A pair of twins is born, and both infants begin to develop normally. By their first birthday, however, the male sibling has begun to diverge from his sister, showing less eye contact and affection. He often wears a spaced-out expression and fixates on certain puzzles and patterns. By age three, his mounting symptoms lead to a diagnosis that has become disturbingly routine in recent years: autism.

What causes the disease, which now strikes 1 in every 166 children, and why does it affect four times as many boys as girls?