A clinically extensive and mathematically powerful study of 1000 families with one autistic child and one unaffected sibling has validated a controversial theory of autism’s complex genetic causation. The study for the first time estimates the minimum number of locations in the human genome — 250 to 300 — where gene copy number variation (CNV) can give rise to autism spectrum disorder (ASD). It also sheds new light on the long observed but little understood “gender bias” of autism