"Theory of mind" describes an area of research that focuses on the child's ability to understand mental concepts such as belief, desire, the difference between appearance and reality, and the existence of other minds. The field has flourished in the past twenty years for at least two reasons: it is of great theoretical interest and it has taken advantage of relatively recent developments in methods for studying infant cognition. The theoretical interest lies in the fact that theory of mind research explores the development of our naïve concepts about mind in the same way that Piaget's theory explores the development of our naïve concepts about the physical world. Theory of mind research has shown that young children make just as many systematic mistakes in the mental arena as they do in the physical. The methodologies that have helped advance theory of mind research are those that allow researchers to probe the cognition of infants who cannot yet speak. The most important of these methods is the looking-time paradigm, in which the amount of time a child looks at an image is used to judge whether that image is "surprising" given the child's representation of the world. More concrete examples are given below.

The false-belief paradigm is the classic example of pre-preschool children's failures in theory of mind. The experiment, stripped down to its essentials, is as follows: a child is shown two dolls, two boxes (one red and one green), and a marble, all of which are in a single miniature scene. One doll puts the marble in the green box, so that it is hidden, and then departs. While the first doll is gone, the second doll removes the marble from the green box and puts it in the red box. The first doll then returns, and the child is asked: Where will the first doll look for the marble? Children before the age of four will typically say that the doll will look in the red box, where the marble actually is. They seem to have a hard time understanding that the doll might have a false belief, one that does not correspond to reality. After the age of four, children begin to give the correct response: the doll will look in the green box, where it had last seen the marble.

This inability to comprehend the idea of false belief extends to other paradigms as well. In one study, children are shown a box of pencils. When asked what they think is inside the box, most children understandably respond "pencils." The box is then opened and the child is shown the box's actual contents: candy. The child is then asked to imagine what someone who had seen the closed box would think was inside. Before the age of four, most children will respond "candy," failing to recognize that people sometimes believe falsely. In fact, when asked what they themselves thought was in the box when they first saw it, children often report that they thought the box contained candy--even though just moments before they had reported thinking it contained pencils. This shows that children have impairments in metacognition, the ability to think about and remember their own thoughts.

A further demonstration of this failure to distinguish between appearance and reality is given by the "filter experiment." In this experiment, the child is shown a white piece of paper and a transparent blue screen. The child correctly indicates that the paper is white and the filter is blue, but when the paper is placed behind the filter the child is unable to make the distinction between the paper's appearance--now blue--and its intrinsic, or "real," color.

The studies mentioned above describe impairments in theory of mind that have almost completely disappeared, in most children, by the time of puberty. Some children, however, are severely impaired on these tasks throughout their lives. Children with autism, for example, are often as intelligent as children with other forms of mental retardation when measured by standard IQ tests, but they are much worse at the false belief, appearance-reality, and metacognitive tasks described above. Some have argued that this failure in theory of mind lies at the root of autistic deficits in language, communication, and empathy.