12.9 How did homosexuality evolve?

Research on the evolution and occurrence of homosexuality has focused on several hypotheses, including some that are adaptive (fitness-enhancing) explanations and several that are non-adaptive explanations. Note that here we are discussing organisms beyond humans, so we use the term sex rather than gender.

Suggested adaptive explanations include (but are not limited to),

1) Social glue: according to the social glue hypothesis, same-sex sexual interactions help to form bonds, reduce tension, repair relationships after conflict, and prevent future conflicts from occurring. Individuals that practice same-sex behavior may therefore have more biological offspring, as long as they do sometimes mate with a member of the opposite sex.

2) Kin selection: this hypothesis centers on the idea that individuals can increase their fitness either by direct mechanisms (having their own offspring) or by indirect mechanisms (investing in, or somehow providing a benefit to, the offspring of their relatives, thereby helping their relatives have more offspring). A homosexual individual might forego having his or her own direct offspring, but could benefit the family (and help get their own genes into the next generation) by investing in siblings, nieces, nephews, etc.

3) Alliance formation: similar to the social glue hypothesis, the alliance formation hypothesis posits that bonds forged during sex lead individuals to greater acts of bravery or sacrifice, to benefit those with whom they’ve been intimate. If same-sex sexual relationships lead to stronger alliances, and these alliances make better warriors or soldiers who are more likely to survive conflicts, that would lend support for the alliance formation. Again, individuals who engage in same-sex behavior must also occasionally engage in opposite-sex sexual behavior in order for this hypothesis to provide an explanation.

4) Practice: according to the practice hypothesis, same-sex activities during immature stages make an individual more adept at courtship and copulation, with opposite-sex partners, as an adult.

5) Enhanced family fertility: according to the enhanced fertility hypothesis, some of the genetic components that can lead to homosexuality in males are also associated with enhanced fertility or success within females that have those gene versions. From this hypothesis we would predict that individuals who share genetic information with homosexual individuals would have greater reproductive success than those who do not. In other words, a gene version involved in homosexuality does not lead to high fitness in a male, but it does lead to high fitness within a female, so the gene is maintained in the population.

Non-adaptive explanations include,

1) Mistaken identity: as the name suggests, some homosexual interactions may simply be the result of mistaken identity.

2) Prison effect: according to this explanation for the occurrence of homosexuality, individuals deprived of the opposite sex may resort to same-sex copulation to meet a biological urge to copulate.

You can probably see some potential problems associated with each suggestion, and hopefully think of some ways to test these hypotheses and the predictions they make. We’ll consider a couple of these hypotheses, but keep in mind that this list is not exhaustive, and a thorough treatment of each hypothesis is not practical here.