CO-EVOLUTION OF NEOCORTEX SIZE, GROUP SIZE AND LANGUAGE IN
HUMANS
Notes by Gabor Zemplen
Dunbar seems to state at the very beginning of the article, that
primates are social animals, and that this intense sociality is 1.
Related to cognitive abilities.
2. Reflected in large brains.
He goes on to say:
> This claim is
> supported by the finding that mean group size is directly related
> to relative neocortical volume in nonhuman primates (Sawaguchi
> & Kudo 1990, Dunbar 1992a).
We will see whether he manages to prove this - to me nonevident -
point. He also claims:
> there is a
> species-specific upper limit to group size which is set by purely
> cognitive constraints: animals cannot maintain the cohesion and
> integrity of groups larger than a size set by the information-
> processing capacity of their neocortex.
This statement as yet seems harmless, but Dunbar soon starts to
conjecture about the size of these groups.
> It is important to appreciate that the causal relationship
> between group size and neocortex size depends on the explanatory
> perspective (or level) adopted. In evolutionary terms, the size
> of a species' neocortex is set by the range of group size
> required by the habitat(s) in which it typically lives. However,
> seen in proximate terms from an individual animal's point of
> view, current neocortex size sets a limit on the number of
> relationships that it can maintain through time, and hence limits
> the maximum size of its group. This means that although the
> evolution of neocortex size is driven by the ecological factors
> that select for group size, we can use the relationship in
> reverse to predict group sizes for living species (Dunbar 1992a).
Dunbar seems to state that:
1. The brain size is set by the group size required by the habitat. To
show that this is true, seems to me to be fairly difficult: nothing
else can account for large brains but group size? ("lonely" orangoutans
seem to falsify this).
- from this it doesn't follow that:
2. Current brain size limits the number of relationships and hence
limits the maximum size of the group. It is true, there must be a
limiting group size corresponding to the size of the brain, but as
Dunbar himself states:
> the cohesion of primate groups is maintained through time by social
> grooming ... and ....
> the amount of time devoted to social grooming correlates well with group size.
he should show that the limiting factor for group size is NOT the
grooming time (which, as he later shows, never amounts to more than
about 20 % of time), but ALWAYS the brain size.
This, given the fact that animals are investigated in their natural
habitatat, he fails to do. (i.e. if
there is a fission in a population can be the result of :
1. not enough grooming (within a primary network)
2. too little brain
3. the nature of the habitat!
Dunbar should exclude two of these.
Dunbar's aim:
> In this paper, I ask what implications these two sets of
> results have for modern humans (Homo sapiens sapiens). If we
> extrapolate from the nonhuman primate regression, what group size
> would we predict for anatomically modern humans, given our
> current neocortex size? I then ask whether there are any
> observed human group sizes that correspond to this predicted
> value. Since the relationships that maintain group cohesion
> among nonhuman primates are serviced by social grooming, I use
> the regression equation for primates to determine how much time
> humans would have to spend grooming each other if they were to
> maintain group cohesion in this way for groups of the size
> predicted from neocortex size. Finally, I ask what implications
> this might have had for the evolution of language.
2. Methods
> This analysis was based on the mean group size observed for
> a given genus rather than the maximum group size. The main
> justification for using the mean group size in these analyses
> lies in the nature of primate social groups. In contrast to the
> relatively simple aggregations typical of many birds and
> herbivores, primate groups are highly structured with individual
> animals embedded in a complex set of social and kinship networks
> (see Dunbar 1988, 1989a). Whereas bird flocks can shed
> individuals through trickle migration as soon as they exceed
> their optimal size, primate groups cannot: they have to wait
> until the group is large enough to permit it to fission into two
> or more daughter groups of a minimum size necessary to ensure the
> safety and survival of their members. This means that primate
> groups tend to oscillate in size over quite a wide range around
> the optimal value. At the point of fission (by definition, their
> maximum observed size), groups tend to be unstable and close to
> social disintegration: this, of course, is why they undergo
> fission at that point. Hence, maximum group size is likely to
> represent the point of complete social collapse rather than the
> maximum size of group that the animals can maintain as a cohesive
> social unit. Consequently, mean group size is likely to be a
> better estimate of the limiting group size for a species than the
> maximum ever observed in any population (for further discussion,
> see Dunbar 1992a).
