The Guardian:

For decades, developmental psychologists have been fascinated with the question of how children develop theory of mind – in other words, how we come to understand that other people can have different types of thoughts, beliefs and knowledge to ourselves. A key milestone in this journey involves developing a notion of false belief; sometimes, the things that people believe about the world are very different from the reality of the situation, and this will have important consequences for how people act. But how do you measure something so seemingly esoteric?

Beginning in 1983, a series of studies used fictional scenarios to try and assess at what age children start to get a grasp of the existence of false beliefs. Perhaps the most influential of these experiments is known as the Sally Anne task, developed by Simon Baron-Cohen, Alan Leslie and Uta Frith, then at the MRC cognitive development unit in London. In the experiment, children were presented with two dolls, Sally (who has a basket) and Anne (who has a box). Sally puts a marble in her basket, and leaves the room. While Sally is away, Anne takes the marble from the basket, and hides it in her box. Finally, Sally returns to the room, and the child is asked three questions:

• Where will Sally look for her marble? (The “belief” question)
• Where is the marble really? (The “reality” question)
• Where was the marble at the beginning? (The “memory” question)

The critical question is the belief question – if children answer this by pointing to the basket, then they have shown an appreciation that Sally’s understanding of the world doesn’t reflect the actual state of affairs. If they instead point to the box, then they fail the task, arguably because they haven’t taken into account that they possess knowledge that Sally doesn’t have access to. The reality and memory questions essentially serve as control conditions; if either of these are answered incorrectly, then it might suggest that the child didn’t quite understand what was going on.

BTW, Baron-Cohen has a cousin.