Why are some people gay?

I was probably in my mid 20s when I first heard the term "gaydar," meaning the ability to sense if someone you just met is gay. I know that, before I was 30, I had no gaydar. It's not that I didn't know gay stereotypes or recognize those in other people. It's more that the idea of same-sex attraction was completely outside of my realm of thinking. So if someone was stereotypically gay or sent off some other signals of his or her homosexuality, I never gave such clues any thought at all. I more-less sized up other males as friendly or unfriendly and females my age as hot or not.

When I was a bit older, and had met more gays, I gave a bit of thought as to why I was straight and why they were not. No one ever had to explain to me to be attracted to girls. It just happened as a normal course of puberty. It was certainly an expression of my biology. It was never the case that I could go either way. Like almost every guy I knew, I had zero attraction to males.

This realization about myself made me sympathetic to gay males. They surely were not homosexuals (or in some cases, I suppose, bisexuals) because they had chosen that orientation. It's really, truly impossible to choose. No biologically straight man wants gay sex in any form. Rather, I became convinced--and still am convinced--that their being gay is an expression of their biological impulses, very much like mine is to be heterosexual.

Yet the scientific proof of what makes some people gay has been lacking. A new theory, reported in the latest issue of Popular Science, says that chemical "switches" attached to genes may explain why someone is gay.

Gayness may not be in our genes, but in the molecules that regulate them. New research suggests that epigenetic factors -- chemical "switches" attached to genes that turn them on or off -- are a more plausible heritable mechanism behind homosexuality than DNA itself. Non-genetic changes to gene expression are called epi-marks, for epigenetics, the field of research dealing with the molecular on/off switches. Epi-marks are normally erased between generations, but there's recent evidence that they're sometimes passed from parent to child.

The key hormone seems to be testosterone in the womb.

Researchers at the National Institute for Mathematical and Biological Synthesis (NIMBioS) looked at how epi-marks that influence testosterone sensitivity in the womb might contribute to homosexuality. Late in pregnancy, natural variations in testosterone levels can alter a fetus' sexual development. Sex-specific epi-marks protect female fetuses from masculinization in the presence of too much testosterone; boys are protected from feminization if too little testosterone is present.

The theory suggests that lesbian girls inherit testosterone-buffering epi-marks from their fathers and gay boys inherit them from their mothers.

According to computer modeling by the group, testosterone-buffering epi-marks passed from a parent to an opposite-sex offspring may result in the reverse effect: Girls who inherit sex-specific instructions from their fathers will be partially masculinized, while boys who get epi-marks from their mothers will be partially feminized. In this model, homosexuality occurs when stronger-than-average epi-marks influencing sexual preference from an opposite-sex parent escape erasure and are then paired with weaker-than-average sex-specific epi-marks produced in opposite-sex offspring.

Many researchers have looked for a gay gene and have not found it.

Study co-author Sergey Gavrilets, a professor of ecology and evolutionary biology and mathematics at the University of Tennessee, Knoxville, and associate director for scientific activities at NIMBioS, says there could still be a "gay gene" or genes, but that there are problems with the idea: "Nobody has been able to present solid experimental evidence for this in spite of significant effort."

Despite their being no evidence for a specific gay gene, that does not mean that gayness does not tend to run in families. It does.

The search for genes that control sexual orientation is based on increasing evidence of a strong genetic component. Studies clearly show that homosexuality runs in families, with an increased rate among siblings and the maternal uncles of gay men, according to a 2011 review.

The largest theoretical problem with a gay gene being the cause of homosexuality is that gays reproduce less often than straights. Therefore, if it were a gene or a sequence of genes which caused homosexuality, it should be quite rare and disappearing over time. That does not appear to be the case. However, a recent Italian study suggests that females related to gay males tend to reproduce much more than other females.

A study published online just last week by Italian researchers Andrea Camperio Ciani and Elena Pellizzari found that the maternal aunts and grandmothers of gay men have more children than those of straight men. A few years ago, Ciani used genetic modeling to explain the 2004 finding that sisters and maternal aunts of homosexual men have more children than the females in the maternal line of straight men. According to that model, at least one unknown gene on the X chromosome predisposes female carriers to higher fertility and male carriers to homosexuality. "The genes evolved for the fecundity benefit in females, at the reproductive cost of an increase in homosexuality in males," Ciani explains.