Scientists at the University of Wisconsin have discovered that a region of the brain is smaller in autistic boys. The differences could explain why autistic people have trouble developing social skills and “reading” other people’s faces for emotions.

A research team led by Dr. Richard Davidson showed 54 boys pictures of human faces. The 28 autistic boys in the study had trouble deciding whether a face was angry, sad, happy or “neutral.” They also spent less time looking at the eyes in the photos than the others in the study. The time they spent looking at the eyes was directly correlated with the size of their amygdala, a small region in the brain.

The amygdala is the size of an almond and known as the brain’s “fear center.” Its function seems to be to organize memories around emotions.

The University of Wisconsin scientists, writing in the December 2006 Archives of General Psychiatry, postulated that autistic children might be so afraid of people that they “stress out” their amygdalae, which shrinks and atrophies from overuse. Looking in other people’s eyes is a negative experience for them and may create stress for their amygdalae. Dr. Davidson and others believe that brain differences such as smaller amygdalae can explain over half of autistic behaviors.

An unrelated study by French researchers, published in Nature Genetics found changes in a gene called SHANK3 might determine whether some people develop autism.

Scientists are increasingly looking for genetic and physical causes of autism. Many of them, including Paul Shattuck of the University of Wisconsin’s Waisman Center, question whether autism is actually increasing at epidemic levels in the United States. True autism may be rarer than we think.

Shattuck says his research indicates that many children diagnosed as autistic today would have been in the mental retardation or learning disabilities categories ten years ago. The problem is that diagnostic practices have changed and are different in different regions of the country. Shattuck notes that the more autism increases, the more mental retardation decreases.