 
London: Hogarth Press. (Original work published 1895)
	Friedman, S. (1972). Newborn visual attention to repeated exposure of 
redundant vs. "novel" targets. Perception & Psychophysics, 12, 291-294.
	Friedman, W.J. (1991). The development for children's memory for the time of 
past events. Child Development, 61, 139-155.
	Friedman, W.J. (1992). Children's time memory: the development of a 
differentiated past. Cognitive Development, 7, 171-187.
	Friedman, W.J. (1993). Memory for the time of past events. Psychological 
Bulletin, 113, 44-66. 
	Frith, U. (1989). Autism: Explaining an enigma. Oxford: Basil Blackwell.
	Fuster, J.M. (1989). The prefrontal cortex. New York: Raven Press.
	Gagliardi, J.L., Kirkpatrick-Steger, K.K., Thomas, J., Allen, G.J., & 
Blumberg, M.S. (1995). Seeing and knowing: knowledge attribution versus stimulus 
control in adult humans (homo sapiens). Journal of Comparative Psychology, 109, 
107-114.
	Gallup, G.G., Jr. (1970). Chimpanzees: self recognition. Science, 167, 86-87.
	Gallup, G.G., Jr. (1983). Toward a comparative psychology of mind. In R.L. 
Mellgren (Ed.), Animal cognition and behavior (pp. 473-510). New York: North-
Holland.
	Gallup, G.G., Jr. (1994). Self-recognition: Research strategies and 
experimental design. In S.T. Parker, R.W. Mitchell, & M.L. Boccia (Eds.), Self-
awareness in animals and humans (pp. 35-50). Cambridge: Cambridge University 
Press.
	Gallup, G.G., Jr., McClure, M.K., Hill, S.D., & Bundy, R.A. (1971). Capacity 
for self-recognition in differentially reared chimpanzees. Psychological Record, 21, 
69-74.
	Gardiner, J. & Java, R.I. (1990). Recollective experience in word and nonword 
recognition. Memory and Cognition, 18, 23-30.
	Gibson, K.R. (1990). New perspectives on instincts and intelligence: Brain 
size and the emergence of hierarchical mental construction skills. In S.T. Parker & 
K.R. Gibson (Eds.), "Language" and intelligence in monkeys and apes (pp.97-128). 
Cambridge: Cambridge University Press.
	Gibson, K.R. (1993). General introduction: Animal minds, human minds. In 
K.R. Gibson & T. Ingold (Eds.), Tools, language and cognition in human evolution. 
Cambridge: Cambridge University Press.
	Goodall, J. (1986). The chimpanzees of Gombe: patterns of behavior. 
Cambridge, MA: Harvard University Press.
	Gopnik, A. (1993). How we know our own minds: The illusion of first-person 
knowledge of intentionality. Behavioral and Brain Sciences, 16, 1-14.
	Gopnik, A., & Astington J.W. (1988). Children's understanding of 
representational change and its relation to the understanding of false belief and the 
appearance-reality distinction. Child Development, 59, 26-37.
	Gopnik, A., & Graf, P. (1988). Knowing how you know: Young children's 
ability to identify and remember the source of their belief. Child Development, 59, 
1366-1371.
	Gopnik, A., & Slaughter, V. (1991). Young children's understanding of 
changes in their mental states. Child Development, 62, 98-110.
	Gordon, R.M. (1986). Folk psychology as simulation. Mind and Language, 1, 
158-171.
	Greenfield, P.M., & Savage-Rumbaugh, E.S. (1990). Grammatical 
combination in Pan paniscus: Processes of learning and invention in the evolution and 
development of language. In S.T. Parker & K.R. Gibson (Eds.), "Language" and 
intelligence in monkeys and apes (pp. 540-578). Cambridge: Cambridge University 
Press.
	Griffin, D.R. (1978). Prospects for a cognitive ethology. Behavioral and Brain 
Sciences, 1, 527-538.
	Harris, P.L. (1991). The work of imagination. In A. Whiten (Ed.), Natural 
theories of mind: Evolution, development, and simulation of everyday mindreading 
(pp. 283-304). Oxford: Basil Blackwell.
	Hart, D., & Fegley, S. (1994). Social imitation and the emergence of a mental 
model of self. In S.T. Parker, R.W. Mitchell, & M.L. Boccia (Eds.), Self-awareness in 
animals and humans (pp. 149-165) Cambridge: Cambridge University Press.
	Hayes, C. (1951). The ape in our house. New York: Harper & Brothers.
