10 March 2011

An adaptive fairytale with no happy ending

ResearchBlogging.orgThe evolution of human traits and behaviors is, as I've noted before, a contentious and personal subject. This is enough of a problem when there's some data to inform the contentiousness. In the absence of meaningful data, it's downright dangerous.

Take, for instance, Jesse Bering's recent post about the evolution of homophobia, which Steve Silberman just pointed out to me.

A grim fairy tale indeed. Photo by K Wudrich.
When evolutionary biologists say a trait or behavior is "adaptive," we mean that the trait or behavior is the way we see it now because natural selection has made it that way. That is, the trait or behavior is heritable, or passed down from parent to child more-or-less intact; and having it confers fitness benefits, or some probability of producing more offspring than folks who lack the trait. Lots of people, including some evolutionary biologists, speculate about the adaptive value of all sorts of traits—but in the absence of solid evidence for heritability or fitness benefits, such speculation tends to get derided as "adaptive storytelling."

Evolutionary biology wasn't always so rigorous, once upon a time. Then Stephen Jay Gould and Richard Lewontin buried adaptive storytelling under an avalanche of purple prose in their landmark 1979 essay "The Spandrels of San Marco" [PDF]. Norman Ellstrand made a similar point with better humor in a satirical 1983 article for the journal Evolution proposing adaptive explanations for why children always start life smaller than their parents [PDF]. Nowadays, when evolutionary biologists want to, say, argue that horned lizards' horns are an adaptation for defense against predators, they have to demonstrate the claimed fitness benefit [PDF].

Evolutionary psychologists, however, seem not to have gotten the memo.

Bering's post focuses on a series of studies by the evolutionary psychologist Gordon Gallup. Gallup was interested in the question of whether there might be an adaptive explanation for homophobia—which, given the fact that many (although far from all) human cultures treat homosexuality as a taboo—is a fair question for research. He hypothesized that treating homosexuality as taboo helped to prevent homosexual adults from contacting a homophobic parent's children, which would reduce, however slightly, the prospects of those children growing up to be homosexual, and ensure more grandchildren for the homophobe.

Gallup supported this adaptive hypothesis with ... evidence that straight people were uncomfortable about homosexuals coming into contact with children [$a]. Here's the opening sentence of that paper's abstract:
In a series of four surveys administered either to college students or adults, reactions toward homosexuals were found to vary as a function of (1) the homosexual’s likelihood of having contact with children and (2) the reproductive status (either real or imagined) of the respondent.
If you've noticed that this doesn't mention evidence of heritability or a fitness benefit to homophobia, that's not because I left it out—that's because Gallup's work contains no data to support either.

What this amounts to is arguing that homophobia is an adaptation favored by natural selection because homophobia is a thing that exists.

Could a complex behavior like homophobia have a genetic basis? Sure. Homosexuality itself is a complex behavior, and it certainly does have some genetic basis. However, the fact that attitudes toward homosexuality have shifted as far and as fast as they have in the last few decades suggests that any genetic effects underlying homophobia are pretty easy to overcome. Behaviors can be inherited culturally, too, since human children learn from their parents. But—note, again, lots of change in the last thirty years or so—cultural inheritance is more fleeting and malleable than biological inheritance.

Careful, Red Riding Hood—that wolf might be gay. Photo by crackdog.
What about Gallup's proposed fitness benefit for homophobes? Well, that would require homophobia to, you know, actually prevent homosexuality. Gallup's argument there hangs on two distasteful assertions. First, that gay men are more likely to be pedophiles, and second, that boys sexually abused by gay men are themselves more likely to grow up gay. In spite of Gallup's assertions otherwise [$a], we have strong evidence from multiple studies that gay men are no more likely to be sexually attracted to children than straight men.

And there is, to my knowledge, no evidence to suggest that abuse can cause homosexuality. Bering cites a recent study that does document an association between childhood abuse and later homosexuality in men. However, the study's authors point out that, "The reason for the connection between childhood sexual abuse and same-sex partnerships among men is not clear from our findings."
... gay men tend, on average, to be more gender non-conforming as boys (Bailey & Zucker, 1995). This tendency could increase their appeal or conspicuousness to sexual predators, which might make them more likely to be victims of abuse (B. Mustanski, personal communication, February 11, 2008). Similarly, it is possible that boys who are developing and exploring a same-sex sexual orientation are more likely to enter situations where they are at risk for being sexually abused (Holmes & Slap, 1998). [In-text citations sic]
Why on Earth would Bering dredge up Gallup's adaptive fairytale a decade and a half after it was published, if it was baseless to begin with and no new evidence supports it? Well, according to Bering, because he's a hard-nosed scientist who isn't afraid to consider uncomfortable possibilities.
Sometimes, science can be exceedingly rude—unpalatable, even. The rare batch of data, especially from the psychological sciences, can abruptly expose a society’s hypocrisies and capital delusions, all the ugly little seams in a culturally valued fable. I have always had a special affection for those scientists like Gallup who, in investigating highly charged subject matter, operate without curtseying to the court of public opinion.
Of course, says Bering, Gallup's work isn't conclusive, but it sure would be interesting if someone tested it.

Except, when Gallup was forming his hypotheses about the evolutionary benefits of gay-hating—he first proposed the idea in a 1983 article—he was hardly thumbing his nose at public opinion. He was, in fact, giving natural selection's approval to the prevailing ugly stereotypes about gay men. And, as any competent evolutionary biologist would recognize, he did it without a shred of relevant evidence.


Ellstrand, N. (1983). Why are juveniles smaller than their parents? Evolution, 37 (5), 1091-4 DOI: 10.2307/2408423

Gallup GG Jr, & Suarez SD (1983). Homosexuality as a by-product of selection for optimal heterosexual strategies. Perspectives in Biology and Medicine, 26 (2), 315-22 PMID: 6844119

Gallup, G. (1995). Have attitudes toward homosexuals been shaped by natural selection? Ethology and Sociobiology, 16 (1), 53-70 DOI: 10.1016/0162-3095(94)00028-6

Gallup, G. (1996). Attitudes toward homosexuals and evolutionary theory: The role of evidence. Ethology and Sociobiology, 17 (4), 281-284 DOI: 10.1016/0162-3095(96)00042-8

Gould, S., & Lewontin, R. (1979). The Spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proc. Royal Soc. B, 205 (1161), 581-98 DOI: 10.1098/rspb.1979.0086

Wilson, H., & Widom, C. (2010). Does physical abuse, sexual abuse, or neglect in childhood increase the likelihood of same-sex sexual relationships and cohabitation? A prospective 30-year follow-up. Archives of Sexual Behavior, 39 (1), 63-74 DOI: 10.1007/s10508-008-9449-3

Young, K., Brodie, E.D., Jr., & Brodie, E.D., III (2004). How the horned lizard got its horns. Science, 304 (5667) DOI: 10.1126/science.1094790

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