Fraternal birth order

In psychology the fraternal birth order effect is the name given to the observation that the more older brothers a man has, the greater the probability is that he will have a homosexual sexual orientation. The fraternal birth order effect is the strongest known predictor of sexual orientation, each older brother increases a man's chances of being gay by about 33%. Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect of the number of older sisters.

Large studies involving adoptees demonstrate that the effect is not due to being raised with older brothers, but appears to have something to do with changes induced in the uterus when gestating a boy that affects subsequent sons. It is hypothesized that the fraternal birth order effect may be caused by increasing levels of antibodies produced by the mother to the HY antigen with each son. The HY antigen (histocompatibility Y-antigen) is found on the surface of the cells of male mammals. The presence of this foreign chemical when bearing a son could trigger the mother's immune response, which may then lead to different brain development patterns in later male children.