Cognitive abilities are domain-specific to the extent that the mode of reasoning, structure of knowledge, and mechanisms for acquiring knowledge differ in important ways across distinct content areas. For example, many researchers have concluded that the ways in which language is learned and represented are distinct from the ways in which other cognitive skills are learned and represented (Chomsky 1988; but see Bates, Bretherton, and Snyder 1988). Other candidate domains include (but are not limited to) number processing, face perception, and spatial reasoning. The view that thought is domain-specific contrasts with a long-held position that humans are endowed with a general set of reasoning abilities (e.g., memory, attention, inference) that they apply to any cognitive task, regardless of specific content. For example, Jean PIAGET's (1983) theory of cognitive development is a domain-general theory, according to which a child's thought at a given age can be characterized in terms of a single cognitive level. In contrast, evidence for domain-specificity comes from multiple sources, including variability in cognitive level across domains within a given individual at a given point in time (e.g., Gelman and Baillargeon 1983), neuropsychological dissociations between domains (e.g., Baron-Cohen 1995), innate cognitive capacities in infants (Spelke 1994), evolutionary arguments (Cosmides and Tooby 1994), ethological studies of animal learning (e.g., Marler 1991), coherent folk theories (Gopnik and Wellman 1994), and domain-specific performance in areas of expertise (Chase and Simon 1973).
Domain-specificity is not a single, unified theory of the mind. There are at least three distinct approaches to cognition that assume domain-specificity. These approaches in-clude modules, theories, and expertise (see Hirschfeld and Gelman 1994; Wellman and Gelman 1997).
The most powerful domain-specific approach is modularity theory, according to which the mind consists of "separate systems [i.e., the language faculty, visual system, facial recognition module, etc.] with their own properties" (Chomsky 1988: 161). Proposals regarding modularity have varied in at least two respects: whether modularity is restricted to perceptual processes or affects reasoning processes as well, and whether modularity is innate or constructed. Modularity need not imply evolved innate modules (see Karmiloff-Smith 1992) but for most modular proponents it does. Nonetheless, all modularity views assume domain-specificity. Chomsky's focus was on language, and more specifically SYNTAX or universal grammar. Evidence for the status of syntax as a module was its innate, biologically driven character (evident in all and only humans), its neurological localization and breakdown (the selective impairment of syntactic competence in some forms of brain damage), its rapid acquisition in the face of meager environmental data (abstract syntactic categories are readily acquired by young children), and the presence of critical periods and maturational timetables (see Pinker 1994).
Fodor (1983) extended the logic of modules to cognitive abilities more broadly. He distinguished between central logical processes and perceptual systems, arguing for modularity of the latter. In Fodor's analysis, modules are innately specified systems that take in sensory inputs and yield necessary representations of them. The visual system as characterized by MARR (1982) provides a prototypical example: a system that takes visual inputs and generates 2.5-dimensional representations of objects and space. Like the visual system, by Fodor's analysis, modules are innately specified, their processing is mandatory and encapsulated, and (unlike central knowledge and beliefs) their representational outputs are insensitive to revision via experience. Experience provides specific inputs to modules, which yield mandatory representations of inputs. Certain experiential inputs may be necessary to trigger working of the module in the first place, but the processes by which the module arrives at its representations are mandatory rather than revisable.
Extending Fodor, several writers have argued that certain conceptual processes, not just perceptual ones, are modular (Karmiloff-Smith 1992; Sperber 1994) or supported by systems of cognitive modules (e.g., Baron-Cohen 1995; Leslie 1994). In these claims each module works independently, achieving its own special representations. Thus, for the most part cognitive modules are like Fodor's perceptual ones, except that "perceptual processes have, as input, information provided by sensory receptors, and as output, a conceptual representation categorizing the object perceived . . . conceptual processes have conceptual representations both as input and as output" (Sperber 1994: 40).
The claim that people ordinarily construct or possess folk theories (as distinct from scientific theories) is a controversial one. However, everyday thought may be considered theory -like in its resistance to counterevidence, ontological commitments, attention to domain-specific causal principles, and coherence of beliefs (Carey 1985; Gopnik and Wellman 1994). Like modules, folk theories are also domain-specific. Folk theories make use of domain-specific ontologies (e.g., a folk theory of psychology concerns mental entities such as beliefs and desires, whereas a folk theory of physics concerns physical entities such as objects and substances). Folk theories also entail domain-specific causal explanations (e.g., the law of gravity is not applied to mental states). However, in contrast to modules, which are generally assumed to be innately constrained, biologically determined, and invariant, theories are thought to undergo radical restructuring over time, and to be informed by knowledge and cultural beliefs. On this construal of domain-specificity, candidate domains include psychology (also known as theory of mind; Wellman 1990), physics (McCloskey, Washburn, and Felch 1983), and biology (Keil 1994); see Wellman and Gelman (1997).
Domain-specificity is also apparent in the remarkable pockets of skill that people develop as a result of extensive experience. With enough practice at a task (e.g., playing chess, gathering factual knowledge about dinosaurs), an individual can develop extraordinary abilities within that task domain. For example, experts can achieve unusual feats of MEMORY, reorganize knowledge into complex hierarchical systems, and develop complex networks of causally related information (Chi, Hutchinson, and Robin 1989). These abilities are sufficiently powerful that child experts can even surpass novice adults, in contrast to the usual developmental finding of adults outperforming children (e.g., Chi 1978). Importantly, EXPERTISE skills cannot be explained as individual differences in the general processing talents of experts, because these achievements are limited to the narrow task domain. For example, a chess expert displays advanced memory for arrangements of pieces on a chessboard, but ordinary memory for digit strings.
Modular, theory-theory, and expertise views of domain-specificity differ from one another in several fundamental ways. They make different assumptions concerning what is innate, the role of input, mechanisms of development, interindividual variability in performance, and what constitutes a domain. For example, modular theories propose that mechanisms of developmental change are biological constraints, theory theories propose that the relevant mechanisms are causal-explanatory understandings, and expertise theories propose that such mechanisms are information-processing skills. Nonetheless, they converge on the proposal that cognitive abilities are specialized to handle specific types of information. For critiques of domain-specificity, see Bates, Bretherton, and Snyder (1988) and Elman et al. (1996).
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