Brief report: Asperger's syndrome and sibling birth order.

Prior investigations suggest that birth order position may be associated with the risk for developing a pervasive developmental disorder. This retrospective chart review examined the birth order status of 29 psychiatrically-referred patients with Asperger's Syndrome (AS). Eighty-six percent of the subjects were first born. The finding was statistically significant when compared to an expected random distribution of AS subjects χ(2) (1, N = 29) = 9.18, p < 0.01. The reasons for such an association are unclear though birth stoppage, obstetric complications, and immunological mechanisms may play a role.