Thus Tomasello's negative conclusions about the limits of chimpanzee cognition are in the service of a positive theory of how human uniqueness evolved.
Some years ago, Tomasello concluded that chimps and other non-human primates have no capacity for understanding the psychological states of other individuals. But more recently, he has decided that new experimental research shows that he was wrong, and that chimps can understand that others perceive and know things and have goals or intentions. For instance, experiments in which chimps are competing for food show that they think about what their competitor can and cannot see, hear, or know. In this sense, chimps do have a theory of mind.
And yet, in another sense, they don't, Tomasello argues, because there is no experimental evidence that chimpanzees understand false beliefs--that other individuals can be not only informed or uninformed but misinformed. By comparison, children as young as 4 years old--and perhaps even younger--do understand that individuals can have false beliefs.
Here's the abstract for one of his articles on this point ("A Nonverbal False Belief Task"):
A nonverbal task of false belief understanding was given to 4- and 5-year-old children (N=28) and to two species of great ape: chimpanzees and orangutans (N=7). The task was embedded in a series of finding games in which an adult (the hider) hid a reward in one of two identical containers, and another adult (the communicator) observed the hiding process and attempted to help the participant by placing a marker on the container that she believed to hold the reward. An initial series of control trials ensured that participants were able to use the marker to locate the reward, follow the reward in both visible and invisible displacements, and ignore the marker when they knew it to be incorrect. In the crucial false belief trials, the communicator watched the hiding process and then left the area, at which time the hider switched the locations of the containers. When the communicator returned, she marked the container at the location where she had seen the reward hidden, when was incorrect. The hider then gave the subject the opportunity to find the sticker. Successful performance required participants to reason as follows: the communicator placed the marker where she saw the reward hidden; the container that was at that location is now at the other location; so the reward is at the other location. Children were also given a verbal false belief task in the context of this same hiding game. The two main results of the study were: (1) children's performance on the verbal and nonverbal false belief tasks were highly correlated (and both fit very closely with age norms from previous studies), and (2) no ape succeeded in the nonverbal false belief task even though they succeeded in all of the control trials indicating mastery of the general task demands.
This research is part of Tomasello's larger research project for understanding the evolutionary psychology of social ontology. As indicated by John Searle and other philosophers, we live in two worlds--a physical reality that is true independently of our subjective awareness and a social reality that depends on our subjective awareness. So, for example, a piece of paper money exists physically as a piece of paper regardless of what we think about it, but its value as money depends on our subjective social agreement. All of our social practices and institutions--money, property rights, government, and so on--are like this. All social animals seem to have some cognitive capacities that allow them to create their social worlds. But human beings seem to be unique in having cognitive capacities that allow them to create social worlds that are far more complex, extensive, and flexible than is the case for other social animals. Any evolutionary science of social cooperation must explain this.
Through his experiments comparing non-human primates and children, Tomasello concludes that while chimps are capable of social coordination, which requires that individuals respond to each other's behavior--as, for example, when chimps engage in group hunting or warfare--they do not show the collaboration through joint intentions based on coordinated plans that arises among young children.
The uniqueness of human institutional ontology is evident in how human beings use language to construct social institutions. But Tomasello thinks that the primary entrance into human social reality is through games of pretend play that display their basic structure among 2-year old children. Through games of pretense, children create social activities through agreement with others on the rules of the game, which shows the crucial step towards human social ontology, a step not taken by chimps. (Of course, to pursue this further, we would have to ask how this is different from the play behavior of other primates.)
Josep Call and Michael Tomasello, "A Nonverbal False Belief Task: The Performance of Children and Great Apes," Child Development, 70 (March/April 1999): 381-395.
Josep Call and Michael Tomasello, "Does the Chimpanzee Have a Theory of Mind? 30 Years Later," Trends in Cognitive Sciences 12 (2008): 187-192.
Hannes Rakoczy and Michael Tomasello, "The Ontogeny of Social Ontology: Steps to Shared Intentionality and Status Functions," in S. L. Tsohatzidis, ed., Intentional Acts and Institutional Facts: Essays on John Searle's Social Ontology (Dordrecht, The Netherlands: Springer, 2010), 113-137.