Some parts of the brain, like the frontal lobe and the amygdala, may develop too fast in early childhood, possibly because cells become overwrapped in an insulating layer that may facilitate growth, says neuroscientists David Amaral of the M.I.N.D. Institute at the University of California at Davis.

Exactly how these changes cause symptoms; social impairment, repetitive behaviors, poor language skills, is uncertain, although the frontal lobe is known to regulate behavior and the amygdala, anxiety.

Researchers believe that autism has both genetic and environmental causes.

Autism afflicts one in 150 kids by age eight. Diagnoses are on the rise, although whether because of better recognition or a growing frequency is unclear.