The heritability of autism is the proportion of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be primarily genetic. Autism has a strong genetic basis, although the genetics of autism is complex and it is unclear whether autism spectrum disorder (ASD) is explained more by multigene interactions or by rare mutations with major effects. Early studies of twins estimated the heritability of autism to be more than 90%; in other words, that 90% of the differences between autistic and non-autistic individuals is due to genetic effects...