As a toddler watches, two evaluators play with a doll. One evaluator puts the doll in a basket, and then leaves the room. The second evaluator takes the doll out of the basket and puts it into a box. Then she closes the box, and says to the toddler, "When the other evaluator comes back, where will she look for the doll?"
A typical toddler understands that the first evaluator didn't see the doll being moved to the box. Her answer is likely to be, "She'll look for the doll in the basket."
An autistic child is less likely to understand that the evaluator who left the room won't know what he knows... and thus, he will respond, "She'll look for the doll in the box."
What's going on?
For many people with autism, it can be extraordinarily difficult to put oneself in another person's shoes in order to imagine what that other person might be thinking or feeling. Researcher Simon Baron-Cohen describes Theory of Mind as "...being able to infer the full range of mental states (beliefs, desires, intentions, imagination, emotions, etc.) that cause action. In brief, having a theory of mind is to be able to reflect on the contents of one's own and others' minds." Baron-Cohen developed a term for lack of theory of mind that he called "mind blindness."
Researchers including Baron-Cohen and Uta Frith believe that mind blindness at some level is present in all people on the autism spectrum. They also feel that the lack of theory of mind is a result of neurological differences. For those individual on the autism spectrum with strong intellectual abilities, it is possible to build some "mind reading" abilities through social skills training. Mind blindness, however, is likely to be an issue for all people on the autism spectrum throughout their lives.
Baron-Cohen, Simon. "Theory of Mind in Normal Development and Autism." Prisme, 2001, 34, 174-183.
Chevallier C, Noveck I, Happé F, Wilson D. "What's in a voice? Prosody as a test case for the Theory of Mind account of autism." Neuropsychologia. 2011 Feb;49(3):507-17.
Frith, Uta. "Mind Blindness and the Brain in Autism." Neuron, Vol. 32, 969–979, December 20, 2001.
Lerner MD, Hutchins TL, Prelock PA. "Brief Report: Preliminary Evaluation of the Theory of Mind Inventory and its Relationship to Measures of Social Skills." J Autism Dev Disord. 2010 Jul 14.