Do humans have two systems to track beliefs and belief-like states?
Ian A. Apperly
University of Birmingham, UK
& Stephen A. Butterfill
In press in Psychological Review
This article may not exactly replicate the final version published in the APA journal. It is not the copy of record. Copyright to the American Psychological Association http://www.apa.org/journals/rev/submission.html
The lack of consensus on how to characterize humans’ capacity for belief reasoning has been brought into sharp focus by recent research. Children fail critical tests of belief reasoning before 3 to 4 years (Wellman, Cross, & Watson, 2001; Wimmer & Perner, 1983), yet infants apparently pass false belief tasks at 13 or 15 months (Onishi & Baillargeon, 2005; Surian, Caldi, & Sperber, 2007). Non-human animals also fail critical tests of belief reasoning but can show very complex social behaviour (e.g., Call & Tomasello, 2005). Fluent social interaction in adult humans implies efficient processing of beliefs, yet direct tests suggest that belief reasoning is cognitively demanding, even for adults (e.g., Apperly, Samson & Humphreys, 2005). We interpret these findings by drawing an analogy with the domain of number cognition, where similarly contrasting results have been observed. We propose that the success of infants and non-human animals on some belief reasoning tasks may be best explained by a cognitively efficient but inflexible capacity for tracking belief-like states. In humans this capacity persists in parallel with later-developing, more flexible but more cognitively demanding theory of mind abilities.
Do humans have two systems to track beliefs and belief-like states?
More than twenty five years of research has taught us a great deal about theory of mind, the ability to ascribe mental states such as beliefs, desires and intentions in order to explain, predict and justify behaviour. We have learned much about the age at which children reach developmental milestones, about the abilities of non-human animals, about the disruption of theory of mind in developmental disorders such as autism or following brain injury, and about the neural systems involved when people engage in this kind of thinking. However, we seem no nearer to reaching any consensus on the cognitive basis of theory of mind abilities, or even of specific aspects of theory of mind, such as the paradigm case of belief ascriptions. One reason for this is that dominant accounts aim to explain the development of theory of mind, or to characterise theory of mind in non-human animals. They give much less consideration to how inferences about mental states are achieved for the wide range of everyday functions that theory of mind is supposed to support.
A central contention in the account we develop here is that theory of mind abilities are subject to competing demands for efficient and flexible processing. On the one hand theory of mind abilities need to be fast enough to guide competitive and cooperative activities in rapidly changing circumstances and efficient enough not to consume cognitive resources necessary for the primary task of competition or cooperation. On the other hand, theory of mind abilities in human adults need to be as flexible as any reasoning abilities in order to support the explicit explanation and prediction of action that is involved in jurisprudence, strategic negotiation, self-awareness and understanding one’s relations to other thinking agents (Harris, 1994; compare Heal, 1998).
Competition between demands for efficient and flexible processing is reflected in a fundamental disagreement concerning belief ascription which dates back to some of the earliest papers on theory of mind. To one way of thinking, belief ascriptions depend on one or more modules, whose operation is fast and efficient and whose fundamental conceptual and processing structures are fixed before or during infancy (e.g. Leslie, 1994a, 1994b). The alternative view is that the ability to ascribe beliefs depends upon flexible but effortful general reasoning abilities, plus knowledge learned during children’s early childhood about what beliefs are, the conditions for their formation, and the role they play in cognition (e.g. Gopnik & Meltzoff, 1997). These alternatives imply very different and incompatible views about the nature of belief ascription, and about the relationships between belief ascription and other cognitive processes. The conflict between these views is brought into focus by recent research on belief reasoning in infants, and it is with these developmental findings that we will begin. However, the same tensions exist when considering the abilities of non-human animals and human adults, which we discuss in later sections. We will argue that both current views of belief ascription have significant evidence in their favour, and that neither is likely to be fully correct. Instead we advocate a view based on lessons from another domain, number cognition: the competing demands of efficient and flexible processing are solved by having two systems1.
