Understanding other people is one of the most fundamental human problems. We know much less, however, about our ability to understand other minds than about our ability to understand the physical world. The branch of cognitive science that concerns our understanding of the minds of ourselves and others has come to be called "theory of mind," though it should perhaps be called "theory of theory of mind." It involves psychological theorizing about our ordinary, intuitive, "folk" understanding of the mind.
A number of different disciplines have collaborated in this effort. Philosophers have debated the nature and origin of our understanding of the mind, our FOLK PSYCHOLOGY, at length. Comparative psychologists have explored the evolution of this capacity. One currently prevalent theory of the evolution of cognition suggests that the capacity to understand, and so manipulate, our conspecifics was the driving force behind the development of distinctively human intelligence (Byrne and Whiten 1988). There has also been extensive work on primates' ability to understand mental states. The most recent work suggests that these abilities are fragmentary, at best, compared to the abilities of humans (Povinelli 1996; Tomasello, Kruger, and Ratner. 1993; see PRIMATE COGNITION). Clinical psychologists have proposed that the disorder of AUTISM involves a deficit in "theory of mind" capacities. Social psychologists explore our understanding of aspects of the mind such as the stability of personality traits (Nisbett and Ross 1980). Anthropologists suggest that fundamental assumptions about the mind may differ across cultures (Shweder and Levine 1984).
The most extensive "theory of mind" research, however, has been developmental (see Astington, Harris, and Olson 1988; Perner 1991; Wellman 1990). Children seem to understand important aspects of the mind from a strikingly early age, possibly from birth, but this knowledge also undergoes extensive changes with development. Early research focused on the child's understanding of belief and reality, and on the period between three and five years of age. Several significant and correlated changes seem to take place at about this time. Wimmer and Perner (1983) found that children of this age had difficulty understanding the fact that beliefs could be false. In one experiment, for example, children saw a closed candy box. When they opened it, it turned out that there were pencils inside it, rather than the candy they had been expecting. The children were asked what another person would think was in the box at first, before they opened it. Three year-olds consistently said that the children would think that there were pencils in the box. They did not understand that the other person's belief could be false. Gopnik and Astington (1988) demonstrated that children make this same error when they are asked about their own immediately past false beliefs. Children say that they, too, thought that there were pencils in the box, just as they predict that the other person will think there are pencils there. Flavell and his colleagues showed that three year-olds have a similar problem understanding the distinction between appearance and reality. For example, children who were shown a sponge painted to look like a rock insisted that the object both really was a sponge and also looked like a sponge (Flavell, Green, and Flavell 1986). Finally, children at three also seem to have difficulty understanding the sources of their beliefs. In one experiment children learned about an object that was hidden under a tunnel by either seeing it, feeling it, or being told about it. Although children could identify the object, they could not identify how they came to know about the object (O'Neill and Gopnik 1991). These investigators suggested that these linked developments indicate a new understanding of the representational character of belief between the ages of three and five.
Some recent studies suggest that the first signs of this understanding of false belief may emerge when children place the problem in the context of their earlier understanding of desire or perception (Flavell et al. 1990; Moses 1993; Gopnik, Meltozoff, and Slaugnter 1994), or when they are confronted with counterevidence to their incorrect views (Mitchell and Lacohee 1991; Bartsch and Wellman 1989), and that they may demonstrate some implicit knowledge of belief before they make that knowledge explicit (Clements and Perner 1994). However, this very early understanding appears to be, at best, fragile and incomplete in comparison with the robust and coherent knowledge of belief demonstrated at four or five.
More recent studies have investigated both a wider range of ages and a wider range of mental states. There is extensive evidence that children understand important aspects of desire well before age three. These include the facts that desires can be unfulfilled, that desires determine emotions, and even that desires may differ in different people (Perner 1991; Wellman 1990; Wellman and Woolley 1990). Similarly, even two-and-a-half year olds seem to understand aspects of visual perception. They understand, for example, that two people may see different things if they are on opposite sides of a screen (Masangkay et al. 1974). Three-year-olds also seem to understand important aspects of pretense and imagination, and they can use this understanding to make a general distinction between mental and physical entities (Harris and Kavanaugh 1993; Wellman and Estes 1986; Woolley and Wellman 1990). For example, they understand that pretending to be a rabbit is different from being one, or that an imagined toy is private and intangible while a real toy is not. Bartsch and Wellman (1995) have conducted extensive studies of early spontaneous conversations about mental states. They have demonstrated that children between eighteen months and three years of age do not just display these abilities in laboratory tasks, but they also spontaneously explain human action in these terms, and shift from desire to belief explanations .
