When do children realise someone can believe something false?

Around age four, most children realise that another person can believe something that is not true. The test that shows it is the Sally-Anne test, a short puppet show that psychologists use to measure a person's ability to attribute false beliefs to others.

In the skit, Sally takes a marble and hides it in her basket. She leaves the room. While she is away, Anne takes the marble out and puts it in her own box. Then Sally comes back, and the child is asked: where will Sally look for her marble? To pass, the child must say the basket, because Sally believes that is where it is. Children under four mostly answer "Anne's box", seemingly unaware that Sally does not know the marble has been moved.

Why does this matter? Passing shows understanding that someone else has beliefs that may not match reality, the core requirement of theory of mind.

Several things help children get there. Learning complement syntax, as in "She thinks that the book is on the table", makes children more likely to pass, and children who hear more references to mental states from caregivers often show earlier gains. Children in places as different as Canada, India, Peru, Samoa and Thailand pass at around the same time.