Evolution and Homosexuality

For a semester at Clark I was asked to organize the weekly 'recitations' for Introductory Psychology. The course itself was quite large, but the recitations were run by upper-level undergraduates, and contained 20-30 students. I transformed several weeks into lab activities. For others, I organized more formal discussion activities. When the main course was discussing evolutionary psychology, a student asked about how homosexuality could be explained from an evolutionary point of view. This student, and several others in class, seemed to see the existence (or at least the relative normality) of homosexuality as proof against the usefulness of evolutionary theory as an explanation of behavior. For that week, I prepared the following discussion paper for students:

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Evolution and Homosexuality

Evolutionary theorists could potentially explain homosexuality using three distinct methods. The first two take the modern notion of homosexuality at face value, the third questions it.

1.    Explain homosexuality as a benefit in and of itself.

The most straightforward way to explain the presence of any trait using evolutionary logic is to tell a story about how individuals with that trait reproduce their genes better than those without the trait. In the case of exclusive homosexuality, that is difficult, because homosexuals do not reproduce. However, it is still possible.

For example, a costly traits may be so helpful to your relatives (i.e., your kin) that it more than makes up for the cost you pay. This is called “kin selection”. Your children will share 50% of your genes, so we can give them a value of .5 in terms of your reproduction. A full sibling’s children share 25% of your genes, so we can give them a value of .25. That means that if you posses a trait that makes you have one less child on average (-.5), but you get three more nephews or nieces in exchange (+.75), natural selection will favor that trait (= .25). On average, the next generation will have more of your genes by virtue of your possessing a trait that makes you have fewer children. This explanation could be even more powerful when applied your own parents, i.e., helping raise your brothers and sisters, with whom you share as many genes as your own children (both .5).

If that was the explanation for human homosexuality, what might you also expect to be true of homosexuality?


2.    Explain homosexuality as a byproduct of other adaptive mechanisms.

There are many types of explanations compatible with evolutionary theory, but that do not explain the traits under questions as adaptations in and of themselves. In one way or another, these explanations explain traits as the byproduct of some other adaptive process. The trait in question could be a necessary byproduct of two evolutionarily sound items; for example, an armpit appears when you combine a torso with an arm, but no animal was ever selected specifically for having armpits! Alternatively, the trait in question could be the result of an adaptive mechanism placed in an unusual context; for example, evolution favored humans that desired sweet and fatty food in an environment where such things were rare; now that we are in an environment where such things are plentiful, this desire can cause serious health problems. Homosexuality could be explainable in terms of biological or psychological mechanisms acting appropriately in odd circumstances, or as a byproduct of selection for other beneficial traits.

If that explanation were correct, what types of traits might humans have been selected for that could result in homosexuality when pushed to the extreme or placed in unusual circumstances?

3.    Reject the notion of homosexuality as it is currently conceived and offer new categories.

Evolutionary thinking often necessitates a rejection of old categories and the creation of new ones. The current systems of dividing the world may not be relevant to answering evolutionary questions. The labels “Homosexual” and “Heterosexual” may be good examples. The modern notions of strict homo vs. hetero-sexuality arose relatively recently. It has never been bizarrely uncommon for women or men to live together or to set up long term relationships with members of the same sex. What is relatively new is the notion that this can divide people into types, some who exclusively do one thing and some who exclusively do another.  A so-called homosexual man need only have sex with a woman once to have a baby, and visa versa. While this is now the stuff of comedic amusement, it may be a much more natural context for homosexuality. There may be no reason to think that so-called homosexuals of the past got pregnant, or impregnated others, less often than so-called hetersexuals.

If this is the case, would there necessarily be any selection for or against preferring the relatively exclusive company of same-sex others? What possible benefits could there be to raising children in a “homosexual” environment? (Hey now, don’t bring moral judgment into this, it is only a question of surviving and thriving.)