Study leader Dr. Michael Spencer of the University of Cambridge's Autism Research Center says people with autism often struggle to read people's emotions.
The researchers studied 40 families who had a teenager with autism and a sibling without autism. A control group consisted of 40 teenagers with no family history of autism. All were given functional magnetic resonance imaging brain scans while viewing a series of photographs of faces which were either neutral or expressing an emotion such as happiness.
By comparing the brain's activity when viewing a happy vs. a neutral face, the scientists were able to observe areas within the brain that respond to this emotion.
The study, published in the journal Translational Psychiatry, found despite the fact that the siblings of those with autism did not have a diagnosis of autism, they had decreased activity in various areas of the brain -- including those associated with empathy, understanding others' emotions and processing information from faces -- compared to those with no family history of autism.
It's known that in a family in which one child has autism, the chances of a subsequent child developing autism are at least 20 times higher than in the general population, Spencer says.
The reason why two siblings can be so differently affected are key unresolved questions in the field of autism research, Spencer says.