The Function of Homosexuality?

Homosexuality is evolutionarily mysterious.  Why should it evolve, since it seems to reduce the probability of reproducing?  The most obvious hypothesis is that it evolved because it somehow increases the chances of survival of family members, because gays somehow spend extra energy to help their (extended) families (who then go on to transmit the relevant genetic material).  The problem is that until now, there was no evidence that gays do any such thing.  But recently, relevant evidence was reported in an article on Samoan “gays” published in last May’s issue of Evolution and Human Behavior.  Apparently, Samoan “gay men” (who actually only have sex with “straight” men, not with each other) do spend considerable energy to help raise their nieces and nephews.  For a short summary, see here.

Update [2/13/08]: Blake Myers pointed out to me another paper on this subject: Camperio-Ciani et al, “Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity”, Proc. R. Soc. Lond. B (2004).  The paper provides evidence that “
female maternal relatives of homosexuals have higher fecundity than female maternal relatives of heterosexuals and that this difference is not found in female paternal relatives”.  So maybe homosexuality is a kind of spandrel (i.e., a by-product of another adaptation)?

I appreciate Raleigh’s concern about possible abuses of this research.  Gay-bashers will use anything they can find, whether relevant or irrelevant to their cause.  Our job is not to leave the field to them by censoring ourselves, but to speak out against them.  And no matter what the evolutionary origin of homosexuality is, it cannot make homosexuality “unnatural” (a word that suggests “ethically wrong”).


  1. Anyone know what the odds are that homosexuality doesn’t have an evolutionary function?

    Sexual reproduction isn’t well understood. Are the mechanisms of sexual attraction well enough understood to support the claim that if homosexuality didn’t have an evolutionary function, there wouldn’t be any?

  2. This may be a terribly unphilosophical (in spirit) contribution, but I would go so far as to say that this line of inquiry ought to be deliberately avoided unless there is some otherwise intractable problem that we think it might solve. Otherwise, there is a risk that we could reach conclusions, perhaps themselves not too important to our own field, but useful to someone who may, for political reasons, want to present homosexuality as unnatural, or as an evolutionary deficiency.

  3. N. Tinbergen´s four questions in the study of behaviour: evolution (why some trait evolve in the way it did), function (what role play that trait), development (how that trait change during the lifespan) and causation (what are the mechanisms sustaining that trait), that finally were reintrigated in proximate causes(causation and development) and ultimate causes (evolution and function); makes homosexuality in terms of darwinian natural selection or Triver´s social evolution a complete mistery.

    First of all, we have to distinghise homosexuality from homosexual behaviour. The former is characterise for a continuous sexual drive toward members of the opposite sex measure in terms of arousal, genital blood flow and other parameters, and the later is characterise for discrete, noncontinuous engagements in some homosexual behaviours such as genital contact or thrilling.

    Homosexuality can only be apply to humans, but homosexual behaviours it is present in other species, according to Paul L. Vasey.

    Vasey studied many macaque societies over time, and he observed homosexual behaviours in females macaques. The main explanations for these homosexual behaviours are in terms of sociosexual explanations: differential sex ratio (less males than females), social alliances fortified through same sex encounters, allomaternal caring (soliciting to other females help for caring the offspring and sex is the currency in exchange).

    Neverthless, none of these explanations called “sociosexual explanations” because they took sex in its outward form but they really are directed to gain social benefits, are satisfactory to Vasey.

    Homosexual behaviour or even homosexuality have to be considered a by-product of one of the main adaptions in all animal taxa: the neurohormonal systems design for reproduction.

  4. Danny Pearlberg

    On a related note, one of the stranger relatively universal attitudes that humans have had is that of hatred towards homosexuals. Strange, because, from the perspective of sexual competition, heterosexuals should be overjoyed at the prosepect of less competition for mating. Of course, as tricky as it is to engage in an evolutionary understanding of sexual attraction without simply resorting to “storytelling”, it’s even more complicated/controversial when it comes to attitudes. I wanted to point this out, though, because the evolutionary mysteriousness of anti-homosexual attitudes seems to never be mentioned alongside the evolutionary mysteriousness of homosexuality itself.

  5. Eric Thomson

    There was someone I met at Wash U in St Louis when I was interviewing for grad school who wrote a thesis on this concept of ‘natural’ and was interesting in the claims by many on the right wing that homosexuality isn’t ‘natural.’ Perhaps Pete Mandik knows who I’m talkinga bout…

  6. I don’t know much about cross-cultural, but there is an enormous literature in classics about Greek and Roman attitudes about sexuality. See for starters David Halperin’s _100 years of homosexuality_ (Routledge 1990).

Comments are closed.

Back to Top
%d bloggers like this: