The "Theory of mind" is put simply, the ability to think about what another person may be thinking of or interpret another's facial expressions or actions. It is an educated guess as to what is on another persons mind. Some people refer to this as "mind reading". We all have the ability to try and guess what another person is thinking or feeling by their facial expression and body language. The theory of mind is often abbreviated to "TOM"

The Theory of mind begins developing early in a person's life, as does the symptoms of autism. So for instance a normally developing child of around 6 months old will already be able to read your face, they may not yet know what the face means, like a frown for example. But they will be aware of your smile. Once a child has realized what the different facial expressions mean, they are able to act accordingly. This is known as non-verbal behavior, and they will be able to take the appropriate actions. For example, a toddler can be guided by its mother's facial expressions to know what she is feeling. The autistic child on the other hand does not read facial expressions and will not normally be interested in watching faces. They do not hold the same fascination, as they do to a normally developing child.

Children as young as age three or four can already "read minds". They begin to realize that other people may have different thoughts from themselves.  They know that other people may not know what they are thinking. They are also able to guess what other people are thinking based on their facial expressions and non-verbal behavior. Young children are not perfect at this, so you may get a child who makes up stories in order to cover up their actions.  

How does the theory of mind relate to autism? Researchers have found that the theory of mind is an area of deficiency for most people with autism and Asperger's Syndrome.  One solution for dealing with this problem in people with autism is the use of social stories, which fills in the missing information for them.