Known as autism spectrum disorder (ASD), it is characterised by varying degrees of impairment in communication skills, social interaction and restricted, repetitive and stereotyped patterns of behaviour.
It had been thought that lack of social skills was due to abnormalities in particular brain areas. But University of London researchers compared the brain scans of 32 people and found that ASD is a problem in the part of the brain that processes faces, reported online edition of BBC News.
The face processing areas of people suffering from the disorder are not well connected to those parts of the brain that control attention, it said.
The researchers took the brain scans of 16 people with ASD and above-average IQs, as well as those of 16 unaffected volunteers. They were shown four images on the screen - two of houses and two of faces.
Scans showed there were marked differences in the brain activity of the two groups, the study appeared in the journal Neuroimage said.
In the unaffected group, paying attention to pictures of faces caused a significant increase in brain activity. But for people with ASD, paying attention to faces made no impact at all on the brain.
"We know that many people with ASD have particular difficulties in this area and we are hopeful that an improved understanding of these processes will enable people to receive appropriate and helpful support," said Richard Mills, director of research for the National Autism Society.
ASD can often be reliably detected by the age of three years, and in some cases as early as 18 months.