The year 1977 marked an important milestone in the history of autism. In this year, the first twin study in autism was published by Folstein and Rutter1; it demonstrated a striking difference in concordance rates between monozygous (MZ) and dyzgyous (DZ) twins. The studies that followed reported even higher MZ concordance rates, up to 90%, for a broader phenotype resembling what is currently labeled as autism spectrum disorder (ASD)2-4 and DZ concordance rates at or close to 0%. This resulted in heritability estimates greater than 90%, suggesting that almost all of the variance in phenotypic expression could be attributed to inherited genetic factors.