Theory of mind stands apart from other theories that seek to explain how we can attribute mental states to those we deem as "others." Specifically, it states that at some point around age 3 or 4, we become aware that other people hold different attitudes, beliefs and knowledge than we do. It becomes apparent to our minds that knowledge can be compartmentalized (we may know something someone else doesn't and vice versa). We realize that there is such a thing as pretense (the ability to create falsehoods). And we realize that other people may feel differently than us, meaning we do not all share the same mental and emotional states and beliefs simultaneously. This is the first, huge step toward metacognition, or thinking about thinking [source: Wiley-Blackwell].
The skills associated with theory of mind don't emerge in all humans. People on the autism spectrum have long been observed to have mindblindness, characterized as an inability to consider others' points of view, wants, needs and desires. This has often been linked to a lack of empathy, although that idea has come under fire in recent years. Instead, researchers have come to believe that people with autism lack theory of mind skills.
To test this, researchers have given autistic children false-belief tests. These tests go something like this:
Sally is playing with her ball in her room. She gets up to go to the kitchen for a while and places the ball in her top dresser drawer. While she's gone, her mother comes in Sally's room and moves the ball from the dresser drawer to the toy box. When Sally comes back from the kitchen, where will she look for her ball?
A neurotypical child would correctly guess that Sally would look in the drawer for her ball, since that's where she left it. This answer shows a child has developed an awareness that others may not have knowledge she has, a hallmark of theory of mind. Although that child knows Sally's mother moved the ball, Sally doesn't. Children with autism generally tend to answer that Sally will look in the toy box, which is where they know the ball has been moved.
Autistic children are even more prone to fail second-order false belief test. These elaborate on the Sally test, where children are asked what they think a second character thinks about another character; for example, what John thinks Todd knows is in his lunch box [source: Baron-Cohen].
Theory of mind is an interesting concept. Perhaps further study of autism itself will yield more answers to how we arrive at that amazing state of being able to think about others.