A part of the brain associated with emotional learning and fear shrinks in people with autism, research suggests.
Autism impairs social interaction, communication, and imagination
Teenagers and young men with autism in the study who had the most severe social impairment were found to have smaller than normal amygdalae.
The researchers from the University of Wisconsin suggested the amygdalae may shrink due to chronic stress caused by social fear in childhood.
The study was published in the Archives of General Psychiatry.
Of 54 male participants aged eight to 25 years who took part in the study, 23 had autism and five had Asperger syndrome.
The size of the amygdalae, two almond-shaped groups of neurons located deep within the brain, was measured by MRI scans.
Individuals were also asked to complete tasks associated with social interaction such as eye tracking and recognising emotional facial expressions.
Men with autism who had small amygdalae were slowest to distinguish emotional from neutral expressions and showed the least fixation of eye regions.
The same individuals were the most socially impaired in early childhood.
The researchers also found a link with age suggesting that amygdala volume decreases from childhood into early adulthood in autistic people with the most severe social impairment.
Study leader Dr Richard Davidson said the findings pointed towards a model of autism where the brain first reacts to stress brought on by fear of people by becoming hyperactive, which eventually leads to cell death and shrinkage.
Children with autism who have the least difficulty with social interaction would have slower amygdala shrinkage than those who struggled the most.
He said the findings could account for more than half the differences in social impairment in people with autism.
An earlier study published by Dr Davidson reported that unaffected siblings of people with autism share some of the same differences in amygdala volume and the way they look at faces and process information about emotions.
He said: "Together, these results provide the first evidence linking objective measures of social impairment and amygdala structure and related brain function in autism.
"Finding many of the same differences, albeit more moderate, in well siblings helps to confirm that autism is likely the most severe expression of a broad spectrum of genetically-influenced characteristics."
Professor Simon Baron-Cohen, director of the Autism Research Centre in Cambridge, said: "This paper provides important evidence that size of the amygdala is associated with autism severity and social skill.
"We now need to discover if this neural difference is observed at the earliest point in development, and what causes this atypical development."