	Heyes, C.M. (1993). Anecdotes, training, trapping, and triangulation: Do 
animals attribute mental states? Animal Behavior, 46, 177-188.
	Howe, M.L., & Courage, M.L. (1993). On resolving the enigma of infantile 
amnesia. Psychological Bulletin, 113, 305-326.
	Hughes, C., & Russel, J. (1993). Autistic children's difficulty with mental 
disengagement from an object: Its implications for theories of autism. Developmental 
Psychology, 3, 498-510.
	Hughes, C., Russel, J., & Robbins, T.W. (1994). Evidence for executive 
dysfunction in autism. Neuropsychologia, 32, 477-492.
	Humphrey, N.K. (1976). The social function of intellect. In P.P.G. Bateson & 
R.A. Hinde (Eds.), Growing points in ethology (pp. 303-317). Cambridge: Cambridge 
University Press.
	Humphrey, N.K. (1986). The inner eye. London: Faber & Faber.
	Ingvar, D.H. (1985). "Memory of the future:" An essay on the temporal 
organization of conscious awareness. Human Neurobiology, 4, 127-136.
	Jacoby, L.L., Kelley, C.M., & Dywan, J. (1989). Memory attributions. In H.L. 
Roediger & F.I.M. Craik (Eds.), Varieties of memory and consciousness: Essays in 
honor of Endel Tulving (pp. 391-422). Hillsdale, NJ: Lawrence Erlbaum.
	Jarrold, C., Carruthers, P., Smith, P.K., & Boucher, J. (1994). Pretend play: is 
it metarepresentational? Mind & Language, 9, 445-468.
	Johnson, C.N. (1988). Theory of mind and the structure of conscious 
experience. In J.W. Astington, P.L. Harris, & D.R. Olson (Eds.), Developing theories 
of mind (pp. 47-63). Cambridge: Cambridge University Press.
	Jolly, A. (1966). Lemur social behavior and primate intelligence. Science, 153, 
501-506.
	Kendrick, D.F., Rilling, M.E., & Denny, M.R. (1986). Theories of animal 
memory. Hillsdale, NJ: Lawrence Erlbaum.
	Kinsbourne, M. (1989). The boundaries of episodic remembering. In H.L. 
Roediger & F.I.M. Craik (Eds.), Varieties of memory and consciousness: Essays in 
honor of Endel Tulving (pp.179-191). Hillsdale, NJ: Lawrence Erlbaum.
	Kinsbourne, M. & Wood, F. (1975). Short-term memory processes and the 
amnesic syndrome. In D. Deutsch & J.A. Deutsch (Eds.), Short-term memory 
(pp.258-293). New York: Academic Press.
	K”hler, W. (1927). The mentality of apes (E. Winter, Trans.). London: 
Routledge & Kegan Paul. (Original work published 1917).
	Kuhl, J. & Kraska, K. (1989). Self-regulation and metamotivation: 
computational mechanisms, development, and assessment. In R. Kaufer, P.L. 
Ackerman & R. Cudeck (Eds.), The Minnesota Symposium on Learning and 
Individual Differences (pp. 343-374). Hillsdale, NJ.: LEA.
	Leslie, A. (1987). Pretence and representation in infancy: the origin of `theory 
of mind'. Psychological Review, 94, 412-426.
	Leslie, A., & Frith, U. (1988). Autistic children's understanding of seeing, 
knowing, and believing. British Journal of Developmental Psychology, 6, 315-329.
	Lindsay, W.L. (1880). Mind in the lower animals. D. Appleton.
	Loftus, E.F. (1993). Desperately seeking memories of the first few years of 
childhood: The reality of early memories. Journal of Experimental Psychology: 
General, 122, 274-277.
	Lyon, D.L., & Flavell, J.H. (1994). Young children's understanding of 
"remember" and "forget". Child Development, 65, 1357-1371.
	Marshall, J.C. (1982). A la representation du temps perdu. Behavioral and 
Brain Sciences, 5, 382-383.
	Meador, D.M., Rumbaugh, D.M., Pate, J.L., & Bard, K.A. (1987). Learning, 
problem solving, cognition, and intelligence. In J. Erwin (Ed.), Comparative primate 
biology: Vol. 2B (pp.17-83). New York: Alan R. Liss.
	Mental Simulation: Philosophical and Psychological Essays. (1992). Mind and 
Language, 7, 1 & 2.
	Menzel, E.W. (1974). A group of young chimpanzees in a one-acre field. In A. 
