Evidence that homosexuality is biological — and not a lifestyle choice like some on the religious Right believe — is building; gay men are more likely than straight men to have counterclockwise hair wholrs, have longer index than ring fingers, and be left handed or ambidextrous. David France wrote in a 2007 New York magazine article:
Because many of these newly identified “gay” traits and characteristics are known to be influenced in utero, researchers think they may be narrowing in on when gayness is set — and identifying its possible triggers. They believe that homosexuality may be the result of some interaction between a pregnant mother and her fetus. Several hypothetical mechanisms have been identified, most pointing to an alteration in the flow of male hormones in the formation of boys and female hormones in the gestation of girls. What causes this? Nobody has any direct evidence one way or another, but a list of suspects includes germs, genes, maternal stress, and even allergy — maybe the mother mounts some immunological response to the fetal hormones.
Immunological response is the ascendant theory, in fact. We know from a string of surveys that in any family, the second-born son is 33 percent more likely than the first to be gay, and the third is 33 percent more likely than the second, and so on, as though there is some sort of “maternal memory,” similar to the way antibodies are memories of an infection. Perhaps she mounts a more effective immunological response to fetal hormones with each new male fetus. To determine whether the fraternal birth order might also suggest that baby brothers are treated differently in a way that impacts their sexual expression, researchers have studied boys who weren’t raised in their biological families, or who may have been firstborn but grew up as the youngest in Brady Bunch–type homes. In every permutation, the results were the same: What mattered was only how many boys had occupied your mother’s uterus before you.