Excerpts are not my writing! This comes from Richard Wrangham’s book:
It was a fun read. Recommended!
Human homosexuality is not adaptive
The hypothesis that human homosexuality is adaptive (genetically advantageous) has not been rejected lightly. Homosexual behavior can be frequently found among wild animals, and traits that are widespread are likely to be adaptive.
So when evolutionary biologists began to study human homosexual behavior, they tended to search for ways to explain how a same-sex preference might have been favored in natural selection. Homosexual behavior among other animals offered some ideas.
Close study reveals how homosexual behavior can be adaptive.
- Scarcity of opposite-sex partners. Among Laysan albatrosses in Hawaii, two parents are needed for chicks to be reared successfully. When there are not enough males, females pair together. Their sexual behavior includes courtship and pseudo-copulation. Females in same-sex pairs are fertilized by an already mated male, who then ignored the resulting eggs and chicks. The female pair brings them up without male help.
- As a prosocial device. In animals whose choice of sexual partner is not a response to a shortage of opposite-sex partners, homosexual behavior sometimes appears to be adaptive by promoting useful social relationships. In troops of Japanese monkeys, females form temporary homosexual mating partnerships even when other males are available. Among savanna baboons, males form alliances that they use in fights against others. Allies reciprocally fondle one another’s genitals, apparently to demonstrate their commitment to the bond.
Researchers have sought evidence that the kinds of reproductive or social benefits that animals gain from same-sex sexual interaction might be found in human. In theory, humans could form same-sex partnerships in response to a short supply of members of the opposite sex. Certainly, partner availability influences us. Women and men in single-sex institutions such as prisons, schools, monasteries, and ships often temporarily shift their sexual activity toward their own sex. Nevertheless, of course, many individuals feel an exclusive attraction to members of their own sex, regardless of the availability of the opposite sex.
Further, homosexual couples tend to have smaller families than same-sex couples, and there is a lack of evidence that their sexual orientation leads them to give exceptional help to their genetic kin. These data suggest that homosexual behavior in humans is not biologically adaptive.
Unfortunately, the conclusion that same-sex behavior is not adaptive has sometimes been associated with a negative view of homosexuality. But normative value and biological purpose are independent considerations. Many tendencies that we regard as morally reprehensible clearly evolved, including numerous kinds of sexual coercion, lethal violence, and social domination. Equally, many morally delightful tendencies did not evolve, such as charity to strangers and kindness to animals. Our decisions about which behavior we like or dislike should never be attributed to adaptive value.
The biological basis of homosexuality
Same-sex sexual attraction is often stable over a lifetime, and there is good evidence that is is partly heritable. These features make human homosexuality different from most animal homosexuality.
One particular area of the brain responds to androgens (sex hormones) in the fetal stage: the third interstitial nucleus of the anterior hypothalamus (INAH3). The INAH3 is larger in heterosexual men than in women, and has been found to be intermediate-sized in homosexual men. In an adult rams, experimentally reducing the comparable nucleus (oSDN) causes them to change his sexual-partner preference from female to male.
Homosexual preference is more likely in males who receive low testosterone exposure before birth. A standard method for assaying prenatal testosterone exposure is to measure the length of the ring finger (the fourth finger) compared to the length of the second finger: increased prenatal exposure to testosterone tends to be associated with relatively long ring fingers. The largest surveys of homosexual men in the United States, China, and Japan have found a tendency for homosexual women to have relatively long ring fingers, whereas homosexual men have relatively short ring fingers. Homosexual men also tend to have somewhat feminized face shapes and shorter, lighter bodies than heterosexual men, most likely from relatively low exposure to testosterone in the womb. In general, females who have been exposed to higher-than-usual levels of androgens, and males who have been exposed to lower-than-usual levels, appear to have a higher likelihood of being homosexual.
Homosexuality as a by-product of self-domestication
The evidence that exclusive homosexual preference is common but not adaptive makes it a prime candidate for being an evolutionary by-product.
Elsewhere, I have presented the self-domestication hypothesis: the theory that H. sapiens domesticated itself; that is, it selected against reactive aggression. Testosterone is involved both in male violence, but also sexual preference.
But since reduced testosterone is a common effect of domestication, homosexual orientation in this species appears to be explicable ultimately as an incidental consequence of selection against reactive aggression.
Some additional evidence are suggestive:
- The only nonhuman animal in which exclusive homosexual preference is known is a domesticated species – namely, sheep.
- At least 19 species of domesticated animals show homosexual behavior, though it occurs in their wild relatives as well.
- Our two closest primate relatives are chimpanzees and bonobos. Chimpanzees are non-domesticated (highly aggressive) and have long ring fingers suggesting high prenatal exposure to testosterone. Bonobos are self-domesticated (placid), and have short index fingers.
- Homo neanderthalensis morphology indicates they were quite an aggressive species (non-domesticated), and they shows a large finger-length ratio. The 100,000-year-old H. sapiens at Qafzeh is in-between the ratios for living humans and the five Neanderthals.
Thus, it may be that self-domestication (the source of our species’ remarkable ability for cooperation) yielded homosexual behaviors as a by-product.