A blog about problems in the field of psychology and attempts to fix them.

Saturday, March 24, 2012

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:


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.)


  1. Eric, you said: "homosexuals do not reproduce." I realize you developed that line of thinking a bit later on, but for clarity's sake, it seems to me it should be asserted at the outset that "homosexuals do not reproduce with other homosexuals; for example, a homosexual man cannot mate with another homosexual man and create offspring through that mating."

    "What possible benefits could there be to raising children in a “homosexual” environment?" In terms of pure evolutionary theory, I don't know. However, from a social-learning perspective, this kind of environment could serve to be one in which tolerance is demonstrated and modeled for one's children.

    "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?"

    I'm not sure if my thinking's on this line, but when I read what you said, this is what came to mind. I have thought that a way to explain gay men and women in terms of selection pressures was to think of same-sex sexual behavior as a kind of population control. As populations grow and perhaps begin in themselves to place too great a demand on that niche's resources (thus making it harder for future generations to survive), the environment might select for homosexuality because two gay men cannot, together, reproduce. Thus they can pursue their carnality in a way that satisfies their own biological drives without placing extra population pressure on the niche.

    "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."

    Interestingly and just as perhaps an aside, women who engage in same-sex sexual behavior it seems are much less likely to be individuals who sexually only "do one thing." Whereas men who engage in same-sex sexual behavior are much more likely to only "do one thing."

    --Jim Jackson

    1. While your theory is intriguing... "the environment might select for homosexuality" argument pre-supposes that natural selection is a "thinking" mechanism. How would natural selection identify when a population was utilizing too much of the available resources? Remember that evolution normally happens over many, many generations and in that time the population could either die off or rectify the situation, either by lessening the population size or by moving to another location with more resources.
      Also, what would be the biological mechanism by which natural selection would "kick-start" the production of more homosexuals?

  2. Jim,
    Thanks for the comments so many posts! It will take me a bit to catch up.

    For your first point here, you are of course correct, but if everything was clear from the offset, there would be nothing to discuss ;- )

    Even though my bias is for option 3, all options are worth getting people to think about, and it is quite likely that multiple forces are at work. For the people I have worked with, being forced to think through perfectly good evolutionary explanations for exclusive homosexuality is often enough of a shocker, and so it is good to let them do that first. They then seem more receptive to the attack on the category itself.

    For the other questions, I had some notes with example potential answers, but I don't think my lists are exhaustive by any means. As for the last question, about why it might be better to be raised in a homosexual home: Surely if one dad is good for something, then two dads should be especially good for certain thing; if one mother is good for something, then two mothers should be especially good for certain things. We could probably make some long lists.

  3. Eric, question: Are you suggesting that if a dad is particularly good in terms of Parent Role/Activity X that a second dad added in would result in 2(Parent Role/Activity X)? That is, doubling their benefit at ONLY Parent Role/Activity X?

    If so, this seems to me suggesting that either dad in a "two dad" household raising children would not then take up another role. That is, Parent Role/Activity Y.

    If my reading into it is accurate, it seems to be suggesting a lack of flexibility on the part of one of the dads (and by extension to a "two mom" household, one of the moms).

    Authoritative parents are usually good at demonstrating flexibility required by the demands of raising children. Thus, my counter argument would be that as long as "both dads" and "both moms" are authoritative parents, the required flexibility will be demonstrated in their parenting.

    As I tell my students when we discuss same-sex parents: Two dads, two moms, one dad and one mom--what really works the best is two parents both parenting authoritatively.

    -Jim Jackson