Mindreader? Yes, you are

Guessing other people's thoughts is an essential human skill. And now we know which part of the brain gives us this 'theory of mind'. Sanjida O'Connell reports

Every time we venture out on the road, we stake our lives on our ability to read minds, a skill we take for granted. Having a "theory of mind", as this ability is known, is nothing to do with telepathy but refers to our ability to understand that other people have thoughts, beliefs and desires. Whether driving or waiting to walk across a road, we rely on our general expectations about the normal desires and decisions of other motorists.

Daniel Dennett, a philosopher from Tufts University, Boston, gives another example of how we continuously and unconsciously use theory of mind: "... watching a film with a highly original and unstereotyped plot, we see the hero smile at the villain and we all swiftly and effortlessly arrive at the same diagnosis: 'Aha!' we conclude (but perhaps not consciously), 'He wants her to think he doesn't know she intends to defraud her brother!'"

Every normal person over the age of five can demonstrate theory of mind. And although it was only discovered just over a decade ago, there is now scientific evidence indicating exactly which part of the brain computes what other people are thinking.

The litmus test of having a theory of mind is whether you can understand that someone else believes something to be true when it is, in fact, false. Dr Heinz Wimmer and Dr Josef Perner, from the University of Salzburg in Austria, were the first psychologists to prove that children under the age of four to five cannot understand these so-called "false beliefs".

They devised a task known as the Sally-Ann test. Children were told a story about two dolls, Sally and Ann. Sally has a basket and Ann has a box. Sally places her marble in her basket and goes out. While she is out, naughty Ann moves Sally's marble from the basket to her box, then she leaves the room. Sally comes back in.

The children were asked where Sally would look for her marble. Adults know that Sally will look for it in her basket. She has a false belief about the marble's location. Children younger than four or five (the exact age varies) give the wrong answer: they point to the box, where the marble really is.

This understanding of false beliefs opens the gates to a full comprehension of other people. It is a skill understood the world over. It can be demonstrated among preliterate people living an ancient hunter-gatherer lifestyle, such as the Baka pygmies of the rainforests of south-eastern Cameroon. Dr Jeremy Avis and Dr Paul Harris from Oxford University have demonstrated that an ability to write is not necessary to an understanding of beliefs and desires, by performing a version of the Sally-Ann test involving mangoes in cooking pots. Again, only children over the age of five could get these questions right. "The fact that belief-desire reasoning emerges at approximately the same age in such diverse settings strengthens the claim that this mode of reasoning is a universal feature of normal human development," says Dr Avis.

Although the ability to understand a false belief happens relatively suddenly for a child, there is a definite developmental progression leading up to it. It starts with one-year-old infants, who begin to follow the direction of another person's gaze when they look at objects. About six months later, they look where someone is pointing, rather than at their finger. This is the first step towards understanding that what their parents are looking at is what they are mentally paying attention to.

Later, children develop a growing awareness that seeing leads to knowing - in other words, if you are looking in a cupboard, you know far more about its contents than someone who is standing next to it but not looking inside. Between three and four, children start to understand that other people have desires and wishes, until finally they comprehend false beliefs.

The exact age at which a child can perform a theory-of-mind task is determined by how many siblings there are in the family, and how extensive the child's vocabulary is: the larger the family and the more words a child knows, the earlier the age at which they can pass the test.

Until two recent studies no one could say which area of the brain was used in this process. In one experiment, a team led by Dr Paul Fletcher and Professor Chris Frith of the Wellcome Department of Cognitive Neurology in London gave brain scans to volunteers who were listening to stories that either required a physical understanding of the word (understanding, for example, that if you knock a person, they may fall) and ones that needed a mental understanding, such as the Sally-Ann task. They used the PET scan, in which a subject is given a dose of radioactive oxygen in water. When a particular part of the brain is especially active, it uses more oxygen. The radioactivity shows up in the scan, which can be coloured to show levels of activity. Both types of stories showed increased brain activity in the temporal lobes, the superior temporal lobe and the posterior cingulate cortex. But only the theory of mind tasks activated an area at the front of the brain known as Brodmann's 8, on the frontal lobe.

The other study, conducted slightly earlier than Fletcher and Frith's, also involved giving PET scans to students while they listened to a theory- of-mind task. Dr Vinod Goel, Dr Jordan Grafman and colleagues at Bethesda, in the US, asked students how Christopher Columbus might have categorised the function of artefacts he discovered on his travels; they had to assume what kind of knowledge a European in the 15th century might have - a rather bizarre version of the Sally-Ann task. All the subjects also used the same part of the frontal lobe.

Brodmann's 8 has widespread connections to the rest of the brain. Professor Frith believes that the part of the brain associated with theory of mind may be needed to integrate information and stimuli drawn from other parts of the brain.

The work is highly important, not only in furthering our own understanding of the mind, but for people with autism. Sufferers of the disorder do not have a theory of mind, so are unable to deal with people socially or communicate effectively. Finding out whether this area of the brain is damaged in people with autism could help us to understand and treat autism, which at present is an incurable disorder.

Sanjida O'Connell's first novel, 'Theory of Mind', is published this month by Black Swan, pounds 6.99.