People's intuitive understanding of their own and other people's minds or mental states, including beliefs and thoughts. It develops by degrees from a very early age in humans: by the age of 3 years children normally have a well-developed theory of mind but cannot yet understand, for example, that people's beliefs can be false: 3-year-olds who open a candy or chocolate box and are surprised to find pencils inside it consistently say that, before the box was opened, other people would have believed that there were pencils inside it and that they believed this also. Research has suggested that a theory of mind is fully developed only in human beings, but that people with autistic disorder are severely impaired in their ability to understand mental states and to appreciate how mental states govern behaviour. Without a theory of mind, social behaviour is disrupted and the cooperative principle breaks down. The concept was introduced by the US psychologists and primatologists David Premack (born 1925) and Guy Woodruff in an article in the journal The Behavioral and Brain Sciences in 1978, where they defined it as follows: ‘In saying that an individual has a theory of mind, we mean that the individual imputes mental states to himself and others’ (p. 515). The specific inability to appreciate other people's mental states is called mind-blindness. See also empathy, mirror neuron. ToM abbrev.