Autism affects men more than women. Eight out of nine autistics and 14 out of 15 people with "high functioning" autism - known as Asperger's syndrome - are men. A third of people with autism suffer from mental retardation, but Asperger's is characterised by normal or high IQ. In both cases, people with either Asperger's or autism have problems understanding how other people think or feel. They lack the ability to put themselves in someone else's position, what psychologists describe as "theory of mind".
Most of us explain and predict other people's behaviour by guessing their mental state - their thoughts, desires and beliefs. As Professor Daniel Dennett, a philosopher from Tufts University in Massachusetts, explains: "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 theoretical diagnosis: `Aha!' we conclude (but perhaps not consciously), `He wants her to think he doesn't know she intends to defraud her brother!'"
A person with autism only sees the hero smile at the villain; moreover, he or she will also find it difficult to empathise with any of the characters. This kind of intelligence is the oil that lubricates social relations. Some psychologists now believe that the ability to predict other people's behaviour on the basis of what we think they are thinking - rather than an ability to use tools or language - was the driving force behind human evolution.
According to this theory, if our brains have increased in size and complexity, fuelled by our need to work out what our nearest and dearest might be thinking of doing, specific areas of the brain should be devoted to social cogitation. Professor Leslie Brothers, from the School of Medicine at the University of California at Los Angeles, suggests that three parts of the brain are crucial: the orbito-frontal cortex, the superior temporal gyrus, and the amygdala, a small walnut-sized region at the base of the brain.
Simon Baron-Cohen and Sally Wheelwright of Cambridge University, and a team of researchers from the Royal London School of Medicine and the University of London, took brain scans of both normal people and those with Asperger's syndrome, while they were solving a problem designed to test social intelligence. The subjects were given photographs of people's eyes and asked to guess what the person was thinking or feeling. Even with so little visual information most of us could score highly on this kind of task; those with Asperger's tend to get about half the questions right. The team's work seems to indicate that the frontal and temporal regions of the brain and the amygdala were used to calculate social intelligence. When the people with Asperger's answered the questions, they used some of the same brain areas, but not the amygdala. Previous studies have indicated that the amygdala's major role lies in processing emotions. "What we have also shown is that it is involved in inference of a broader range of mental states from the face and especially the eyes," says Dr Baron-Cohen.
The scientists believe there is a genetic basis for autism; previous research suggested that if a child has autism, Asperger's and autistic- like traits will tend to run in the family. A survey of the fathers and grandfathers of people with autism also found that they were twice as likely to be engineers than the relatives of non-autistic people. The stereotype of scientists is still of predominately male, brilliant but impractical, cold, unemotional with limited communication skills - autistic- like traits, in other words.
Dr Baron-Cohen chose to look at engineering since this is a predominantly male occupation which requires a high level of understanding of physics, but does not rely on any kind of social intelligence. The team expanded its investigation by asking students at Cambridge University whether they had a relative who was autistic or had autistic-like traits.
The students were grouped into two categories: those who were reading humanities, literature and arts, and those who were scientists, training to be mathematicians, physicists and engineers. It was found that science students were six times more likely than humanities students to have autistic relatives.
The team delved further by giving highly intelligent eight- to 12-year -old children with Asperger's "folk physics" and "folk psychology" tests. Folk physics is the kind of science that can be understood without knowledge of physics. "They're the kind of thing that can be solved just from being alive. The prediction was that these children's knowledge of how machines work would be superior to their knowledge of how people work," Dr Baron- Cohen says. "The [folk physics] tests are not everyone's cup of tea. Actually I had quite a lot of problems with the tests myself," he says. He had less difficulty with the questions designed to test folk psychology - our innate ability to be socially intelligent. As predicted, the children performed poorly when they had to guess other people's thoughts or emotional states, but were better than the average 16-year-old at solving mechanical problems.
So could autism be linked to science? The best-selling novelist Nick Hornby is firmly in the artistic camp, but his semi-autobiographical novels deal with male characters who cannot express their emotions and have Asperger's- like traits, such as an obsessive attention to detail and a train-spotting mentality.
For example, Robert, the central character in Hornby's novel High Fidelity believes you cannot be a decent person without at least 500 records. Robert continuously makes lists of the top five singles of all time, top five Elvis Costello records, top five Monday morning hits. When he is asked to go to his girlfriend's father's funeral, he asks his mates for their best five pop songs on death. It is no coincidence, perhaps, that Hornby has an autistic son.
According to Dr Baron-Cohen it is not that autism is linked to being a scientist per se, but that the skills required for being a good physicist or engineer are the ones that we currently think of as typically male. Professor David Skuse, from the Institute of Child Health at University College London Medical School, is about to publish evidence suggesting where a gene or genes for autism might be found. He believes that these genes are carried on the X sex chromosome. Girls have two of these, one from each parent, whereas boys only have one, which they inherit from their mothers.
"The threshold hypothesis we are suggesting is that normal girls who carry their father's X chromosome also have a protective factor on that chromosome which helps to prevent them from developing autism," Professor Skuse says. "We believe it is an imprinted gene, which is switched off when inherited from one parent and switched on when inherited from the other. In this particular instance we are suggesting the gene is always switched on when it is transmitted by a father and always switched off when it is transmitted by a mother." This would explain why boys are more prone to autism than girls.
If Professor Skuse's hypothesis proves to be correct, the consequences are two-fold: first it means that autism or autistic-like traits must be far more common that most of us imagine. Second, since these genes can protect women from autism, but do not protect men, there might be some slight evolutionary advantage that is conferred if men have a mild form of autism and women do not.
Dr Baron-Cohen and Professor Skuse believe autism and Asperger's may be an extreme form of the male brain, a suite of behavioural attributes that are at one end of a spectrum of otherwise normal human responses. "A little bit of autism could be useful. For instance, if men are a bit less socially responsive, this would allow them to be more dominant." It could also help them to excel in the typically "male" professions of engineering and science, said Professor Skuse.
Sanjida O'Connell's novel, `Angel Bird', is published this month