A reader recently asked me to post about the evolution of homosexuality. Given that homosexuality can be a socially contentious issue, it seems reasonable to review what objective scientific research says about it. So this week I thought I would present a well-known study on the topic, “The Evolution of Human Homosexual Behavior” by anthropologist R. C. Kirkpatrick. Instead of presenting a complete review of the paper, I’ll summarize and quote from those parts that help answer three common questions: Is homosexuality natural?; What causes homosexuality?; and Why did homosexuality evolve?
Is homosexuality natural?
A common assertion about homosexuality is that since it cannot result in children, it is unnatural. However, if “unnatural” means “not found in nature”, this assertion is certainly wrong. Kirkpatric provides a review of homosexual behavior from contemporary and historical human societies, both developed and primitive, and finds that homosexual behavior is in fact common. For example, 3 – 30% of individuals in contemporary human societies report having participated in some form of homosexual behavior (defined as genital contact or manipulation by pubescent adolescents and adults, including bisexuality and excluding pedophilia; see the table below). In addition to humans, homosexual behavior is also found in many mammals, including the great apes, monkeys, dolphins, and lions. Or in Kirkpatrick’s words, “Homosexual behavior is too widespread to be a fluke or an aberration.” The frequency of homosexual behavior among humans and other mammals demonstrates that it is not unnatural.
What causes homosexuality?
A second common assertion is that since homosexuality is unnatural, it must therefore be a choice. Numerous scientists and anthropologists have tried to determine the causes of homosexuality by correlating behavioral and environmental factors to sexual orientation. So far, three main factors have been linked to homosexual behavior in humans: genes, hormones, and the environment.
- For genes, Kirkpatrick reviews studies of twins, which biologists often use to help disentangle the influence of genes and environment. These studies find that the brother of a homosexual male is more likely to be homosexual if the two are a set of monozygotic twins (which are genetically identical) rather than dizygotic twins (which are genetically different). This difference suggests that there is indeed some genetic component to human homosexual behavior.
- For hormones, Kirkpatrick reviews studies that find no inherent difference in the types of hormone receptors that homo- and heterosexual individuals have; however, he concludes that the evidence for hormonal influence in homosexual behavior is generally inconclusive. But to this I’ll add that more recent research has shown that hormones can play an important role in determining sex and sexual preference, at least as far as frogs are concerned.
- Finally, for the environment, Kirkpatrick reviews evidence from several different types of studies. Perhaps the most interesting are those on birth-order: in general, men with more older brothers are more commonly homosexual than men with fewer older brothers. Scientists posit that prenatal hormones cause this effect, as the immune response of the mother to male infants gets stronger with each successive birth, more effectively “feminizing” the later infants.
However, at the end of this review Kirkpatrick concludes that “what is most striking about correlates of homosexual behavior is the small amount of variation explained by any single factor. Genes, hormones, childhood experience, and adult experience are multivariate and interact to produce multivariate life histories . . . [and] sexual behavior and emotion are continuous variables, not dichotomous.” But the consensus remains: there are many factors that determine homosexual behavior, including several which cannot be controlled by the individual (genes and the early environment of the fetus). These results suggests that many aspects of homosexuality are innate rather than a choice.
Why did homosexuality evolve?
Taken together, the studies presented above suggest that homosexual behavior has evolved. But given that there is an obvious evolutionary downside to homosexuality — the inability to reproduce and pass on genes to the next generation — an important question arises: Why did homosexuality evolve? Kirkpatrick lists three main hypotheses regarding the evolution of homosexual behavior. I lump these into two. Below, I introduce both hypothesis and summarize Kirkpatrick’s analysis of each.
- Inclusive fitness: The hypothesis of inclusive fitness posits that the genes for homosexuality are maintained through indirect rather than direct reproduction. Homosexuals may be unable to reproduce, but their siblings and relatives can, and these siblings and relatives carry many of the same genes as the homosexual individual. This hypothesis assumes that if the relatives of homosexuals produce more offspring than average (presumably because homosexuals help raise these children), then the alleles which cause homosexuality can be maintained and even increase in future generations. Although this is an appealing hypothesis, unfortunately there is just no evidence that families with homosexual members produce more children than families without them. Therefore, Kirkpatrick doesn’t think inclusive fitness is a likely explanation for the evolution of homosexual behavior.
- Alliance formation: The hypothesis of alliance formation posits that homosexual behavior is not the result of a reproductive strategy to increase fitness, but instead is a part of a survival strategy. The basis for this hypothesis is that sex has many functions, of which reproduction is only one. An additional function of sex is to promote emotional and material bonds between individuals and groups, which may have been especially important during the early hunter/gather period of human evolution. This hypothesis states that by linking both same-sex couples for survival and resource sharing, and opposite-sex couples for reproduction, homosexual behavior can increase fitness better than mating strategies based on one sex alone. An important prediction of this hypothesis is that bisexuality should be more common than strict homosexuality, which is true. He also finds evidence that humans that participate in homosexual behavior also produce children, and that contemporary British women that have participated in homosexual behavior still produce the same number of children as those who report no homosexual behavior. Kirkpatrick concludes that the alliance formation hypothesis is a powerful explanation for the maintenance of homosexual behavior in humans.
I hope this review helps dispel some of the more common myths about homosexuality. Scientific research suggests that homosexual behavior is common in humans and nature; that is it the result of numerous biological, environmental, and sociological factors; and that it may have evolved as strategy to increase survival through the formation of same-sex alliances.
Kirkpatrick, R. (2000). The Evolution of Human Homosexual Behavior Current Anthropology, 41 (3), 385-413 DOI: 10.1086/300145