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Sexual Orientation
The "Johnny Depp Effect" - An evolutionary explanation for homosexuality
How is homosexuality maintained via natural selection?
Posted June 10, 2009
Who is more attractive as a mate? A guy who is kind or a guy who is cruel? A guy who is sensitive or a guy who lacks empathy? A guy who is physically attractive or a guy who is homely? A guy who can appreciate art or the guy who only appreciates ESPN?
Now consider who is more likely to be gay. A guy who is kind or a guy who cruel? A guy who is sensitive or a guy who lacks empathy? A guy who is physically attractive or a guy who is homely? A guy who is...well, you get the point...
This exercise in mate preferences (and stereotypes, please excuse us) serves to illustrate a simple point: Gay men are attractive. In fact, their sexual orientation might sometimes be the only trait that makes them less than the ideal mate for many women. But you already knew this. If you ever agreed with the lonely-hearted lady who lamented, "all the good ones are gay or taken," then you understand.
Two recent articles suggest that these women are on to something. These types of findings are also exciting for the purveyor of natural selection (e.g., evolutionary psychologists) because they might offer a solution to an old Darwinian puzzle. Questions like, "Why do people commit suicide?" "Why do people adopt children?" and the question relevant to this post, "Why are some people sexually attracted to the same sex?" are music to the ears of anyone trying to convey the basic tenets of evolution by natural selection to a class full of students. These questions reflect that the "asker" has a more than cursory understanding of natural selection and EP, enough so to raise their eager little hand and attempt to "stump the teacher." Puzzles, however, do have solutions, and these Darwinian puzzles have some tentative answers.
Again, scientists have known for some time that sexual orientation has a genetic component. They've noticed that it runs in families and that identical twins-who share 100% of their genes-are more likely to share a sexual orientation than are fraternal twins-who share only 50% of their genes. But how are genes associated with homosexuality maintained in a population if homosexuals don't reproduce?
It was once hypothesized that such a trait could be maintained via kin selection. That is, instead of homosexuals reproducing directly, they get their genes into the future by investing (e.g., providing resources and child care) in the children of genetic relatives, increasing their kin's chance of survival and reproduction as a result. Reproducing indirectly is actually not too uncommon in nature (see, for example, the hymenoptera). By this reasoning, homosexuals could have positively affected the survival and reproductive success of their family members and passed on their genes indirectly, through their related gene-vehicles (i.e., their kin). [By the way, we do not recommend referring to your kin as gene-vehicles, even after several glasses of wine.]
Hypotheses demand empirical tests, and when the kin selection hypothesis of homosexuality was tested by David Bobrow and Michael Bailey of Northwestern University and later by Qazi Rahman and Matthew Hull of the University of East London, it was not supported. Homosexuals did not provide more care and resources to family members than heterosexuals. Whether assessing subjective measures (e.g., feeling of closeness to the family, generosity, etc.) or objective measures (e.g., distance residing from relatives, frequency of contact, etc.), few significant differences emerged between homosexuals and heterosexuals, and when there were slight differences, they were in the opposite direction (e.g., homosexuals reported giving fewer monetary resources to kin than heterosexuals reported).
Even without data protesting against this kin selection hypothesis, we find it illogical. The extension of the kin selection argument to account for the origins and maintenance of homosexuality would suggest that homosexuality is a switch in reproductive strategy (duh)-a tradeoff from mating effort to parenting effort. That is, if an individual does not reproduce directly, he should focus his efforts and resources on alloparenting so as to increase the likelihood of related offspring's survival and reproduction. Sexual preferences are inherently about mating, however, and so the kin selection argument does not make clear why individuals who choose to invest their resources in relatives' offspring would be anything but asexual (as they are in hymoptera).
So if this Darwinian puzzle is not solved by invoking kin selection, what's another solution? Two recent studies reported in the journal Evolution and Human Behavior demonstrate how genes associated with homosexuality might lead to reproductive success in their heterosexual carriers.
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.
These recent findings are scientifically intriguing and they likely have profound implications for the LGBT community (which we purposefully skirted here as we are donning our science and description hats and not our policy and prescription caps). This Darwinian puzzle, of course, is far from solved. For instance, it is unclear how this tipping-point model applies to lesbianism. The maintenance of homosexuality in the population is a 500-piece puzzle and we might have some of the borders in place: We're not sure what the picture fully looks like, but we're beginning to make out the parameters. Future replications will confirm or disconfirm this model, and new hypotheses generated from the model will bolster or undermine the current work. For example, one hypothesis that is generated from this model is that attractive men should be more likely to have gay sons than unattractive men. Men generally viewed as attractive partners should carry more of the feminine-pulling alleles than unattractive men, thus making the former more likely to produce gay sons. John Depp Jr.'s sexual orientation is yet unknown-the kid's only 7-but you can bet that Johnny Sr.'s kindness, empathy, and tenderness will make for a fabulous father-son relationship.
References
Hutchinson, G. E. (1959). A speculative consideration of certain possible forms of sexual selection in man. American Naturalist, 93, 81-91.
McKnight, J. (1997). Straight science? Homosexuality, evolution and adaptation. New York: Routledge.
Miller, E. M. (2000). Homosexuality, birth order, and evolution: Toward an equilibrium reproductive economics of homosexuality. Archives of Sexual Behavior, 29, 1-34.
Zietsch, B. P. et al. (2008). Genetic factors predisposing to homosexuality may increase mating success in heterosexuals. Evolution and Human Behavior, 29, 424-433.
