12.5 Is sexual orientation genetic?

Asking this question is a bit like asking, “are we born gay? Or straight?” This question can be problematic for some, because the motivation for asking the question may not be scientific. For example, individuals who have a social problem with homosexuals may be motivated to see sexual orientation as a choice, making homosexuality a characteristic one could choose not to exhibit. And in recent history, eugenicists (individuals who promote selective reproduction among “favored” types of humans) used a presumed genetic basis for homosexuality as an argument in favor of sterilizing gay people. The question can also be problematic because the stated or implied focus is typically on the cause of homosexuality, rather than heterosexuality. (We’ll say more about that in a bit.)

But, for now, let’s focus on the biology of homosexuality’s origins. While no serious scientist is claiming that same-sex mating preferences arise in a simple Mendelian fashion, or that there is a single “gay gene,” many have found evidence of a possible genetic basis. Some intriguing data are from the literature on twins. For example, X and Y discovered that identical twins (who arise from the same sperm and egg, and have nearly 100% identical genetics) are more alike with respect to sexual orientation than are non-identical twins (who arise from different eggs and sperm). However, identical twins don’t overlap completely in sexual preferences, a finding that suggests other factors—besides genetics—may be at work.