The average age at which children are diagnosed with autism is between three and four, but scientists have long suspected that the disorder starts much earlier. A key piece of evidence is a phenomenon known as brain overgrowth. Autistic toddlers tend to have large brains for their age, and researchers have shown a correlation between the degree of excess growth and the severity of autism symptoms. Eric Courchesne, director of the Autism Center of Excellence at the University of California, San Diego, helped to pioneer the overgrowth hypothesis. Now he and his colleague Cynthia Schumann have published data that suggest the excess brain growth starts in the first year of life, if not sooner.

The study, published in a recent issue of the Journal of Neuroscience, is the first to evaluate brain growth and autism throughout early development. Using cross-sectional MRI scans, the U.C.S.D. researchers found overgrowth in autistic subjects as young as one and a half. At two and a half, the autistic subjects’ brains were 7 percent larger on average than the control group’s. Al­though why, exactly, excessive brain growth is related to autism remains a mystery, the new work helps to confirm that signs of the disorder appear early—knowledge that could lead to detection and treatments, such as behavior therapy, at a younger age. “The earlier the intervention, the better the outcome,” Courchesne says.