Using brain cortical organoids (BOCs) and comprehensive social brain imaging, social eye tracking and social behavior testing, Courchesne and colleagues discovered that profound autism begins during embryogenesis. The greater the overgrowth of embryonic BCOs, the more severe the autism social symptoms at toddler ages. Toddlers who have profound autism, which is the most severe type of autism, have the most extreme BCO overgrowth during embryonic development.