Does family birth order predict homosexual orientation?
If you read the popular press, you might think so. A recent report from Canadian psychologist, Anthony Bogaert, in the Proceedings of the National Academy of Sciences suggests a link for males, but not females, between adult sexual orientation and the number of older biological brothers. In other words, the more older biological brothers one has the more likely one is to be gay. Dr. Bogaert speculates a pre-natal cause for this observation, in part because the brothers had to be biologically related. Growing up with adopted or step-brothers did not predict sexual orientation. If the effect proves valid, then some gay little brothers might owe their sexuality in some way to the blood brothers who came before them. However, the potential causal mechanism(s) for the fraternal birth order effect (FBOE) is unknown. Dr. Bogaert offers a theory he dubbed the maternal immune response. He speculates that when pregnant with a male, a woman’s body may identify proteins in male cells as foreign. W
