Here is another interesting new paper in PNAS. The abstract:
The most consistent biodemographic correlate of sexual orientation in men is the number of older brothers (fraternal birth order). The mechanism underlying this effect remains unknown. In this article, I provide a direct test pitting prenatal against postnatal (e.g., social/rearing) mechanisms. Four samples of homosexual and heterosexual men (total n = 944), including one sample of men raised in nonbiological and blended families (e.g., raised with half- or step-siblings or as adoptees) were studied. Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.
The same issue has a commentary that reviews the research in this area. The authors propose a mechanism summarized in their Figure 1:
A mother carrying a first son has very little exposure to the proteins he is making because of the placental barrier. But upon delivery and the inevitable mixing of fetal and maternal blood, her immune system will now see proteins it has never seen before, including proteins encoded on her son’s Y chromosome. If she mounts an immune response to these proteins, then any subsequent sons will be exposed, via active transport across the placenta, to maternal antibodies directed against the male-specific proteins. These maternal antibodies might then perturb development of the younger son, decreasing birth weight and affecting his brain to increase the probability that he will grow up to be gay. Whether this is what is really happening for sexual orientation remains to be seen, but it is a provocative hypothesis.
I leave it to others to evaluate this specific result and the broader hypothesis in mechanistic and evolutionary terms. (One thought of mine: a potential mechanism more generally for greater male variability in all traits? An easily testable hypothesis.) As I read them, these papers are not in conflict with Greg Cochran’s germ theory of homosexuality.
Coffee Mug Addendum: There is a colorfully written commentary in Endocrinology for free that sums up some of the research into neurobiological and endocrinological mechanisms determining sexual preference. Sheep seem to be a good model system for this issue, and the above papers imply some fairly explicit predictions about birth-order effects in homosexual sheep.