The “tipping point” model as an explanation for the maintenance of homosexuality

This is interesting and suggests that there may be a definitive answer to the long-standing question, “Is there an evolutionarily based explanation for homosexuality?”

Overly simplified, this “tipping-point” model (originally introduced by G. E. Hutchinson in 1959, and then later popularized by Jim McKnight in 1997 and Edward Miller in 2000) posits that genes associated with homosexuality confer fitness benefits in their heterosexual carriers. If only a few of these alleles are inherited, a males’ reproductive success is enhanced via the expression of attractive, albeit feminine traits, such as kindness, sensitivity, empathy, and tenderness. However, if many of these alleles are inherited, a “tipping point” is reached at which even mate preferences become “feminized,” meaning males are attracted to other males. In explaining this model, Miller asked readers to imagine a genetic system in which there are five different genes that place an individual along a masculine-feminine continuum. Each of the five genes has two alleles, one that pulls the individual to the masculine side and one that pulls to the feminine side. If a man inherited all of the feminine-pulling alleles (of which he has a 3.125% chance: .55), he will become homosexual. If he inherited less than all five of the feminine-pulling alleles, however, he would not be homosexual. Although originally proposed in simple form in 1959, this model was finally empirically tested in 2008 and 2009.

Behavioral geneticists at the Queensland Institute of Medical Research lead by Brendan Zietsch (joined by sexual orientation expert Michael Bailey and evolutionary geneticist Matthew Keller) found that psychological femininity in heterosexual men elevated the number of opposite-sex sexual partners, suggesting that their femininity was often attractive to women (think Johnny Depp). In addition, these researchers and those at Abo Akedemi University in Finland (lead by Pekka Santtila) independently predicted that if the “tipping point” model was correct, then heterosexual men with a homosexual twin should have more of the attractive feminine-pulling alleles and thus more opposite-sex sexual partners than members of heterosexual twin pairs. The Finnish group also measured the number of children and age at first intercourse between heterosexual men with a homosexual twin brother and heterosexual men with heterosexual twin brothers. While the findings did not reach statistical significance, data suggested that heterosexuals with a homosexual twin had slightly more opposite-sex sexual partners, slightly more children, and were a bit younger at the age of first intercourse than heterosexual twin pairs.

In other words, a certain amount of “femininity” makes straight men more appealing to women and increases the chances that they’ll pass along more of their genes. Too much and then men are simply gay, but the fact that they don’t reproduce is mitigated by the face that their close cousins–comparatively “woman-like” straight men–are busy passing along a slew of the same genes that apparently contribute to homosexuality. (Obviously this description is fraught with hazards. “Woman-like” as I’m using it here implies not a queeny bearing, but greater tendencies toward kindness, empathy, and other positive traits that can be found in men who present as perfectly “masculine.”)

It’s an intriguing idea, anyway, not that the homobigots–usually people whose comprehension of simple genetics is zero–will either understand or accept it if it ever comes to prominence.

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  1. #1 by Warren on June 11, 2009 - 7:23 pm

    But these kinds of models don’t explain either bisexuals or the ancient Greeks.

  2. #2 by kemibe on June 11, 2009 - 7:38 pm

    They are also suspiciously circular in terms of their reasoning. There’s a lot of assuming what they set out to establish.

  3. #3 by Gingerbaker on June 12, 2009 - 10:45 am

    First thing that comes to mind is “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme”

    http://www.aaas.org/spp/dser/03_Areas/evolution/perspectives/Gould_Lewontin_1979.shtml

  4. #4 by catgirl on June 12, 2009 - 11:57 am

    I wonder how this works out for lesbianism. Are men more attracted to women with “masculine” traits? This isn’t an unreasonable hypothesis since men seem to like long eyelashes on women. Or is there something completely different at work for lesbians?

    Do you know if anyone has studied the effect of avuncular societies on male homosexuality? I wonder if these societies were conducive to a certain percentage of homosexuality because men would would provide more resources for their sisters’ children if they have none of their own, and the genes for homosexuality could be passed through their sisters and would have a greater chance of surviving. I guess we could study the rates of homosexuality among avuncular cultures compared to other types, but it’s hard to know if the societies have functioned that way long enough for the evolution of the trait of homosexuality.

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