For each additional brother that precedes him, a boy's chance of growing up to be gay increases by a third. A fascinating article and commentary in the current issue of Proc. Nat. Acad. Sci. now make the point "that the social influence of an older brother is irrelevant to whether his younger brother will develop a homosexual orientation. It is the number of older biological brothers the mother carried, not the presence of older brothers while growing up, that makes some boys grow up to be gay. Older stepbrothers in the home have no effect, although older biological brothers raised apart still exert their influence. These data, by elimination, strengthen the notion that the common denominator between biological brothers, the mother, provides a prenatal environment that fosters homosexuality in her younger sons."
One explanation that has been proposed is a maternal immunization hypothesis. "When a mother is carrying her first son, the placental barrier protects each from exposure to the other’s proteins. But inevitable mixing of blood upon delivery will expose the mother for the first time to male-specific proteins..., including those encoded on the Y chromosome. If her immune system produces antibodies to these proteins, then the placenta may actively transport those antibodies (indeed, all IgGs) to subsequent offspring in utero, potentially affecting development of later-born sons, but not later-born daughters."