12.9 How did homosexuality evolve?

The literature 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.

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
  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. 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 social glue, 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.
  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 are also associated with enhanced fertility or success in getting mates. 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.

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 copulations to meet a biological urge to copulate.

You can probably imagine ways to test all of the above explanations, as well as potential problems associated with each suggestion. 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.


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The Evolution and Biology of Sex Copyright © 2020 by Sehoya Cotner and Deena Wassenberg is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.