12.5 Proximate causes: Is sexual orientation genetic?

Proximate vs. Ultimate explanations: How vs. Why

Biologists can search for explanations for adaptations at two levels: Proximate and Ultimate. A proximate explanation is HOW a certain trait came to be within an individual. For example, a frog may be green because it has several gene versions that cause the production of green pigments in its skin cells. An ultimate explanation is WHY a trait evolved. A population of frogs may be green because they live in a green and leafy environment, and their green color makes them camouflaged and protected from predators.

Another way to think of this is that proximate explanations are mechanistic. Ultimate causes are evolutionary.

We’ll first discuss potential proximate causes of homosexuality when asking how people become (or simply are) homosexual. Then, we’ll turn to ultimate causes when asking why is homosexuality relatively common.

A possible proximate cause: Genetics

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.

But, for now, let’s focus on the biology of homosexuality’s origins. While no serious scientist claims that same-sex mating preferences arise because of a single “gay gene,” many have found evidence of a possible genetic basis.

Some intriguing data are from studies of twins. For example, in a 1993 study, researchers discovered that identical twins (who arise from the same sperm and egg, and have nearly 100% identical DNA) are more alike with respect to sexual orientation than are non-identical twins (who arise from different eggs and sperm and on average share 50% of their DNA). However, identical twins don’t overlap completely in sexual preferences, a finding that suggests other factors—besides genetics—may be at work. As in many areas of biology, the answer to our question is not simple.