This seems to me to be a very dodgy reasoning. In fact, reading this I
would suggest that the MAX. group size is used, as this should fairly
exactly show what size is JUST TOO BIG for a population. Knowing this,
we would be able to tell that group size just a bit below this number
can surely be handled by the brain (or there is still enough
grooming?).
3. Results
3.1. Group Size in Modern Humans
> Strictly
> speaking, of course, extrapolation from regression equations
> beyond the range of the X-variable values on which they are based
> is frowned on.
And rightly frowned on! Especially if some novel mechanisms might
contribute to the 1. group size of humans, 2. the brain size, and 3.
grooming.
> . Equations based on alternative indices of neocortex size
> (see Dunbar 1992a, Table 2) yield predicted group sizes that
> range from 107.6 (EQ residual of neocortex volume regressed
> against body weight) to 189.1 (Jerison's Extra Neocortical
> Neurons index) and 248.6 (absolute neocortex volume), all of
> which are within (or close to) the 95% confidence limits on the
> neocortex ratio equation.
100-250 is itself a very broad range (you are more likely to hit
something than to miss everything). Also I don't see how Dunbar can
ever TEST his claim that there is a set average grooup size for humans.
His hypothesis will be reluctant to respond to any falsificatory data.
He admits that
> it is nonetheless clear that most hunter-gatherers live
> in complexly structured social universes that involve several
> different levels of grouping.
This and other utterances show that on the following pages what he aims
at doing is to find supportive data - anything that will match his
expectations. Knowing that he could chose a number freely between 100
and 250, his job will not be exceedingly difficult.I will thus not
discuss these.
> Plotting these values on a graph produces what appears to
> be a clear trimodal distribution of group sizes with no overlap
> between grouping levels (Fig. 2). The average size of the
> smallest and largest grouping levels (means of 37.7 and 1154.7,
> respectively) correspond quite closely to the figures for bands
> and tribal groups (1000-2000) that are widely quoted in
> the anthropological literature (e.g. Steward 1955, Service 1962).
> The average size of the intermediate level groups for those
> societies for which accurate census data are available is 148.4
> (range 90-221.5, N=9). If all the available data are considered
> (taking median values in cases where only ranges are given), the
> mean is 134.8 (N=15); if only nomadic hunter-gatherers are
> considered, the mean is 156.4 (N=4). None of these estimates
> differs significantly from the predicted value (z< +0.431,
To be honest, the reason why the graph looks trimodal to me, is because
three different symbols are used for the suitable groups...
To see just one example that even looking for white swans to prove the
statement "All swans are white" doesn't always work (and than we are
not even bringing up an Australian black swan against the case)
> the mean size of the 51 communities (or
> Bruderhoefe) in the Schmedenleut section of the Hutterites (a
> fundamentalist group who live and farm communally in South Dakota
> and Manitoba) is 106.9 individuals (Mange & Mange 1980).
> According to Hardin (1988), the Hutterites regard 150 individuals
> as the limiting size for their farming communities: once a
> community reaches this size,
> steps are taken to split it into two daughter communities
Later he writes:
> In the context of the present analysis, the reason given by
> the Hutterites for limiting their communities to 150 is
> particularly illuminating. They explicitly state that when the
> number of individuals is much larger than this, it becomes
> difficult to control their behaviour by means of peer pressure
> alone (Hardin 1988).
An also:
> He (Forge 1972) argued that the figure 150 was a key threshold
> in community size in these societies. When communities exceed
> this size, he suggested, basic relationships of kinship and
> affinity were insufficient to maintain social cohesion; stability
> could then be maintained only if formal structures developed
> which defined specific roles within society. In other words,
> large communities were invariably hierarchically structured in
> some way, whereas small communities were not.
This shows a maximum, not a mean, and Dunbar seems to accept it. If
this is the case he extrapolated from MEAN group sizes, and got a MAX.
group size.
