There have been considerable advances in the comparative study of primate social learning during the last two decades. As Hurley outlines, one of the key findings to emerge in this time has been the clear and consistent documentation of differences in the ways human children and chimpanzees respond to another's modelled actions. Young children tend to fixate on copying the specific behavioural means used by the demonstrator (i.e., the person modelling the target actions), even if a simpler method is available. By contrast, chimpanzees tend to focus on outcomes, preferring to discover their own means of bringing about the demonstrated end result. This difference is representative of the oft-debated distinction between imitation and emulation. Hurley presents a detailed, thought-provoking model that is aimed, in part, at identifying the possible neural underpinnings of these alternative approaches to social learning. But as elegant as her model is, it misses a crucial component of human imitation: motivation.
A critical means of distinguishing imitation from emulation is to identify whether an observer preferentially aims to reproduce the specific behavioural means a demonstrator used to bring about an outcome or whether she chooses to use her own means. Does the observer focus on copying actions or outcomes? This notion of separating actions from outcomes when evaluating copying behaviour was presaged by Uzgiris (Reference Uzgiris1981), who drew attention to what she saw as two core functions of copying behaviour: a cognitive function that promotes learning about events in the world and an interpersonal function that promotes children's sharing of experience with others. According to Uzgiris, young infants are primarily driven by a need to acquire new skills and behaviours and, as such, when they are shown how to do something, they focus on what was done (i.e., the outcome). However, as they move into their second year, infants become increasingly motivated to engage in social interaction and hence, as a means of realizing the congruence that exists between themselves and others, they begin to focus on the way something was done (i.e., the means used). To put it another way, young infants emulate out of a motivation to learn about the world, whereas toddlers show an increasing proclivity for imitation based on a desire to interact with, and to be like, others (for a similar view on why adults imitate, see Dijksterhuis Reference Dijksterhuis, Hurley and Chater2005).
Recent studies have provided evidence for this proposal of an age-related shift when copying from a focus on outcomes to a focus on actions (Nielsen Reference Nielsen2006; Tennie et al. Reference Tennie, Call and Tomasello2006). In a cross-sectional study, Nielsen (Reference Nielsen2006; Experiment 1) tested 12-, 18-, and 24-month-olds. An adult demonstrated how to open a series of novel boxes (which contained a desirable toy) by using a miscellaneous object to activate a switch located on the front of each box. The 24-month-olds imitated in attempting to open the boxes by using the object, as was shown to them. In contrast, the 12-month-olds emulated the demonstrator's actions and only attempted to open the boxes with their hands (18-month-olds showed reactions that were intermediate between the older and younger age groups). In a follow-up experiment (Nielsen Reference Nielsen2006; Experiment 2), 12-month-olds did imitate the adult's object use, but only after she had “attempted but failed” to activate the switches by hand. Thus, it appears that 12-month-olds did not fail to imitate because they could not use the object, but rather because they did not interpret this action to be the most efficient alternative available (see also Gergely Reference Gergely2003; Gergely et al. Reference Gergely, Bekkering and Király2002).
Following Uzgiris (Reference Uzgiris1981), I reasoned that the 24-month-olds might persist in imitating a demonstrator's inefficient object use in order to satisfy social motivations. Testing this interpretation, Nielsen et al. (in press) compared the responses of 24-month-olds to live and videotaped demonstrators on the boxes task used in the Nielsen (Reference Nielsen2006) study. The rationale for using videotaped demonstrators was that they can act in a social and engaging manner but, by virtue of the medium, do not afford opportunity for spontaneous, contingent interaction. If the social motivation hypothesis is valid, children should be less inclined to imitate when the opportunity for social interaction is reduced. They should be less inclined to imitate a videotaped adult than one who is available for interaction. This is exactly what happened. The children imitated the adult's object use significantly less when she appeared on video compared to when she was “live” (Experiment 1). Critically, in a second experiment, when given the opportunity to interact with the adult on a TV monitor via a closed-circuit system (i.e., where socially contingent interaction could take place), the amount of imitation children exhibited returned to “live” levels, indicating that it was the nature of their interaction with the demonstrator that affected the children's copying behaviour, not the medium.
Hurley makes a laudable effort at trying to account for human imitative behaviour by integrating complex motor ability and a capacity for goal reading into the shared circuits model (SCM). Both elements are certainly crucial in determining how we copy others. Nevertheless, as attested to by the previously discussed studies, human social learning can be strongly impacted by interpersonal motivations. These motivations are all too frequently neglected in discussions of, and attempts at explaining, imitation. Unfortunately, the SCM is no exception. There is focus on the way in which a capacity for imitation may get developed and, in layer 5, on how this could then lead to a faculty for understanding other minds. But this is not the same as acknowledging the strong interpersonal motivations that can drive imitative behaviour in the first place.
A growing number of experiments have provided remarkable insights into the neural substrates of imitation. The SCM offers a means of unifying much of this literature and promises to make a major contribution to the field. Nevertheless, one must not lose sight of the fact that human copying behaviour is extremely complex. Its expression is affected by multiple factors and here I have tried to draw attention to interpersonal ones. If we continue to ignore these factors, our understanding of the mechanisms that lie at the heart of human imitation is destined to remain incomplete.
