Un articolo di Nicholas Wade («Pas de Deux of Sexuality Is Written in the Genes», The New York Times, 10 aprile 2007) ci aggiorna, fra l’altro, su quello che la scienza va dicendo in questi giorni a proposito dell’origine dell’orientamento sessuale:
The male brain is sexually oriented toward women as an object of desire. The most direct evidence comes from a handful of cases, some of them circumcision accidents, in which boy babies have lost their penises and been reared as female. Despite every social inducement to the opposite, they grow up desiring women as partners, not men.Come si vede, anche se le cose sono lungi dall’essere chiare, siamo comunque molto distanti dalle spiegazioni preferite da certi psicoanalisti...
“If you can’t make a male attracted to other males by cutting off his penis, how strong could any psychosocial effect be?” said J. Michael Bailey, an expert on sexual orientation at Northwestern University.
Presumably the masculinization of the brain shapes some neural circuit that makes women desirable. If so, this circuitry is wired differently in gay men. In experiments in which subjects are shown photographs of desirable men or women, straight men are aroused by women, gay men by men.
Such experiments do not show the same clear divide with women. Whether women describe themselves as straight or lesbian, “Their sexual arousal seems to be relatively indiscriminate — they get aroused by both male and female images,” Dr. Bailey said. “I’m not even sure females have a sexual orientation. But they have sexual preferences. Women are very picky, and most choose to have sex with men.” […]
Similar differences between the sexes are seen by Marc Breedlove, a neuroscientist at Michigan State University. “Most males are quite stubborn in their ideas about which sex they want to pursue, while women seem more flexible,” he said.
Sexual orientation, at least for men, seems to be settled before birth. “I think most of the scientists working on these questions are convinced that the antecedents of sexual orientation in males are happening early in life, probably before birth,” Dr. Breedlove said, “whereas for females, some are probably born to become gay, but clearly some get there quite late in life.” […]
Researchers have devoted considerable effort to understanding homosexuality in men and women, both for its intrinsic interest and for the light it could shed on the more usual channels of desire. Studies of twins show that homosexuality, especially among men, is quite heritable, meaning there is a genetic component to it. But since gay men have about one-fifth as many children as straight men, any gene favoring homosexuality should quickly disappear from the population.
Such genes could be retained if gay men were unusually effective protectors of their nephews and nieces, helping genes just like theirs get into future generations. But gay men make no better uncles than straight men, according to a study by Dr. Bailey. So that leaves the possibility that being gay is a byproduct of a gene that persists because it enhances fertility in other family members. Some studies have found that gay men have more relatives than straight men, particularly on their mother’s side.
But Dr. Bailey believes the effect, if real, would be more clear-cut. “Male homosexuality is evolutionarily maladaptive,” he said, noting that the phrase means only that genes favoring homosexuality cannot be favored by evolution if fewer such genes reach the next generation.
A somewhat more straightforward clue to the origin of homosexuality is the fraternal birth order effect. Two Canadian researchers, Ray Blanchard and Anthony F. Bogaert, have shown that having older brothers substantially increases the chances that a man will be gay. Older sisters don’t count, nor does it matter whether the brothers are in the house when the boy is reared.
The finding suggests that male homosexuality in these cases is caused by some event in the womb, such as “a maternal immune response to succeeding male pregnancies,” Dr. Bogaert wrote last year in the Proceedings of the National Academy of Sciences. Antimale antibodies could perhaps interfere with the usual masculinization of the brain that occurs before birth, though no such antibodies have yet been detected.
The fraternal birth order effect is quite substantial. Some 15 percent of gay men can attribute their homosexuality to it, based on the assumption that 1 percent to 4 percent of men are gay, and each additional older brother increases the odds of same-sex attraction by 33 percent.
The effect supports the idea that the levels of circulating testosterone before birth are critical in determining sexual orientation. But testosterone in the fetus cannot be measured, and as adults, gay and straight men have the same levels of the hormone, giving no clue to prenatal exposure. So the hypothesis, though plausible, has not been proved.