As we study younger and younger infants it becomes more difficult to be certain that they are making genuinely mentalistic ascriptions to others. However, studies of nonverbal behavior suggest that even infants understand certain aspects of the mind. Studies of infant IMITATION (Meltzoff and Moore 1977) suggest that there are innate links between children's perception of the actions of others and their perception of their own internal kinesthetic states; newborns who see another person produce a particular gesture will produce that gesture themselves (Meltzoff and Gopnik 1993). Similarly, very young infants show special preferences for human faces and voices, and engage in complex nonverbal communicative interactions with others (Trevarthen 1979). By nine months infants begin to follow the gaze of others and to point objects out to them (Butterworth 1991). In the behavior known as "social referencing," infants who are faced with an ambiguous situation turn to check the adult's facial expression and regulate their actions in accordance with it (Campos and Sternberg 1980). These very early abilities suggest that there is a strong innate component to our "theory of mind."
However, other aspects of our understanding of the mind do not appear to be in place until well after five years of age. For example, although children understand EMOTIONS at a very early stage, they only understand the difference between real emotion and emotional expression at around six (Harris 1989). Similarly, understanding the inferential character of the mind appears to be quite difficult. Children who can understand simple cases of false belief still have difficulty when the questions involve multiple interpretations of ambiguous stimuli or more complex sources of information (Chandler and Helm 1984; Taylor 1988; Wimmer, Hogrefe and Sodian 1988). Finally, children appear to understand fundamental facts about conscious phenomenology, such as the existence of "the stream of consciousness," at a surprisingly late age. Six-year-olds, for example, reported that people could consciously decide to turn over in the midst of a deep dreamless sleep, or conversely, that a person who was awake but just sitting still doing nothing would have no thoughts or internal experience at all (Flavell, Green, and Flavell 1995).
As always in developmental psychology we have a better sense of when various developments take place than of the mechanisms that underlie these changes. Several different accounts of these underlying mechanisms have been proposed in the literature (see Carruthers and Smith 1995). Leslie (1994) and Baron-Cohen (1995) have suggested that the developments reflect the maturation of an innate theory of mind module, by analogy with similar modular theories of language and perception. In fact, in Leslie's view several different modules mature in succession. Harris (1991) has argued for a "simulation" theory of many theory of mind developments. On this view advances in the child's understanding of the mind reflect an increasing ability to simulate or imagine the experiences of others. Dunn suggests that socialization and social interaction may play a crucial role in children's development of a theory of mind (Dunn et al. 1991). In support of this view, there is evidence that younger siblings have a more advanced theory of mind than older siblings (Perner, Ruffman, and Leekam 1994) and that parent's conversations about mental states influence children's understanding of the mind (Dunn et al. 1991). Probably the most widely held view, however, is what has been called "the theory theory" (Perner 1991; Wellman 1990; Gopnik 1993; Flavell, Green, and Flavell 1995). Originally advanced in philosophy (see SIMULATION VS. THEORY-THEORY) this view also is part of a more general theoretical approach that explains children's COGNITIVE DEVELOPMENT by analogy to scientific theory change (Carey 1985; Gopnik and Meltzoff 1997; Wellman 1990). On this view children develop a succession of theories of the mind that they use to explain their experience and the behavior of themselves and others. Like scientific theories, these intuitive or naive theories postulate abstract coherent mental entities and laws, and they provide predictions, interpretations, and explanations. The theories change as children confront counterevidence, gather new data, and perform experiments. One consequence of this view is that the philosophical doctrine of first-person authority is incorrect; our knowledge of our minds is as theoretical as our knowledge of the minds of others. More broadly, the recent research suggests that empirical evidence from developmental psychology may be brought to bear on classic problems in the philosophy of mind.
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