Page 249 - Foundations of Cognitive Psychology : Core Readings

Principles of Categorization 255

among each other. Categories below the basic level will be bundles of common
and, thus, predictable attributes and functions but contain many attributes that
overlap with other categories (for example, kitchen chair shares most of its
attributes with other kinds of chairs).
Superordinate categories have lower total cue validity and lower category
resemblance than do basic-level categories, because they have fewer common
attributes; in fact, the category resemblance measure of items within the super-
ordinate can even be negative due to the high ratio of distinctive to common
features. Subordinate categories have lower total cue validity than do basic
categories, because they also share most attributes with contrasting subordinate
categories; in Tversky’s terms, they tend to be combined because the weight of
the added common features tends to exceed the weight of the distinctive fea-
tures. That basic objects are categories at the level of abstraction that maximizes
cue validity and maximizes category resemblance is another way of asserting
that basic objects are the categories that best mirror the correlational structure
of the environment.
We chose to look at concrete objects because they appeared to be a domain
that was at once an indisputable aspect of complex natural language classi-
fications yet at the same time was amenable to methods of empirical analysis.
In our investigations of basic categories, the correlational structure of concrete
objects was considered to consist of a number of inseparable aspects of form
and function, any one of which could serve as the starting point for analysis.
Four investigations provided converging operational definitions of the basic
level of abstraction: attributes in common, motor movements in common, ob-
jective similarity in shape, and identifiability of averaged shapes.

Common Attributes
Ethnobiologists had suggested on the basis of linguistic criteria and field ob-
servation that the folk genus was the level of classification at which organisms
had bundles of attributes in common and maximum discontinuity between
classes (see Berlin 1978). The purpose of our research was to provide a system-
atic empirical study of the co-occurrence of attributes in the most common tax-
onomies of biological and man-made objects in our own culture.
The hypothesis that basic level objects are the most inclusive level of classifi-
cation at which objects have numbers of attributes in common was tested for
categories at three levels of abstraction for nine taxonomies: tree, bird, fish,
fruit, musical instruments, tool, clothing, furniture, and vehicle. Examples of
the three levels for one biological and one nonbiological taxonomy are shown
in table 10.1. Criteria for choice of these specific items were that the taxonomies
contain the most common (defined by word frequency) categories of concrete
nouns in English, that the levels of abstraction bear simple class-inclusion rela-
tions to each other, and that those class-inclusion relations be generally known
to our subjects (be agreed upon by a sample of native English speakers). The
middle level of abstraction was the hypothesized basic level: For nonbiological
taxonomies, this corresponded to the intuition of the experimenters (which also
turned out to be consistent with Berlin’s linguistic criteria); for biological cate-
gories, we assumed that the basic level would be the level of the folk generic.