The Familial Risk of Autism

Autism spectrum disorder (ASD) aggregates in families, but the individual risk and to what extent this is caused by genetic factors or shared or nonshared environmental factors remains unresolved.

To provide estimates of familial aggregation and heritability of ASD. DESIGN, SETTING, AND PARTICIPANTS: A population-based cohort including 2,049,973 Swedish children born 1982 through 2006. We identified 37,570 twin pairs, 2,642,064 full sibling pairs, 432,281 maternal and 445,531 paternal half sibling pairs, and 5,799,875 cousin pairs. Diagnoses of ASD to December 31, 2009 were ascertained. MAIN OUTCOMES AND MEASURES: The relative recurrence risk (RRR) measures familial aggregation of disease. The RRR is the relative risk of autism in a participant with a sibling or cousin who has the diagnosis (exposed) compared with the risk in a participant with no diagnosed family member (unexposed). We calculated RRR for both ASD and autistic disorder adjusting for age, birth year, sex, parental psychiatric history, and parental age. We estimated how much of the probability of developing ASD can be related to genetic (additive and dominant) and environmental (shared and nonshared) factors.