Some other data
> suggest that there is a critical threshold at a maximum
> settlement size of 500 beyond which social cohesion can only be
> maintained if there is an appropriate number of authoritarian
> officials. Bearing in mind that Naroll's threshold is expressed
> as the maximum observed settlement size, it seems likely that the
> equivalent mean settlement size will not be too far from the
> value of 150 suggested by the above analyses.
> Other evidence suggests that 150 may be a functional limit
> on interacting groups even in contemporary western industrial
> societies.
If critical settlement size is 500, then above this settlements tend to
split: 2 times 250. Therefore the average size should be between 250
and 500. Or not?
Also if 150 is functional limit, then why do I expect it to be the
average as well (see before)?
Another example:
> Even academic
> communities appear to abide by this rule. Price & Beaver (1966),
> for example, found that research specialities in the sciences
> tend to consist of up to 200 individuals, but rarely more. Becher
> sampled network sizes (defined as the number of
> individuals whose work you pay attention to) in 13 academic sub-
> disciplines drawn from both the sciences and the humanities and
> concluded that the typical size of the outer circle of
> professional associates that defines a sub-discipline is about
> (with a range between 100-400). It seems that disciplines
> tend to fragment with time as their numerical size (and, of
> course, literature) grows.
This seems to be a strong point against Dunbar. If our brain size
predicts a group size around 150, how come that I can belong to several
of these groups (200 academic collegues, 150 friends watching soccer
games, 200 students, 200 ex-classmates, old girlfriends, relatives,
etc. And all these are valid groupings, my brain is big enough to
memorize them all, so where is the correlation that limited mz group
size to 150? If what I see in modern society is true, than brain size
is not limiting group size, and his whole starting point seems
unclear.
Later Dunbar quotes highly interesting findings:
> Terrien & Mills (1955), for example, found that the
> larger the organisation, the greater the number of control
> officials that is needed to ensure its smooth functioning.
Hm...
I would also like to show that Dunbar is fairly liberal about his
statistics - he seems to quote numbers when the look good, and not when
they don't.
> The number of different acquaintances listed
> assumed to be an index of the subject's total social network.
> The mean number of acquaintances selected was 134 (though the
> variance around this figure was considerable).
I don't know the data, but not quoting it and still using it seems to weaken Dunbar's rhetoric.
We can, however, truly agree with Dunbar:
> More research in this area is clearly needed to
> clarify this (network size).
3.2. Grooming and the Evolution of Language
> Since our main concern is with how time spent grooming
> functions to maintain group cohesion, I have considered only
> those catarrhine species which do not have fission-fusion social
> systems.
Why? Do we rightly suppose that humans have a stable social system and
not a fission-fusion system?
1. I do not think this to be the case,
2. I don't think Dunbar shows this to be the case, and
3.doesn't even give sufficient reasons why this fact can or could be
neglected.
4. I don't think that he is justified to make any predictions from his
data concerning Homo s.s.
The statement:
> A figure of around 20% seems
> to be an absolute upper limit on the amount of time that primates
> can afford to devote to social interaction.
together with this, previous one:
> A comparative analysis of the determinants of time spent
> grooming by primates has demonstrated that grooming time is a
> linear function of group size, at least within the catarrhine
> primates (Dunbar 1991).
seems to complicate the case. One can presume that
1. Grooming time is not stable (this is disregarded by Dunbar later)
2. Grooming can limit population size - this threatens the first
correlation: that there is strong connection between group size and
brain size (or ratio).
> 1992b). Faced with this problem (too much grooming time calculated fro humans)
> , there are, in principle, only
> two solutions: either reduce group size to the point where the
> amount of grooming time is manageable or use the time that is
> available for social bonding in a more efficient way.
This dichotomy seems very promising - may be we can even test our
hypothesis: small hunter- gatherer-groups might not have developed a
language - they didn't need it, while large communities (closer to 150
people) will have much more sophisticated languages. Well... I can't
recall anything like this difference. In fact, as Dunbar goes on:
> Given that minimum group sizes are ecologically imposed
> (see Dunbar 1988), there may be little that a particular species
> can do to manipulate its group size in a particular habitat. The
> only option will thus be a more efficient use of the time
> available for social bonding.
he seems to eliminate one branch of this dichotomy without ANY
EXPLANATION for doing so. I can very easily imagine, that certain
habitats do not require Homo groups that are above 30 members (small
family tribes in Borneo - tough luck they also have a pretty
sophisticated language)
> My
> suggestion, then, is that language evolved as a "cheap" form of
> social grooming, so enabling the ancestral humans to maintain the
> cohesion of the unusually large groups demanded by the particular
> conditions they faced at the time.
