"Classical" Theory of Categorisation

From: Wendy Smith (WS93PY@psy.soton.ac.uk)
Date: Tue Feb 25 1997 - 08:52:51 GMT

Models of Category Learning: The Defining Attributes Theory

The "Classical" Theory of Categorisation

 The classical theory of categorisation is considered to be the
 "defining attributes" theory, and if this has to be summed up in
one phrase, it would be: Singly Necessary and Jointly Sufficient. The
idea is that a category can be defined by a set of attributes. Each
attribute is singly necessary, which means that if an item does not
have one of the attributes, it isn't a member of that category, no
matter what other attributes it does have. The set of attributes is all
that is necessary to be a category member. If an item has all the
attributes deemed sufficient, it is a member of that category, no
matter what other attributes it does or doesn't have.


 The theory has several implications. First, category membership
 is all-or-none. Either you are a bird, or you're not a bird; you
can't be a kind of a bird. Second, leading on from this, all members
of the category are equally representative. A penguin is just as much
or as little a bird as a turkey, an ostrich or a sparrow. Third, again
following from the all-or-none nature, the boundaries are clear cut,
there are no straddlers. Fourth, people have a knowledge, or mental
representation, of these attributes, and are able to apply this to
novel items to determine category membership.


 The theory met with several criticisms. The first criticism
 revolves around the finding of "typicality" effects. It was
found (e.g. Rosch & Mervis, 1975) that some items were deemed more
typical of a category than others, and that membership appears graded
rather than all-or-none. Items which were more typical could be
categorised more quickly than less typical examples. These criticisms
led to another view of categorisation (probabilistic theories,
including family resemblances and prototype theories - which Vered will
be doing later, so I don't want to pre-empt her too much).

 The second criticism, stemming from the first, revolves around the
 findings that the boundaries between categories are not always
clear cut. Not only were inconsistencies between subjects found, but
also within subjects, when they categorised a set of items, then
categorised the same items a month later (McCloskey & Glucksberg,
1978). This suggested that rather than clear cut boundaries and fuzzy
middles, perhaps the boundaries were fuzzy, and the centres clear

 Third, it can be very difficult to establish the set of defining
 attributes. The famous example is that of "games"; everyone
(supposedly) knows exactly what is or isn't a game, but no-one can give
a clear set of defining attributes. In addition, if people hold
beliefs about what attributes are and are not necessary, how stable are
these beliefs? First, they appear to change with increasing experience
(Rey, 1983); and, second, the beliefs can be frankly wrong (McNamara &
Sternberg, 1983). So, the theory was that people use a knowledge of
the defining attributes to decide category membership; but this
knowledge may not exist, and, even if it
 does exist, may not be correct; furthermore, even if it is correct, it
 can change over time as a function of experience.


 The strongest criticisms levelled appeared to revolve around the
 issue of typicality. Typicality ratings are discrimination tasks,
not categorisation tasks. If the discrimination task shows an
influence due to categorisation, then what we are seeing is a
demonstration of categorical perception. The situations described in
the vast majority of these studies, therefore, seem equivalent to our
"post-learning" discrimination tasks. Perhaps what we really need to
be asking is whether, if defining attributes is the correct mechanism
for categorisation, it would be able to predict the sort of results we
see. The implications from the theory were that there would be clear
boundaries, but fuzzy middles. This would seem to suggest that,
following categorisation, there would be a compression within
categories (because all items within a category are equivalent), and
separation between categories (because all items are either a member of
a category, or not a member), relative to the situation before
categorisation. This is compatible with the findings of absolute C.P.,
but not so compatible with those of relative C.P. It is particularly
poor in the situation where there is separation only, and suggests that
the classical theory cannot be a full explanation of categorisation.

 The second criticism concerned the fuzziness of the boundaries.
 Before this criticism can be evaluated, we need to decide exactly
what is meant by fuzziness about the boundary? For example, take the
boundary between "mammals" and "fish", and the problem of categorising
a whale and a cat. It will probably take a longer time, and result in
more uncertainty, to categorise the whale, relative to categorising the
cat. But what does this mean? Is it that a whale is less of a mammal
than a cat, or is it that a whale may or may not be a mammal, I just
don't have enough knowledge to categorise it? That is, let's say the
defining attributes of a mammal are: hair covered body, viviparous,
produces milk to feed the young. I have enough experience with cats to
know that these conditions are satisfied, but I'm not so familiar with
whales. This doesn't make a whale less of a mammal, or the boundary
between fish and mammals less clear cut, it just means I don't know
what attributes a whale possesses. Experience does appear to play a
role in determining which features are used; it may also play a role in
the ease with which features can be identified. The nature of the
experience may also be important, highlighting the role of feedback.
If there are no consequences to categorising correctly or incorrectly,
the boundary may remain fuzzy. If, on the other hand, there are
serious consequences, then a clear boundary may emerge very quickly.

