Classical Categorisation

From: Smith, Wendy (
Date: Tue Jan 23 1996 - 09:24:22 GMT

Miller (1956) demonstrated that by "chunking" information, we could
store more information than by not chunking it. Those individuals with
memories which retained everything (eg S described by Luria) at first
seem gifted with their abilities of recall. However, on closer
examination, they are actually handicapped. They are less able to
understand the material they are presented, because they cannot extract
the relevant items. Each situation is therefore unique, because they
cannot generalise, based on these items. Categorisation would appear to
involve the ability to generalise, select invariants, and reduce
information. The question of interest to psychologists is how do we do

The classical view of categorisation can be considered as the defining
attribute view, based mainly on the work of Frege. Briefly, it suggests
that a category can be described by a set of defining attributes. The
attributes are singly necessary and jointly sufficient to allow an item
to be identified as a member of a category. This means that the
boundaries are clearly defined: what is and is not a category member is
clear cut. It also means that each category member is equally
representative of the category. It was also considered that the
categories could be organised hierarchically, in which the more
specific instances include all the attributes of the more abstract
levels, and include extra attributes which define it more narrowly. For
example, let us say that a "bird" has the defining attributes "beak,
feathers, wings" . Each of these attributes are necessary for
inclusion. If an item does not have a beak, it is not a bird, even if
it has feathers and wings. The attributes are jointly sufficient. In
this example, if an item has a beak, feathers and wings, it is a bird -
even if it has four legs and barks. Also, this member is just as
representative as a sparrow, or a penguin. A penguin has all the
defining attributes of a bird, but in addition it has the attributes of
"black and white, swims"

There were criticisms of the defining attributes view. First, it
considered all attributes within the definition as equally salient.
However, in speed tests and descriptive tasks, some attributes appeared
more relevant than others (Conrad, 1972). Second, it considered all
members of a category to be equally representative. however, when
people were asked to rate typicality, some members were consistently
voted more typical. Typical members were also categorised and learned
faster than atypical members. It was difficult to determine what the
defining attributes were for many categories, the most celebrated
example being "games" (Wittgenstein). This led to the proposal that
members of a category shared family resemblances rather than a set of
defining features. Second, the notion of clear cut boundaries between
categories was called into question. Sometimes, people could not agree
whether an belonged in one category or another.

However, these criticisms can be regarded as a manifestation of how the
questions are asked, they are looking at the metaphysical aspects of
categories rather than the psychological aspect of how we categorise.
Rating for typicality is a similarity judgement, and not
categorisation. Judging the salient properties in the manner above can
also only be performed after the categories have been established. Both
judgements can be influenced by the categorisation - this is known as
categorical perception. The knowledge of the categories will affect a
person's judgement of the categories. Although the defining attributes
view may have had limitations, the criticisms above did not address the
important ones, such as how the defining attributes were decided upon.

An alternative view of categorisation was proposed to account for these
so-called deficits, especially the typicality effects and the fuzziness
of the boundaries. Theories within this view were called "prototype
theories" (eg Rosch). The prototype theories all suggested that
categorisation was based on a prototype. A prototype is an ideal, or
central, member. Although there may be necessary attributes, membership
does not depend on them as a defining set. The are not "jointly
sufficient". Membership is determined by the similarity of an item's
properties to those of a prototype for that category. Members of a
category will therefore show a typicality gradient. The boundaries
between categories are not clearly defined, but "fuzzy". Again, the
categories were arranged hierarchically, and Rosch suggested there was
a basic level of categorisation at which all manner of operations
(perception, language etc) will converge.

There are criticisms of the prototype view. First, not all categories
have prototype characteristics, particularly abstract categories such
as "instinct" (Hampton, 1981). Second, when categorising, people do not
just look for properties which co-occur together, but for properties
which co-occur with the consequences of getting the categorisation
right. For example, there are two large, animals, which look identical
in appearance, except for one feature. One animal has sharp canines,
because it is a carnivore (and likes eating humans), and the other has
flat teeth for grinding leaves. The correlation between fur, eye
colour, ear size and tail length etc are all largely irrelevant as far
as the consequences of miscategorisation are concerned. The only
feature which matters is teeth, and that it co-occurs with whether the
animal will eat humans, not that it co-occurs with all the other

However, by far the biggest problem with the prototype view is that it
is approaching the problem from the wrong direction: it is looking at
the metaphysical problem of what constitutes a category. Typicality
judgements may be useful for answering this type of question, but it is
not the same task as categorisation, and presupposes that a category
already exists. The real question that should be asked by psychologists
is how we categorise. The only situation in which prototypes help
answer that question is in continuous categories, and categorisation of
unidimensional, continuous stimuli is not very successful outside a few
special cases where inborn feature detectors appear to be present.

Before we address how we categorise, perhaps we need to ask why we
categorise, why is it useful? The answer is quite simple: there are
consequences of miscategorisation. Miscategorising toadstools as
mushrooms has the consequence of poisoning, therefore being able to
categorise mushrooms and toadstools is useful in avoiding the
consequence of miscategorisation.

So, how do humans categorise? As far as correlations between properties
are concerned (prototype theories) this is not useful. The Ugly
Duckling Theorem illustrates that when all properties are considered,
everything becomes infinitely unique, and of no use in determining
possible consequences, or reducing uncertainty about what action to
take. Instead, some of the properties have to be selected, and others
ignored when a person categorises. Furthermore, the properties are
selected by their correlation with the consequences of categorising one
way or another. The properties which get the categorisation "right" are
the ones which are selected. "Right" is determined by the consequences.
If there are no consequences, the categories are trivial, and perhaps
better described as subjective tastes.

Categories can be arranged hierarchically, but this is arbitrary, and
so is the entry point. This often depends upon context. The "basic
level" of Rosch's theory is really a default context. The uniformity of
response probably springs from uniformity of experience within a

Clear boundaries are also important. Categorisation means responding
differentially to certain categories of input; therefore boundaries
signal a change in behaviour and are all or nothing. Categorisation
involves selecting which features are invariant, and disregarding the
rest. This is more reminiscent of the defining attribute theory that
the prototype theories. Feature detection would appear a more plausible
mechanism for categorisation than prototype matching.

In conclusion, the classical view of categorisation was of defining
attributes. Several things were thought to be wrong with this. The
main problems were that it was difficult to establish the defining
attributes in many cases, some category members were found to be more
typical than others, and were c ategorised and learned faster, and it
was thought that categories were based on family resemblances rather
than features. The alternatives proposed were the prototype theories.
Although these may address the metaphysical problem of what a category
is, they do not fare well in explanatory strength of how we actually

This archive was generated by hypermail 2b30 : Tue Feb 13 2001 - 16:23:57 GMT