OBJECTIVE: Familial recurrence risk is an important population-level measure of the combined genetic and shared familial liability of autism spectrum disorder (ASD). Objectives were to estimate ASD recurrence risk among siblings and cousins by varying degree of relatedness and by sex.

METHOD: Population-based cohort study of 1998-2007 livebirths from California, Denmark, Finland, Israel, Sweden and Western Australia followed through 2011-2015. Subjects were monitored for an ASD diagnosis in their older siblings or cousins (exposure) and for their own ASD diagnosis (outcome). The relative recurrence risk was estimated for different sibling- and cousin-pairs, for each site separately and combined, and by sex.

RESULTS: During follow-up, 29,998 cases of ASD were observed among the 2,551,918 births used to estimate recurrence in ASD and 33,769 cases of childhood autism (CA) were observed among the 6,110,942 births used to estimate CA recurrence. Compared to the risk in unaffected families, we observed an 8.4-fold increase in the risk of ASD following an older sibling with ASD and an 17.4-fold increase in the risk of CA following an older sibling with CA. A 2-fold increase in the risk for cousin recurrence was observed for both disorders. We also found a significant difference in sibling ASD recurrence risk by sex.

CONCLUSION: Our estimates of relative recurrence risks for ASD and CA will assist clinicians and families in understanding autism risk in the context of other families in their population. The observed variation by sex underlines the need to deepen our understanding of factors influencing ASD familial risk.