 Context effects may also cause fuzziness. If a given item can be
 categorised in more than one way (e.g. edible versus inedible, but
also perfumed versus non-perfumed), then, in the context-free situation
described in many of these studies, there may be some
confusion/disagreement concerning the placement of an item.

 The third criticism, relating to the role of knowledge and belief,
 does have to be addressed, but perhaps not in the way it is
usually phrased. Furthermore, this criticism is not specific to this
approach; the assumption of knowledge also exists in other theories,
e.g. prototype theories and theory-based theories (Margolis, 1994).

 The problem is often presented as an inability to state the
 defining attributes, even when accurate categorisation is taking
place. But do we have to be able to verbalise a set of attributes to
be able to use them, or can we use them in an implicit fashion?
Implicit use could explain correct categorisation in the presence of
both apparently non-existent and incorrect knowledge. Ultimately, if a
person can categorise correctly, i.e. reap the rewards of successful
categorisation and avoid the costs of miscategorisation, then whether
or not they can define the category is immaterial. Perhaps an
important point here is the term "correctly". How do we decide what is
and isn't correct? Arguing over what does and what doesn't constitute
a category is more an intellectual question for experts in that field
than a practical question for psychologists.

 The criticism still has a valid point, though, in that if we
 assume knowledge is required, whether this is implicit, explicit,
or whatever, then how do we get the knowledge in the first place? And
exactly how do we use this knowledge to determine category membership?
Defining attributes may provide a structure, or mental representation,
for the knowledge which is needed, but the classical theory does not
provide a clear mechanism for how this knowledge is acquired and used.


 This theory may be singly necessary, but is not jointly sufficient
 as an explanation for categorisation mechanisms.


 I've put this section as a series of questions which arose out of
 the reading.

What, as psychologists, do we mean by the term "categorisation"?

Is there a difference between deciding what a category is, and deciding
the mechanics of how we categorise?

How important is the relationship between why we categorise and how we

How can we study this? - " It is always difficult to decide whether a
particular observation is a function of the information represented by
the concept, the structure or the form of that information....or of the
processes that operate on
 that concept" (Komatsu, 1992, p501)

Is categorisation the same when we have full knowledge of the stimulus
set, and when we have not yet met all the stimuli?

Is the mental representation the same in both these cases?

Is there a difference between "intellectual" type categorisation and
"practical" categorisation?

Do we have a concept first, then begin to categorise, or does the
concept arise out of the categorisation process?

What role does context play in deciding which factors may or may not be

Can the "rules" change as experience with the stimulus set increases,
and, if so, how and at what level does this happen?

What would this method of representation have to say about C.P? For
example, the representation appears to be uniform across all members,
i.e. they are represented as a set of defining attributes. This would
appear to suggest that, following learning, C.P. should be demonstrated
by a strong compression, as well as clear separation. This isn't
always found, so what does that mean for the theory?


Komatsu, L K (1992) Recent Views of Conceptual Structure Psychological
Bulletin 112 (3) 500 - 526

McCloskey, M E & Glucksberg, S (1978) Natural Categories: Well defined
or Fuzzy Sets? Memory and Cognition 6 462 - 472

McNamara, T & Sternberg, R (1983) Mental Models of Word Meaning
Journal of Verbal Learning and Verbal Behaviour 22 449 - 474

Margolis, I (1994) A Reassessment of the Shift from the Classical
Theory of Concepts to Prototype Theory Cognition 51 73 - 89

Rey, G (1983) Concepts and Stereotypes Cognition 15 237 - 262

Rosch, E & Mervis, C B (1975) Family Resemblances: Studies in the
Internal Structure of Categories Cognitive Psychology 7 573 - 605

er. l.m.

This archive was generated by hypermail 2b30 : Tue Feb 13 2001 - 16:24:05 GMT