Research has shown that between the ages of three and four, in what Piaget (1954) calls the preoperational stage, children naturally develop a "belief-desire theory of mind" (Wellman, 1990). This theory (Gopnik & Wellman, 1992) isseen in the fact that children behave as if (and increasingly talk as if) their beliefs and desires are what cause their behavior, as well as the behavior of others. It appears that the development of such theories of mind (or folk psychologies) are particularly encouraged in cultures where parents spend time discussing motives and intentions, as well as mental states, as they do in the United States (Sabbagh & Callahan, 1998). An important part of this theory of mind is a sense of free will. That is, one perceives the thoughts and feelings that occur before an action as the causes of that action. Even though this sense of free will persists into adulthood it is an illusion, as discussed below. The tendency to attribute mental states to ourselves and to others is quite powerful, even in adults, as seen in a classic study done by Heider and Simmel (1944). The researchers showed a short film, which consisted of different simple shapes such as triangles or squares, moving about on the screen. When asked to report what they saw, the majority of the subjects attributed motives and intentions to the inanimate shapes. It seems highly likely that this tendency would be much stronger when people are observing animate subjects, including people. At the same time that people have a tendency to attribute psychological states to themselves and to others, related work has shown that they do not necessarily do so accurately. In a review, Nisbett and Wilson (1977) reported that in a large number of studies conducted by many different investigators, people showed minimal capacity to provide an accurate report of their own cognitive processes. They instead may offer contrived or confabulated explanations of behavior. These two studies foreshadow the points to be made here: first, that it is people's natural tendency to try and explain the behavior of themselves and of others, and second, that the explanations that they come up with are not necessarily accurate. Folk psychology as seen in adults is an explicit or implicit theory about the causes of behavior. One instantiation of this theory is seen in everyday talk about mental states. We make statements that link sensory experiences to mental states; mental states to other mental states; and mental states to behavior. We might say that the smell of cookies baking made Jessie feel hungry. Because she was food deprived, Jessie wanted to eat a cookie. But she also felt bad about eating a cookie because Jessie thought she was overweight. Nevertheless, Jessie then went to the cookie jar and got a cookie because Jessie wanted a cookie. This example illustrates one key aspect of many of the folk psychological beliefs, which is that people freely plan their actions, and that their intention or wish to obtain something is in fact what leads to the action. This is the notion of free will.