Autism has long been considered among the most heritable of developmental brain disorders. But a new study of autistic twins indicates that shared environment has a larger impact on the development of autism and autism spectrum disorders than previously thought, and is in fact more important than genetics.
Researchers from Stanford and UC San Francisco examined the prevalence of autism among both identical and non-identical twin pairs. The frequency of identical twins both being diagnosed with autism was lower than would be predicted from statistical models of heritability. The scientists estimate that environmental factors common to twins explains 55% of susceptibility to autism, whereas genetics accounts for only 37%.
Another study published in the same issue of Archives of General Psychiatry suggests that the use of antidepressants by mothers doubles the risk of a child developing autism. Other environmental factors that could be important in autism risk include diet, pollution, parental age, birthweight and maternal infections during pregnancy.