Schrier & F. Stollnitz (Eds.), Behaviour of non-human primates, Vol. 5 (pp. 83-153). 
New York: Academic Press. 
	Mitchell, R.W. (1994). Multiplicities of self. In S.T. Parker, R.W. Mitchell, & 
M.L. Boccia (Eds.), Self-awareness in animals and humans (pp. 81-107). Cambridge: 
Cambridge University Press.
	Miyamoto, M., Slightom, J.L., & Goodman, M. (1987). Phylogenetic relations 
of humans and African apes from DNA sequences in the psi-nu-globin region. 
Science, 230, 369-373.
	Nelson, K. (1992). Emergence of autobiographical memory at age 4. Human 
Development, 35, 172-177.
	Ogden, J.A., & Corkin, S. (1991). Memories of H.M. In W.C. Abraham, M.C. 
Corballis, & K.G. White (Eds.), Memory mechanisms: A tribute to G.V. Goddard (pp. 
195-215). Hillsdale, N.J: Lawrence Erlbaum.
	Olton, D.R. (1984). Comparative analysis of episodic memory. Behavioral and 
Brain Sciences, 7, 250-251.
	O'Neill, D.K., & Gopnik, A. (1991). Young children's ability to identify the 
sources of their beliefs. Developmental Psychology, 27, 390-397.
	Ozonoff, S., Pennington, B.F., & Rogers, S. (1991). Executive function 
deficits in high-functioning autistic children: Relationship to theory of mind. Journal 
of Child Psychology and Psychiatry, 32, 1081-1105.
	Parker S.T., & Gibson, K.R. (1979). A developmental model for the evolution 
of language and intelligence in early hominids. Behavioral and Brain Sciences, 2, 367-
408.
	Parker, S.T., Mitchell, R.W., & Boccia M.L. (Eds.). (1994). Self-awareness in 
animals and humans. Cambridge: Cambridge University Press.
	Passingham, R.E. (1982). The human primate. San Francisco: W.H. Freeman.
	Patterson, F. (1991). Self-awareness in the gorilla Koko. Gorilla, 14, 2-5.
	Pepperberg, I.M., Garcia, S.E., Jackson, E.C., & Marconi, S. (1995). Mirror 
use by African Grey Parrots. Journal of Comparative Psychology, 109, 182-195.
	Perner, J. (1991). Understanding the representational mind. Cambridge, MA: 
MIT Press.
	Perner, J., Frith, U., Leslie, A., & Leekam, S. (1989). Exploration of the 
autistic child's theory of mind. Child Development, 60, 689-700.
	Perner, J., & Ruffman, T. (1995). Episodic memory and autogenetic 
consciousness: developmental evidence and a theory of childhood amnesia. Journal of 
Experimental Child Psychology, 59, 516-548.
	Pfungst, O. (1965). Clever Hans, the horse of Mr. Von Osten (C.L. Rahn, 
Trans.). C.L. Rahn. New York: Holt, Rinehart & Winston. (Original work published 
1911) 
	Piaget, J. (1954). The construction of reality in the child. New York: Basic 
Books.
	Pillemer, D.B., & White, S.H. (1989). Childhood events recalled by children 
and adults. Advances in Child Development and Behavior, 21, 297-240.
	Pinker, S. (1994). The language instinct. New York: William Morrow.
	Pinker, S., & Bloom, P. (1990). Natural language and natural selection. 
Behavioral and Brain Sciences, 13, 707-784.
	Povinelli, D.J. (1989). Failure to find self-recognition in Asian elephants 
(Elephans maximus) in contrast to their use of mirror cues to discover hidden food. 
Journal of Comparative Psychology, 103, 122-131.
	Povinelli, D.J. (1993). Reconstructing the evolution of mind. American 
Psychologist, 48, 493-509.
	Povinelli, D.J. (in press). The undublicated self. In P. Rochat (Ed.), The self in 
early infancy. Amsterdam: North-Holland-Elsevier.
	Povinelli, D.J., Nelson, K.E., & Boysen, S.T. (1990). Inferences about 
guessing and knowing by chimpanzees (Pan troglodytes). Journal of Comparative 
Psychology, 104, 203-210.
	Povinelli, D.J., Parks, K.A., & Novak, M.A. (1991). Do rhesus monkeys 
(Macaca mulatta) attribute knowledge and ignorance to others? Journal of 
Comparative Psychology, 105, 318-325.
	Povinelli, D.J., Nelson, K.E., & Boysen, S.T. (1992). Comprehension of social 
role reversal by chimpanzees: evidence for empathy? Animal Behavior, 43, 633-640.
