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The Minds of Humans and Apes: Alternative Outcomes on an Evolutionary Experiment

Are there are any truly unique characteristics of the human mind? If so, what techniques are needed to identify them? Traditionally, the human mind has been viewed as a brighter, more talkative version of a generalized mammalian mind. But this view may be flawed. The results of our comparative research program with chimpanzees and human children suggest that the human species may have evolved fundamentally new psychological abilities - abilities that arose not as graded improvements of existing cognitive faculties, but as specifications that were woven into ancestral neural systems.

Six years ago, I established two laboratories, one for studying chimpanzees, the other for studying human children. Through systematic -experimental comparisons, we have explored the similarities and differences in how humans and their closest living relatives understand the social and physical worlds. In this essay, I focus on one aspect of our research: the ability to reason about mental states. In particular, I summarize our work on how chimpanzees and children understand visual perception and attention. The results of this research have revealed what seems at first to be a perplexing mix of similarities and differences. On the one hand, our research has provided the first experimental demonstration of gaze following in nonhuman primates and has shown that this gaze-following system is remarkably sophisticated. Indeed, not only do chimpanzees follow the gaze of others, they do so in response to eye movements alone, and even appear to understand that the gaze of others can be obstructed by opaque barriers. On the surface, such findings seem to imply that chimpanzees, like 18-month-old human infants, have some Understanding of the mental state of attention. However, the results of over three dozen other experiments have consistently indicated that despite their excellent understanding of the geometry of gaze direction, our apes do not appear to understand that gaze is connected to internal states of attention.

In a particularly revealing series of studies, we investigated whether chimpanzees understand the psychological difference between someone who $1110, see them and someone who cannot. To do so, we capitalized on the chimpanzee's natural begging gesture (one of their common communicative signals). We trained our apes to gesture through a hole in a plexiglas partition directly in front of a familiar experimenter who was either to their left or right. The apes quickly learned to gesture through the correct hole toward the experimenter. In order to determine whether their use of their begging gesture was mediated by a concept of 'seeing' we examined their reactions to encountering two experimenters, one who Mild see them (and therefore could respond to their gesture), and one who could not. We constructed several clear cases (at least from our human point of view) of seeing' versus 'not seeing.' Remarkably, the animals were just as likely to gesture to the person who could not see them, as to the person who could see them. In contrast, on the easy surrounding trials, the apes nearly always gestured correctly directly in front on the single experimenter. Thus, despite their general interest and motivation, the animals appeared oblivious to the distinction between seeing and not seeing. An extended series of follow-up studies revealed that although the apes could learn to discriminate between the seeing and not seeing conditions, what they learned was unrelated to the mental state of attention. They learned rules such as 'gesture to the person who is facing forward,' or 'gesture to the person whose is face is visible,' and not rules such as 'gesture to the person who can see me.' With additional experience, however, our apes learned to discriminate even the most subtle conditions we created (such as between a person with eyes open versus a person with eyes closed). However, even in these cases, further tests revealed that the chimpanzees based their choices on superficial physical cues, not on who could see them. In contrast, 2-3-year-old children consistently chose the person who could see them.

Our chimpanzees' emphasis on behavioral propensities, and not mental states, appears to be quite general, extending well beyond the realm of gaze and attention. For example, we have compared chimpanzees and children's understanding of the intentions underlying cooperative actions, the difference between intended versus unintended actions, and referential acts such as pointing. In each case, our apes have responded in ways that suggest that although they are very attuned to the world of behavior, they are largely oblivious to the world of the mind. Our comparative work with children reveals that these are capacities that emerge between 2 and 4 years of age.

The results of our research, then, suggest that there may be profound psychological differences between humans and chimpanzees. I explore the possibility that although humans may have evolved a new system for representing the actions of others in mentalistic terms, this new system did not replace the ancestral psychology of the social primates. Like a tapestry into which new colors were added, new systems or subsystems may have been created while conserving an ancestral repertoire of social behaviors. As our research on gaze-following reveals, two species may share a very similar behavior, controlled by similar lowlevel mechanisms, but interpret that behavior in very different ways. Thus, although humans form internal representations of mental states such as desires, knowledge, and beliefs, and these representations provide us with a useful means of anticipating what others will do, the basic blueprint of the behaviors with which we can respond evolved long before those higher-level representations were possible. However, once our species evolved the capacity to represent other organisms not just as behaving beings, but as subjective beings who attend, want, and think, it became necessary to use our behaviors to respond to the world of the mind may have set the stage for the emergence of those psychological arenas in mind, as well as the world of behavior. Our species' evolutionary discovery of the which we now seem to differ most from other species, such as culture, pedagogy, and ethics.