Abnormal activity in neurons that help individuals imitate others may underlie some of the social deficits found in autism, US researchers believe.
The researchers used functional MRI scans to look at the brain
A Nature Neuroscience study found autistic children had less brain activation in an area involved in understanding others' state of mind.
The degree of activation of the 'mirror neurons' housed in this area correlated with measures of social impairment.
The lower the activation, the stronger the impairment the children had.
Autism affects a person's ability to communicate with others and to respond appropriately to environmental cues.
In animals, mirror neurons have been shown to fire both when the animal observes another performing an act and when they perform the same act themselves.
Dr Mirella Dapretto and colleagues studied the brain activity patterns of 10 children with autism as the children either imitated facial gestures or passively watched facial gestures.
The facial gestures reflected emotions including fear, anger, sadness and happiness.
The researchers compared these outcomes with those of 10 children of the same age and IQ but who did not have autism.
Although the autistic children were able to perform the task, they had lower activation in a brain area containing mirror neurons - the inferior frontal gyrus pars opercularis - both when watching and imitating facial gestures, compared to the other children.
The children with autism also had reduced activity in emotion centres of the brain - the insula and the amygdala.
Dr Dapretto, of University California, Los Angeles, said: "Our findings suggest that a dysfunctional mirror neuron system may underlie the social deficits observed in autism.
"Together with other recent data, our results provide strong support for a mirror neuron theory of autism. This is exciting because we finally have an account that can explain all core symptoms of this disorder."
The researchers believe that children with autism must use other parts of the brain in order to be able to perform the task that they tested.
For example, the autistic children might pay more attention to visual and motor clues without experiencing the internally felt emotional significance of the imitated facial expression.
Professor Michael Rutter from the Institute of Psychiatry, London, said: "The mirror neurons are interesting and there is some good research in animals looking at how they function.
"The general notion of linking mirror neurons with the social deficit in autism is quite reasonable.
"But we need more research into the brain systems that might be involved. These might involve mirror neurons, but we need more studies."
The National Autistic Society said: "There is strong evidence to suggest that autism can be caused by a variety of physical factors, all of which affect brain development - it is not due to emotional deprivation or the way a person has been brought up.
"We welcome research into all areas which may throw light on the causes behind autism."
Dr Bernard Rimland, of the Autism Research Institute, said it would be wrong to think the abnormalities were necessarily hard-wired.
He said research had shown that dietary intervention, for instance, could have an effect on symptoms.