"Big-Brother Effect" or Biological Error?

What it might mean: Psychological influences are probably not at work, because the pattern holds even for gay men who weren’t raised with their older brothers. Instead, the mother's womb might be key. After giving birth to a boy, her immune system might create antibodies to foreign, male proteins in her bloodstream. Subsequent sons in the womb could be exposed to these "anti-boy" antibodies, which might affect sexual development in the brain.

Accordingly, you'd expect the percentage of gay men in a society to vary depending on demographic differences in family size: One study calculated that a one-child-per-family law would reduce male homosexuality by about 29% from current levels.

* Left hand vs. right hand. The hand you use to sign your name might have something to do with what gender you are drawn to.

The numbers: More lefties -- or at least more somewhat-ambidextrous folks -- crop up in the gay population than among straight people, several studies have shown. An analysis of more than 23,000 men and women from North America and Europe in 2000 found that being non-right-handed seems to increase a man's chances of being gay by about 34%, and a woman's by about 90%.

What it might mean: One guess is that different-than-normal levels of testosterone in the womb -- widely theorized to play a role in determining eventual sexual orientation -- could nudge a fetus toward brain organization that favors left-handedness as well as same-sex attraction.

Another theory is that development of a fetus might be disturbed by factors such as a mother's illness, steering the fetus into being less than strictly right-handed -- and, in some cases, less than strictly heterosexual.

It's a politically sticky idea, says Qazi Rahman of Queen Mary-University of London. "It's essentially saying that homosexual preference . . . is some kind of biological error," he says. (It might tick off the left-handed folks too.)