This study, conducted by a team from Stanford University School of Medicine and other institutions, aimed to provide rigorous quantitative estimates of the genetic heritability of autism and the effects of shared environmental factors among twin pairs. The study identified 192 twin pairs with at least one twin diagnosed with an autism spectrum disorder (ASD) between 1987 and 2004 through the California Department of Developmental Services. Structured diagnostic assessments, including the Autism Diagnostic Interview–Revised and the Autism Diagnostic Observation Schedule, were used to evaluate the twins. The results showed that for strict autism, probandwise concordance rates were higher in male monozygotic pairs (58%) than in female monozygotic pairs (60%), and higher in male dizygotic pairs (21%) than in female dizygotic pairs (27%). For ASD, the probandwise concordance rates were even higher, with 77% in male monozygotic pairs and 50% in female monozygotic pairs. The study also found that a significant portion of the variance in liability to autism can be attributed to shared environmental factors (55% for autism and 58% for ASD), in addition to moderate genetic heritability (37% for autism and 38% for ASD). The findings suggest that susceptibility to ASD has moderate genetic heritability and a substantial shared twin environmental component.This study, conducted by a team from Stanford University School of Medicine and other institutions, aimed to provide rigorous quantitative estimates of the genetic heritability of autism and the effects of shared environmental factors among twin pairs. The study identified 192 twin pairs with at least one twin diagnosed with an autism spectrum disorder (ASD) between 1987 and 2004 through the California Department of Developmental Services. Structured diagnostic assessments, including the Autism Diagnostic Interview–Revised and the Autism Diagnostic Observation Schedule, were used to evaluate the twins. The results showed that for strict autism, probandwise concordance rates were higher in male monozygotic pairs (58%) than in female monozygotic pairs (60%), and higher in male dizygotic pairs (21%) than in female dizygotic pairs (27%). For ASD, the probandwise concordance rates were even higher, with 77% in male monozygotic pairs and 50% in female monozygotic pairs. The study also found that a significant portion of the variance in liability to autism can be attributed to shared environmental factors (55% for autism and 58% for ASD), in addition to moderate genetic heritability (37% for autism and 38% for ASD). The findings suggest that susceptibility to ASD has moderate genetic heritability and a substantial shared twin environmental component.