The largest and most rigorous twin study of its kind to date has found that shared environment influences susceptibility to autism more than previously thought.

The study, supported by the National Institutes of Health, found that shared environmental factors — experiences and exposures common to both twin individuals — accounted for 55% of strict autism and 58% of more broadly defined autism spectrum disorders (ASD). Genetic heritability accounted for 37% of autism and 38% of ASD. Random environmental factors not shared among twins play a much smaller role.