Research over the years has established several facts. The fraternal birth order effect is a phenomenon that can be described in one of two ways: Shel March 11, at 1: Oh good grief, do you tell people your shin hurts, or your tibia? Instead, researchers found a correlation between such unions and having an excess of older sisters. If a prenatal factor underlies the fraternal birth order effect, then a postnatal factor such as rearing time with older siblings be they biological or non-biological should have no impact on the sexual orientation of younger male siblings. Which brings up the interesting question:
The fraternal birth order effect has been confirmed to interact with handedness  as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males.
Study confirms link between the number of older brothers and increased odds of being homosexual
Blanchard and Ellis studied adult, homosexual and heterosexual, men and women the probands whose mothers knew the sex of every child or fetus that they were pregnant with prior to the proband. Proceedings of the National Academy of Sciences. In a few studies, homosexual subjects have occasionally displayed both a larger number of older brothers and a larger number of older sisters in comparison to heterosexual men. You can unsubscribe at any time and we'll never share your details to third parties. While there were defiantly women warriors, and many women today participate in combat and war, the evolution of males is set to be the physical provider. A conceptual analysis and quantitative review". In animals the behavior is pretty common.
Madzast December 10, at After all, being gay or bi does not reduce the function of ones anatomy or ability to reproduce. The cross-cultural consistency with which these effects have been documented is consistent with the conclusion that culturally invariant, biological processes underlie the development of homosexuality in males. Your feedback will go directly to Science X editors. J Sex Marital Ther.