Running Head : Classifying with Essentialized Categories Classifying with Essentialized Categories

A central tenet of modern categorization research is that real-world categories have no defining features, and that features are better thought of as probabilistically related to their category. Nevertheless, the belief in properties that are invariably associated with categories has persisted. One view— psychological essentialism—further states that essential properties and structures constrain or generate the observable features of objects. We argue that categorization is often an instance of causal inference, as classifiers reason from observable features to essential ones (and then to category membership). We demonstrate that observable features viewed as causally related to underlying structures carry more weight on classification than unrelated features, and that this phenomenon is responsible for changes in some key macroscopic properties of categories, such as their variability. But because these inferences are made with less than complete certainty, we suggest that essentialism is compatible with a probabilistic view of classification, suitably amended. An account of these findings in the form of a generative model of classification is offered. Generative Essentialism 3 Classifying with Essentialized Categories A central dogma of modern categorization research is that real categories have no defining features. Once upon a time, investigators assumed that people determine category membership on the basis of a small number of defining features that covary more or less perfectly with category labels, and their research focused on how people form and test the rules that govern classification (Bourne, 1970). But the Roschian revolution in the 1970s demonstrating people's apparent ignorance of defining features, and the existence of unclear cases and better or worse examples of a category, led most of the field to the alternative view that features are better thought of as probabilistically related to their category (Rosch & Mervis, 1975). This view has come to dominate all major theories of category representation, regardless of whether they assume a single summary representation (e.g., a prototype) or the memory traces of all the exemplars of a category that a classifier has observed in the past. Nevertheless, the belief in properties that are invariably associated with categories has persisted in various forms. For example, Miller and Johnson-Laird (1976) distinguished between categories’ core properties (which could be used during, e.g., reasoning, determining the meaning of conceptual combinations, and so on) versus their identification procedures which inferred category membership on the basis of perceptual information (also see Armstrong, Gleitman, & Gleitman, 1983; Osherson & Smith, 1981; Smith, Shoben, & Rips, 1974). More recently, the view known as psychological essentialism also states that people view categories as having properties, or some kind of underlying structure, that is essential, that makes an object the kind of thing it is (Medin & Ortony, 1989; Gelman, 2003). This view differs from core properties in two important ways however. First, it does not assume that people necessarily know what those properties or structures are; thus, it is consistent with the apparent absence of real defining features. Second, it assumes that essential properties and structures constrain, or generate, the features of objects that can be observed. Evidence in support of psychological essentialism comes from a variety of sources. For example, Gelman and Markman (1986) demonstrated that children's inductive inferences reflect an assumption that members of the same kind are likely to share many hidden properties. When preschool children were told, Generative Essentialism 4 for instance, that a flamingo feeds its baby mashed-up food and that a bat feeds its baby milk, the children were more likely to judge that a blackbird would feed its baby mashed-up food (despite the fact that the blackbird was more perceptually similar to the bat than the flamingo), apparently because the children believed that objects are likely to share properties when they are the same kind (e.g., flamingos and blackbirds are both birds). A second line of evidence concerns children's beliefs about the innate potential of kinds. For example, preschool children judged that a baby kangaroo raised by goats would be more likely to be good at hopping than climbing (Gelman & Wellman, 1991). Finally, the assumption of underlying structure has also been shown to affect children's judgments of category membership. Keil (1989) performed a transformation experiment in which he told second-graders about, for instance, doctors who dyed a raccoon's fur black, bleached a white stripe down its back, and put a sac of super smell odor in its body. The children judged that the transformed animal was still a raccoon, despite that it now looked like a skunk (see Rips, 1989, for a similar finding with adults). In fact, evidence that the internal structure of animals is more important than the "outsides" has even been found with 3 year old children (Gelman & Wellman, 1991). The fact that all these findings involve young children suggests that their judgments were based on their default assumptions about the internal structure of the categories rather than specific knowledge (e.g., that raccoons have raccoon DNA) that might be acquired, for example, through formal education. But although these studies provide strong evidence that even children view categories as possessing internal structure, the second part of the essentialist claim—that internal structure generates or causes observable properties—is less well established. For example, the transformation experiments demonstrate that "insides" matter, but not that the insides are necessarily responsible for the outsides. It is also uncertain whether the effect of internal structure on classification is limited to unusual hypothetical situations (e.g., transformations) or whether it also extends to acts of classification that people perform everyday. Clearly, the case for the importance of essentialism would be strengthened if it were found to influence the classification of objects that have not undergone transformations. The purpose of the present research, then, is to assess how classification is affected when an object's normal (untransformed) features are known to be causally related to an unobserved property or Generative Essentialism 5 structure that is defining of, or essential to, membership in a category. Of course, one might start off wondering how such causes could have any influence at all, exactly because they are unobserved. Nevertheless, underlying causes might make their influence felt indirectly by changing how observable features are used in classification. In this article we assess the hypothesis that classifiers can engage in a kind of two-step inferential process in which they first reason backwards from observable features to the essential properties or structures that are their causes, and then from those essential properties to category membership. On this view, one "diagnoses" category membership in the same way that one diagnoses the presence of a disease from the presence of the symptoms it causes. We will refer to this proposal as the classification as diagnostic reasoning view. It may be that diagnostic reasoning of this kind is sometimes decisive in judgments of category membership. For example, an object is definitely not a category member if it is missing a feature that is always generated by the category, and an object definitely is a category member if it has a feature that is generated by no other category. However, we suggest that such causal inferences are frequently made with less than complete certainty. The conclusion against category membership for an object missing a feature will be less than certain if the categorizer recognizes that the causal process that generates the feature is less than 100% reliable (or if knowledge about its reliability is unavailable). Similarly, the conclusion in favor of category membership for an object displaying a feature will be less than certain if the categorizer suspects that the feature can be generated by the causal processes associated with some other category. In other words, although essentialism stipulates defining features, this does not entail that categorization decisions concerning essentialized objects are therefore all-or-none, because the observable features' diagnostic value, or evidentiary weight, will vary on a continuum depending on the details of the causal beliefs involved. Moreover, the importance of causal inference to categorization will also depend on how fully essentialized the category is, that is, the extent to which the internal properties and structures are indeed considered defining of category membership. In fact, research suggests that not all categories are essentialized to the same degree, or at all. The strongest evidence for essentialism is found with natural kinds, especially biological kinds (Gelman, 2003). There is also considerable evidence that social kinds Generative Essentialism 6 such as races and ethnicities are essentialized (Eberhardt & Randall, 1997; Hirschfeld, 1996; Rothbart & Taylor, 1992; Yulill, 1992). Evidence for essentialism is more ambiguous for artifacts, because whereas more complex artifacts like cars and computers possess considerable internal mechanisms, simple artifacts like pencils and wastepaper baskets do not. (Although it has been argued that an artifact's essential feature is the intention of its human designer, Bloom, 1998; see Malt & Johnson 1992; 1998 for a dissenting view). Clearly, one can expect the two-step process in which classifiers reason from observable features to internal structure and then to category membership to be operative only to the extent that internal structure is present and thought to be defining of category membership. When a category is essen