How blind and deaf people approach a cognitive test regarded as a milestone in human development has provided clues to how we deduce what others are thinking.
Understanding another person’s perspective, and realising that it can differ from our own, is known as theory of mind. It underpins empathy, communication and the ability to deceive – all of which we take for granted. Although our theory of mind is more developed than it is in other animals, we don’t acquire it until around age four, and how it develops is a mystery.
You can test for theory of mind via the false-belief test, in which two children are shown playing. One puts a toy under the bed and leaves the room. The second then removes it and puts it in the toy box. On returning, where will the older child look for the toy? Those under the age of four choose the box, while older children correctly say under the bed.
Where does this leap in understanding come from? According to one hypothesis, children gradually deduce that other people have internal experiences that are different from their own by observing the facial expressions and gestures of others over time.