Weiner-Vol-4 c22.tex V3 - 08/10/2012 4:07pm Page 607 CHAPTER 22 Concepts and Categorization ROBERT L. GOLDSTONE, ALAN KERSTEN, AND PAULO F. CARVALHO INTRODUCTION 607 THEORIES 618 WHAT ARE CONCEPTS? 608 CONNECTING CONCEPTS 620 WHAT DO CONCEPTS DO FOR US? 609 THE FUTURE OF CONCEPTS AND HOW ARE CONCEPTS REPRESENTED? 611 CATEGORIZATION 624 CATEGORY BOUNDARIES 616 REFERENCES 625 INTRODUCTION principles that explain many or all acts of categorization. This assumption is controversial (see Medin, Lynch, & Issues related to concepts and categorization are nearly Solomon, 2000), but is perhaps justified by the poten- ubiquitous in psychology because of people’s natural tial pay-off of discovering common principles governing tendency to perceive a thing as something. We have a concepts in their diverse manifestations. powerful impulse to interpret our world. This act of inter- The desirability of a general account of concept learn- pretation, an act of “seeing something as X” rather than ing has led the field to focus its energy on what might be simply seeing it (Wittgenstein, 1953), is fundamentally an called “generic concepts.” Experiments typically involve act of categorization. artificial categories that are hopefully unfamiliar to the The attraction of research on concepts is that an subject. Formal models of concept learning and use are extremely wide variety of cognitive acts can be under- constructed to be able to handle any kind of concept irre- stood as categorizations (Murphy, 2002). Identifying the spective of its content. Although there are exceptions to person sitting across from you at the breakfast table this general trend (Malt, 1994; Ross & Murphy, 1999), involves categorizing something as your spouse. Diag- much of the mainstream empirical and theoretical work nosing the cause of someone’s illness involves a disease on concept learning is concerned not with explaining how categorization. Interpreting a painting as a Picasso, an arti- particular concepts are created but, rather, with how con- fact as Mayan, a geometry as non-Euclidean, a fugue as cepts in general are represented and processed. baroque, a conversationalist as charming, a wine as a Bor- One manifestation of this approach is that the mem- deaux, and a government as socialist are categorizations bers of a concept are often given an abstract symbolic at various levels of abstraction. The typically unspoken representation. For example, Table 22.1 shows a typical assumption of research on concepts is that these cogni- notation used to describe the stimuli seen by a subject tive acts have something in common. That is, there are in a psychological experiment or presented to a formal model of concept learning. Nine objects belong to two We are grateful to Alice Healy, Robert Proctor, Brian Rogosky, categories, and each object is defined by its value along and Irving Weiner for helpful comments on earlier drafts of this four binary dimensions. In this notation, objects from Cat- chapter. This research was funded by National Science Foun- egory A typically have values of 1 on each of the four dation REESE grant DRL-0910218, and Department of Educa- tion IES grant R305A1100060. Correspondence concerning this dimensions, whereas objects from Category B have values chapter should be addressed to [email protected] or Robert of 0. The dimensions are typically unrelated to each other, Goldstone, Psychology Department, Indiana University, Bloom- and assigning values of 0 and 1 to a dimension is arbitrary. ington, Indiana 47405. Further information about the laboratory For example, for a color dimension, red may be assigned a can be found at http://cognitrn.psych.indiana.edu value of 0 and blue a value 1. The exact category structure 607 Weiner-Vol-4 c22.tex V3 - 08/10/2012 4:07pm Page 608 608 Thought Processes TABLE 22.1 A Common Category Structure, Originally Used by categories or vice versa is an important foundational con- Medin and Schaffer (1978) troversy. If one assumes the primacy of external categories Dimension of entities, then one will tend to view concept learning Category Stimulus D1 D2 D3 D4 as the enterprise of inductively creating mental structures Category A A11110that predict these categories. One extreme version of this A21010view is the exemplar model of concept learning (Estes, A310111994; Medin & Schaffer, 1978; Nosofsky, 1984; see also A41101Capaldi & Martins, this volume), in which one’s inter- A50111nal representation for a concept is nothing more than the Category B B11100set of all the externally supplied examples of the concept B20110 to which one has been exposed. If one assumes the pri- B30001 B40000macy of internal mental concepts, then one tends to view external categories as the end product of applying these internal concepts to observed entities. An extreme version of Table 22.1 has been used in at least 30 studies (reviewed of this approach is to argue that the external world does by J. D. Smith & Minda, 2000), instantiated by stim- not inherently consist of