	Povinelli, D.J., Parks, K.A., & Novak, M.A. (1992). Role reversal by rehsus 
monkeys, but no evidence of empathy. Animal Behavior, 44, 269-281.
	Powell, S.D., & Jordan, R.R. (1993). Being subjective about autistic thinking 
and learning to learn. Educational Psychology, 13, 359-370.
	Premack, D. (1988). `Does the chimpanzee have a theory of mind?' revisited. 
In R.W Byrne & A. Whiten (Eds.), Machiavellian intelligence (pp. 160-179). Oxford: 
Clarendon Press.
	Premack, D., & Dasser, V. (1991). Perceptual origins and conceptual evidence 
for theory of mind in apes and children. In A. Whiten (Ed.), Natural theories of mind: 
Evolution, development and simulation of everyday mindreading (pp. 253-266). 
Oxford: Basil Blackwell.
	Premack, D., & Premack, A.J. (1994). How 'theory of mind' constrains 
language and communication. Discussions in Neuroscience, 10, 93-105.
	Premack, D., & Woodruff, G. (1978). Does the chimpanzee have a theory of 
mind? Behavioral & Brain Sciences, 4, 515-526.
	Roitblat, H.L. (1982). The meaning of representation in animal memory. 
Behavioral & Brain Sciences, 5, 353-406.
	Rovee-Collier, C.K., Sullivan, M.W., Enright, M., Lucas, D., & Fagen, J. 
(1980). Reactivation of infant memory. Science, 208, 1159-1161.
	Ross, L.D., Green, D., & House, P. (1977). The false consensus bias in self-
perception and social perception process. Journal of Experimental Social Psychology, 
13, 279-301.
	Russel, J., Jarrold, C., & Potel, D. (1994). Executive factors in preschoolers' 
strategic deception. British Journal of Developmental Psychology, 12, 301-314.77``
	 E. (1972). Episodic and semantic memory. In E. Tulving & W. Donaldson 
(Eds.), Organization of memory (pp. 381-403). New York: Academic Press.
	Tulving, E. (1983). Elements of episodic memory. London: Oxford University 
Press.
	Tulving, E. (1984). Precis of Elements of episodic memory. Behavioral and 
Brain Sciences, 7, 223-268.
	Tulving, E. (1985). How many memory systems are there? American 
Psychologist, 40, 385-398.
	Tulving, E. (1993). What is episodic memory? Current Directions in 
Psychological Science, 2, 67-70.
	Wellman, H.M. (1991). From desires to beliefs: Acquisition of a theory of 
mind. In A. Whiten (Ed.), Natural theories of mind: Evolution, development, and 
simulation of everyday mindreading (pp. 19-38). Oxford: Blackwell.
	Westergaard, G.C., & Suomi, S.J. (1994). The use and modification of bone 
tools by Capuchin monkeys. Current Anthropology, 35, 75-77.
	Whiten, A. (1991). Natural theories of mind: Evolution, development, and 
simulation of everyday mindreading. Oxford: Blackwell.
	Whiten, A., & Byrne, R.W. (1988). Tactical deception in primates. Behavioral 
& Brain Sciences, 11, 233-273.
	Whiten, A., & Byrne, R.W. (1991). The emergence of metarepresentation in 
human ontogeny and primate phylogeny. In A. Whiten (Ed.), Natural theories of 
mind: Evolution, development, and simulation of everyday mindreading (pp. 267-
281). Oxford: Blackwell.
	Wimmer, H., & Perner, J. (1983). Beliefs about beliefs: Representation and 
constraining function of wrong beliefs in young children's understanding of deception. 
Cognition, 13, 103-128.
	Wimmer, H., Hogrefe, G.J., & Perner, 
1. H.M.'s amnesia is based not merely on failure of storage or retrieval, but also on an 
inability to actively reconstruct the past.  He can recall some episodes from about 16 
years prior to the operation that led to his amnesia, but these are recounted in highly 
stereotyped fashion.  He is apparently unable to "update" these memories (Ogden & 
Corkin 1991), but recalls them, like semantic knowledge, without further 
reconstruction.
2. It is usually assumed that the common ancestor of chimpanzee, gorilla, and human 
was also a knuckle-walker.  Controversially, Savage-Rumbaugh (1994a) suggests that 
the common ancestor depended primarily on brachiation, and that gorillas and 
chimpanzees evolved knuckle-walking independently--a case of convergent evolution.