Mice carrying an autism-associated mutation show impaired social interactions and dramatic changes in brain size when their immune systems are activated, according to research presented yesterday at a poster session at the Society for Neuroscience meeting in Chicago.

As many as one in every three people with autism develop a macrocephalus, or extremely enlarged head, at some point in their lives, an observation largely accepted as fact. But how or why this happens ― and whether it happens consistently enough to be useful in diagnosing autism ― remains contentious.

Gray matter, that mysterious brain substance, is thought to control everything from motor function to mental acuity. In recent years several studies have suggested that an excess of gray matter during childhood is to blame for the symptoms of autism.