Our central conjecture, that theory of mind involves two systems, has been canvassed in general terms by a variety of theorists with otherwise very different convictions (including Byrne, 2002; Csibra & Gergely, 1998; Leslie, 1994a; Perner, 1991; Povinelli, Bering, & Giambrone, 2000; Russell, 2007, Tager-Flusberg & Sullivan, 2000; Suddendorf & Whitten, 2003). However, it has not been developed in any detail for the case of belief reasoning that occupies such a central position in research on theory of mind. In the current paper we examine the cognitive demands of belief reasoning, develop the hypothesis that meeting these demands requires two kinds of cognitive system, we evaluate this hypothesis in the light of converging evidence from infants, adults, and non-humans, and identify the possible future evidence that could distinguish between alternative two-system solutions. Our project of characterising the cognitive basis of belief reasoning has implications for all of the subject areas where theory of mind has been investigated, including typical and atypical development (e.g., Baron-Cohen, Tager-Flusberg & Cohen, 2000; Doherty, 2008; Wellman et al., 2001), cognitive psychology (e.g., Apperly, Samson & Humphreys, 2009; German & Hehman, 2006; Keysar, Lin & Barr, 2003), cognitive neuroscience (e.g., Apperly, Samson & Humphreys, 2005; Frith & Frith, 2003; Saxe, Carey & Kanwisher, 2004) and comparative psychology (e.g., Call & Tomasello, 2008; Penn, Holyoak & Povinelli, 2008).
The development of belief reasoning.
Reasoning about beliefs and other mental states has a protracted developmental course, in which the acquisition of a conventional linguistic system for describing different mental states and structuring their content appears to play a critical role (Astington & Baird, 2005). One important and much-studied benchmark in this development is the ability to understand false beliefs. Children do not typically succeed on standard false belief tasks until around 4 years of age (Wellman et al., 2001; Wimmer & Perner, 1983) 2. Further developments in children's theory of mind abilities occur later still. For example, children do not succeed until 5 or 6 years of age on tasks that require recognition that beliefs only represent a subset of the features of their referents, (Apperly & Robinson, 1998, 2003; Hulme, Mitchell, & Wood, 2003; Sprung, Perner, & Mitchell, 2007).
However, there is evidence that 2- and 3-year-old children, who fail standard false belief tasks, may be aware of false beliefs. For example, Garnham and colleagues asked 3-year-old children where Sam will look for some cheese which was secretly moved while he was sleeping. Although the 3-year-olds incorrectly said that Sam will look where the cheese actually is, they nevertheless appear to show some awareness of false beliefs by looking at the location where a character believes some cheese is hidden when prompted by with ‘I wonder where he’s going to look’ (Clements & Perner, 1994; Garnham & Perner, 2001; Garnham & Ruffman, 2001). Using a modified, non-verbal version of Garnham’s procedure, Southgate and colleagues (Southgate, Senju, & Csibra, 2007) have shown similar looking behaviour in 2-year-olds which is also indicative of false belief understanding3.
Most strikingly, Onishi & Baillargeon (2005) provided evidence that 15-month-old infants understand that others can have false beliefs by using a violation-of-expectation paradigm. In one condition infants were shown the following sequence of events: a watermelon slice is placed into a green box while an actor watches; then the actor’s view is blocked as the watermelon slice moves to a yellow box (so the actor’s belief becomes false); finally, the actor reappears and reaches into one of the boxes. Infants looked significantly longer at the display when the actor reached into the yellow box than when the actor reached into the green box. The opposite pattern of looking was found in another condition where the actor observed the watermelon’s movement (so had a true belief). This and other control conditions suggest that infant’s looking times may correlate with whether or not the actor acts in accordance with her beliefs. Related results have since been independently obtained with 13-month-olds (Surian et al., 2007; see also Scott & Baillargeon, 2008)4.
There have been two main lines of response to the evidence that infants are aware of false beliefs. Some authors argue that infants understand false belief; they therefore deny that there is a fundamental change at around four years when children first pass standard measures of false belief (Onishi & Baillargeon 2005; Leslie 2005), and insist that infants' early false belief understanding provides the "conceptual foundation" for later abilities to reason about false beliefs (Surian et al., 2007, p. 585). Opposing this view, others insist that apparent success on theory of mind tasks in infancy can be explained without supposing infants have any understanding of belief at all. For example, some suggest infants’ looking times may be explained by their adopting behavioural rules such as ‘people look for objects where they last set eyes on them’ (Perner & Ruffman 2005, p.214; Ruffman & Perner 2005, p.462).
How might this conflict be decided? Both sides claim that parsimony favours their position (Onishi & Baillargeon 2005, p.257; Perner & Ruffman 2005, p.214). This suggests that considerations of parsimony will not be decisive. In our view both sides of this conflict are mistaken, but progress can be made by taking a broader perspective on the abilities of infants, taking into account the cognitive limitations of infants, and the abilities of non-human animals and human adults. Our first step is to draw a lesson from the case of number cognition, where there is also an apparent discrepancy between precocious abilities in infants and persisting difficulties in older children, and where taking a broader perspective on infants’ abilities has significantly advanced our understanding of number cognition in general.