rocks, dogs, and tables; these are uli as diverse as geometric forms, yearbook photographs, mental concepts that organize an otherwise unstructured cartoons of faces (Medin & Schaffer, 1978), and line external world (Lakoff, 1987). drawings of rocket ships. These researchers are not partic- ularly interested in the category structure of Table 22.1 and Equivalence Classes are certainly not interested in the categorization of rocket ships per se. Instead, they choose their structures and stim- Another important aspect of concepts is that they are uli so as to be (a) unfamiliar (so that learning is required), equivalence classes. In the classical notion of an equiva- (b) well controlled (dimensions are approximately equally lence class, distinguishable stimuli come to be treated as salient and independent), (c) diagnostic with respect to the same thing once they have been placed in the same theories, and (d) potentially generalizable to natural cate- category (Sidman, 1994). This kind of equivalence is too gories that people learn. Work on generic concepts is very strong when it comes to human concepts because, even valuable if it turns out that there are domain-general prin- when we place two objects into the same category, we ciples underlying human concepts that can be discovered. do not treat them as the same thing for all purposes. Still, there is no a priori reason to assume that all concepts Some researchers have stressed the intrinsic variability of will follow the same principles, or that we can generalize human concepts—variability that makes it unlikely that from generic concepts to naturally occurring concepts. a concept has the same sense or meaning each time it is used (Barsalou, 1987; Thelen & Smith, 1994). Still, the extent to which perceptually dissimilar things can be WHAT ARE CONCEPTS? treated equivalently given the appropriate conceptualiza- tion is impressive. To the biologist armed with a strong Concepts, Categories, and Internal Representations mammal concept, even whales and dogs may be treated as equivalent in many situations related to biochemistry, A good starting place is Edward Smith’s (1989) char- child rearing, and thermoregulation. acterization that a concept is “a mental representation Equivalence classes are relatively impervious to super- of a class or individual and deals with what is being ficial similarities. Once one has formed a concept that represented and how that information is typically used treats all skunks as equivalent for some purposes, irrel- during the categorization” (p. 502). It is common to distin- evant variations among skunks can be greatly deempha- guish between a concept and a category. A concept refers sized. When people are told a story in which scientists to a mentally possessed idea or notion, whereas a cate- discover that an animal that looks exactly like a raccoon gory refers to a set of entities that are grouped together. actually contains the internal organs of a skunk and has The concept dog is whatever psychological state signifies skunk parents and skunk children, they often categorize thoughts of dogs. The category dog consists of all the the animal as a skunk (Keil, 1989; Rips, 1989). People entities in the real world that are appropriately catego- may never be able to transcend superficial appearances rized as dogs. The question of whether concepts determine when categorizing objects (Goldstone, 1994a), nor is it Weiner-Vol-4 c22.tex V3 - 08/10/2012 4:07pm Page 609 Concepts and Categorization 609 clear that they would want to (Jones & Smith, 1993). or is it like fly paper but used to catch bison? Interpreta- Still, one of the most powerful aspects of concepts is tions of word combinations are often created by finding their ability to make superficially different things alike a relation that connects the two concepts. In Murphy’s (Sloman, 1996). If one has the concept “Things to remove (1988) concept-specialization model, one interprets noun- from a burning house,” even children and jewelry become noun combinations by finding a variable that the second similar (Barsalou, 1983). The spoken phonemes /d/ /o/ noun has that can be filled by the first noun. By this /g/, the French word chien, the written word dog, and account, a “Robin Snake” might be interpreted as a snake a picture of a dog can all trigger one’s concept of dog that eats robins once Robin is used to the fill the “eats” (Snodgrass, 1984), and although they may trigger slightly slot in the Snake concept. different representations, much of the core information In addition to promoting creative thought, the com- will be the same. Concepts are particularly useful when binatorial power of concepts is required for cognitive we need to make connections between things that have systematicity (Fodor & Pylyshyn, 1988). The notion of different apparent forms. systematicity is that a system’s ability to entertain com- plex thoughts is intrinsically connected to its ability to WHAT DO CONCEPTS DO FOR US? entertain the components of those thoughts.