Smash hit

The children who had high foetal testosterone also did better on a test in which they had to pick out a pattern from a background image. It is well established that autistic children are particularly good at this task.

In total, about 20 per cent of the variation between children’s autistic traits appeared to match foetal testosterone levels, with the remainder likely caused by genetic and environmental factors.

Professor Baron-Cohen cautioned that the results do not prove that the link between male hormones and autistic traits is causal – both could be the result of something else – and that his team has not yet examined autistic children, only autistic traits in the normal position.

Even so, he said the work adds strong biological evidence to his extreme male brain theory. “The extreme male brain hypothesis was developed at a psychological level, based on observed sex differences,” he said. “Simply put, girls tend to show better empathy and boys tend to have a stronger interest in systems. Children with autism seem to have an exaggerated version of typical male preferences. They have a strong interest in systems and have difficulty empathising.

“We are now moving from a psychological level down to a biological level, finding that elevated levels of testosterone may contribute to the number of autistic traits that you have. It is an exciting development.

“What we knew before was that foetal testosterone affected brain development, but we only knew that from animal research. This study has looked at that relationship in humans. We knew before that it correlated with social development at early points in childhood, but we had not looked at early autistic traits.”

Professor Baron-Cohen has now won funding from the Medical Research Council to conduct a similar study using two large databases in Denmark, which will allow him to determine whether foetal testosterone levels are raised among children in whom autism is later diagnosed.

Denmark has an archive of 90,000 amniocentesis samples and a national register of people diagnosed with psychological or developmental disorders such as autism. “We are going to put these two datasets together, to look at who has turned out to have a formal diagnosis of autism and then pick out their amniocentesis sample and look at testosterone.

“The children we have looked at so far do not have autism: they are developmentally normal and all we have looked at are autistic traits. Now we are going to get the numbers to look at autism itself.”

It has not been possible to assess autistic children yet because amniocentesis, in which a needle is inserted into the womb to collect amniotic fluid surrounding the foetus, carries small risk of miscarriage. It is thus ethical to test for foetal testosterone only when a mother is having amniocentesis for another reason. This has limited the number of children who could be included in the research, leaving too few with autism to produce significant results.

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