Autistic men have striking abnormalities in a region of the brain that deals with social skills, according to research published today.

Detailed maps of autistic men's brains show they have substantially fewer neurons than expected in a region called the amygdala, which plays a major role in understanding others' actions and emotions.

The finding adds weight to a theory put forward by some scientists that stunted development in the amygdala gives rise to autism. Further research is needed, however, to confirm whether the lack of neurons is a direct cause of autism, or is merely a consequence of it.

Scientists at the University of California, Davis studied the brains of 19 dead men, nine of whom had autism. Using a technique called stereological analysis, they were able to count the numbers of neurons in different parts of the men's brains and compare them.

The researchers found abnormally low numbers of brain cells in the almond-shaped amygdala and a structure known as its lateral nucleus.

"This is the first quantitative evidence of an abnormal number of neurons in the autistic amygdala," said David Amaral, who led the research, which appears in the Journal of Neuroscience.

Simon Baron-Cohen, director of the Autism Research Centre at Cambridge University, said the findings supported the theory that changes in the amygdala were associated with autism.

"The amygdala is part of a network called the social brain and it is involved in making sense of other people's actions and interpreting other people's expressions of emotion. It's a possibility that these abnormalities in the amygdala in autism causally relate to their social difficulties," he said.

"The caveat is that it's possible these abnormalities may be a consequence of lack of social experience, so we need more research to test whether this is cause or consequence," he added.

A recent survey found that autism and related disorders affect as many as one in a hundred British children. Although research published in 1998 suggesting a link between autism and the triple MMR (measles, mumps and rubella) vaccine has since been discredited, scientists remain unclear what causes the condition.

Scientists are working on new techniques that will allow them to analyse the brains of children at much younger ages, which could clarify when and how changes in the amygdala occur. Researchers are keen to investigate the effects of hormones such as testosterone which is believed to be capable of altering the development of the amygdala in babies in the womb.

"While we have known that autism is a developmental disorder, where, how and when the autistic brain develops abnormally has been a mystery," said Thomas Insel, director of the US national institute of mental health.

"This new finding is important because it demonstrates that the structure of the amygdala is abnormal in autism. Along with other findings on the abnormal function of the amygdala, research is beginning to narrow the search for the brain basis of autism."