Finding the Switch

[aphar]

Homosexuality may persist because the associated genes convey surprising advantages on homosexuals' family members.

Studies suggest there is a genetic basis for homosexuality in only 50 percent of gay men.

Camperio and his colleagues compared the family trees of gay men to those of straight men, and confirmed that homosexuals had more gay male relatives on their mother's side than on their father's side—which suggests an X-linked trait. But the Italian researchers also found something more intriguing: Compared with the straight men, the gay men had more relatives, period.
[T]he mothers of homosexuals in the study did indeed have between a quarter and a third more children than the mothers of heterosexuals. Camperio also uncovered another dramatic finding: In families with gay sons, the aunts from the mother's side had many more children than the aunts on the father's side—the large families, in other words, were on the maternal side.
Camperio spent the next few years going to gay men and begging them to let him interview their mothers and aunts—a daunting task in deeply Catholic Italy. In the end, it took him three years to get 30 subjects. When he interviewed the women, though, he found they had fewer miscarriages, fewer infections, and used fewer contraceptives than the mothers and aunts of heterosexuals, though the differences were only slight. One difference, though, was not slight at all: The homosexuals' mothers and aunts had had between three and four times as many sexual partners. They seemed to really like having sex with men.
Camperio says his results can explain no more than 20 percent of the incidence of homosexuality.

The gay men in Camperio's study [...] also had more older brothers—and not just because they came from larger families. It's true across the board: The more older brothers a man has, the more likely he is to be gay. The "fraternal birth order effect" [...] has since been replicated by a dozen other studies.

For every older brother a man has, his chances of being gay go up by around a third. In other words, if you have two older brothers, you're nearly twice as likely to be gay—regardless of whether the older brothers are themselves gay. It is not possible to explain that as an effect of genetics.

...[I]n another study, Bogaert found that it was only biological older brothers that contributed to the effect. Men who grew up with older stepbrothers or adopted brothers—brothers born of different wombs—were no more likely to become gay. Meanwhile, men with biological older brothers who died in infancy or who were raised separately—including brothers they had never even met and sometimes didn't even know about—did manifest the effect. In other words, the effect could not be explained through upbringing.

If it wasn't genetic and it wasn't upbringing, then what could it possibly be? The answer is the prenatal environment—the result of something that occurs as the fetus develops in the womb.

So what happens in the womb to make a fetus gay? ... Blanchard and Bogaert suggest the older-brother effect could result from a mother's immune reaction against her male fetuses. During her first male pregnancy, the mother's body reacts against some factor related to male fetal development. Her immune system detects male-specific proteins produced by the boy's Y chromosome—perhaps proteins located on the surface of his brain cells—and deems them foreign invaders. As a result, her body generates antibodies against them. Each successive male pregnancy strengthens this immune response. The next time she's pregnant, the anti-male antibodies cross through the placenta and influence the fetus's brain, interfering with the masculinization of his brain and making him gay.

It may even be that women with strong immune systems are more likely to produce gay sons. The reproductive advantages to her of having such a healthy constitution might outweigh the disadvantages of occasionally producing a son who will have no kids himself. Even if the immunization scenario is true, however, it explains only 15 to 30 percent of the cases of male homosexuality.

Some facts:

1. 2-6% of men are gay.


2. Gay men have longer, thicker penises than straight men, on average.


3. As children, most gay men display gender-bending behavior, like dressing up in their sisters' clothes or playing with dolls.


4. In general, gay men are worse than straight men at certain cognitive skills, like reading maps, spatial orientation, finding missing objects, and packing trunks.


5. Lesbians are better than straight women at certain spatial, navigational, and language tasks.


6. The voices rated as most masculine are those of gay men (and the most feminine are those of lesbians).


7. Gay men often have distant fathers, but not because levels of childhood affection have an effect on sexual orientation, but because some fathers may become distant in reaction to childhood gender nonconformity of boys who are born gay.


8. The more older brothers you have, the more likely you are to be gay.


9. Sexual orientation correlates with whether you are right- or left-handed: Homosexuals are 39 percent more likely to be left-handed or ambidextrous.


10. The ratio of the lengths of the second to fourth fingers predicts sexual orientation.

Comments

[ymarkov.livejournal.com]

Interesting! How does this play out with lesbians?

Immunization scenario
Some time ago US News & World Report wrote about a study indicating that women who frequently engage in oral sex with the fathers of their children have a statistically significant lower risk of miscarriage. The speculation was that digesting sperm acquaints their immune systems with the fathers' DNA markers, making an immune reaction against a baby less likely.

[aphar.livejournal.com]

women's sexuality seems to be more complicated and fluid—women are much more likely to report fantasizing about both sexes, or to change how they report their sexual orientation over time—which makes it harder to study.