I have nothing personal against this suggestion, but fail see why I
should accept it - the idea is more persuasive than any of the as yet
suggested supporting evidence.
3.3. Language as a Bonding Mechanism
> The fact that language
> can be interpreted as fulfilling the same role as social grooming
> suggests that, rather than being the selective factor driving
> brain evolution, ecologically-related information-exchange might
> be a subsequent development that capitalised on a window of
> opportunity created by the availability of a computer with a
> substantial information-processing capacity.
3.4. Efficiency of Language as a Bonding Mechanism
> Since the predicted size for
> human groups is 147.8, this implies that language (the human
> bonding mechanism) ought to be 147.8/53.5=2.76 times as efficient
> as social grooming (the nonhuman primate bonding mechanism).
But this implies a SET grooming %, something that Dunbar has never
shown to be the case. His efficiency-rate, 2.76 this new advocate for
numerology (I suggest cabbalists consider dropping the Pythagorean
Tetraktis, and adopt 2.76, 148, etc) seems totally out of thin air.
What if language is so good, that not only the group size grows, but
also the grooming- equivalent time consumption decreases, too? So
language is AT LEAST 2.76 times as efficient as grooming (if we believe
Dunbar's calculations), but the actual number may be 3.14, 6.6666, or
10.0.
Also this whole thing about language-evolution supposes that
1. There is a stable group size at a given habitat (no language, only
grooming).
2. This means a stable neocortex size (just enough for me to "remember
me mates")
3. Now suppose that a better group would be more efficient.
For this two things are needed:
1.Before the group grows big enough to split it has to develop some
rudimentary form of language to keep the larger group together (though
there is no selection pressure for this)
2. It also has to increase brain size - to enable members to remember
the extra individuals of the larger group (also no selection pressure
for this until the critical size is reached).
For me this seems like a very unlikely scenario: a group grows in
number, large brains and language appear at the sime time - to play a
crucial role at a crucial minute: the critical group size.
> In other words, human conversation
> group sizes should be limited to about 3.8 in size (one speaker
> plus 2.8 listeners).
Why limited? Should not this be the average?
I do not want to quote the conversational experiments. The number
ofpossible problems just seem to be too big,it is unclear what
unmentioned factors (4-person tables, time of investigation for
conversation (exam period or normal)are significant, no clear
categories are given, significance of time and place are not discussed
(investigate peeing behaviour near public lavatories, and you find Homo
s.s. to spend 40 percent of his time peeing), and again, only positive
instances are listed.
4. Discussion
In toto I believe that Dunbar's pet idea is interesting. He, however
fails to give an acceptable argumentat, let alone a persuasive one.
His starting point - extrapolating from brain - group- size data is
weak. But even if we accept it, he later writes in a seemingly
innocent passage:
> Clearly, the gelada have in no sense evolved language in the
> sense we would use this term of humans, but then neither have
> they developed the large cohesive groups chararcteristic of our
> species. However, it may be that the large groups in which this
> species sometimes gathers forced the evolution of a supplementary
> vocal mechanism for servicing relationships in a context where
> they are already at the limit of available grooming time (see
> Iwamoto & Dunbar 1983, Dunbar 1991). It is worth noting that
> this much has been achieved without the need to increase
> neocortex size: indeed, the gelada have a rather small neocortex
> compared to their baboon cousins (genus Papio) which probably
> explains the lack of cohesiveness in their larger-scale groups
> compared to those of the baboons.
This means that the brain size is not a good limiting factor for group
size, and this questions the validity of Dunbar's first correlation -
brain size is not a giveaway sign for group size (probably not even for
average size - as even determining that for the gelada seems tricky and
theory- laden). If this is not valid, than the dodgy extrapolation
certainly isn't - so no "average group size for humans" can be validly
inferred - and this weakens the claim that for the large group- size a
new, grooming-equivalent method was essential.
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