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.


Blind alley

To test this idea, neuroscientist Rebecca Saxe at the Massachusetts Institute of Technology and colleagues scanned the brains of 10 congenitally blind adults as they answered questions about the beliefs of people described to them.

While most blind adults develop a mature theory of mind, it wasn’t clear whether they used the same parts of their brain as sighted people do to reason about the mental states of others.

Saxe’s team showed that the same brain regions were indeed activated in the blind adults as in 22 sighted volunteers. They conclude that the way the brain reasons about the beliefs of others does not depend on visual observations.

New language

Another suggestion is that theory of mind comes from our ability to use language, which allows children to listen to people talking about their beliefs and emotions. This is backed up by the fact that language fluency and the ability to pass the false belief test emerge at around the same age.

However, previous studies have not teased apart whether language makes understanding false beliefs easier, or is a “necessary prerequisite”, says Jennie Pyers, a psychologist at Wellesley College in Massachusetts.

A community of deaf people in Nicaragua, who only developed a sign language in the 1970s, provided Pyers’s team with a unique opportunity to compare two sets of people with very different levels of language ability: the first generation of signers, who created the rudimentary sign language, and adolescent signers who had worked out a more complex system of signs.

Language key

Pyers’s team showed both groups videos of false belief tests and asked them to answer by pointing at one of two images. The adolescent signers were more likely to show an understanding of false belief than the older generation.

What’s more, adults who later learned the more complex language from the youngsters got better at the false belief tests. The researchers say this suggests that language may be the key to passing the false belief test.

“Hearing language is particularly important for understanding others, while other kinds of experience, such as the visual modality, are less important,” says Alison Gopnik, a cognitive psychologist at the University of California, Berkeley.

Journal references: Saxe: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0900010106

Pyers: Psychological Science, DOI: 10.1111/j.1467-9280.2009.02377.x