麻豆传媒

YOUR early life as a fetus may have influenced your sexuality as an
adult鈥攁nd your fingers tell part of the story. Researchers in California
have used relative finger lengths to show that sexual orientation is partly
determined by events in the womb.

In animals, prenatal exposure to the male sex hormone testosterone seems to
influence sexual orientation. But it is not easy to measure fetal hormone levels
in humans. One indirect way is to look at the size of a person鈥檚 fingers. In
women, the index finger, called the second digit or 2D, is about the same length
as the ring finger, 4D. In men, the ring finger is often considerably longer,
leading to a lower 2D:4D ratio. This sex difference is clear from infancy, and
researchers attribute it to masculinising hormones during fetal development.

Marc Breedlove and his colleagues at the University of California at Berkeley
wanted to know if gay people had different finger length ratios from straight
people, so they surveyed 720 adults during street fairs in San Francisco. The
researchers collected information about gender, age, sexual orientation,
handedness and older siblings. They also carefully measured the lengths of the
volunteers鈥� fingers.

They found something striking in gay women: their index to ring finger ratios
resembled those of heterosexual men. This suggests that at least some lesbian
women were exposed to higher than average levels of male hormones before
birth.

What they found in men is less clear-cut. The 2D:4D ratio in gay men was not
significantly different from that in straight men. But a series of studies in
the 1990s had shown that the more older brothers a boy had, the more likely he
was to be gay, so the researchers sorted the volunteers according to numbers of
older brothers. They found that all the men who had two or more older brothers
had significantly smaller 2D:4D ratios. 鈥淚t was a big surprise to me that the
finger measures would follow the epidemiology so closely,鈥� says Breedlove.

The findings suggest that homosexuality is partly due to higher levels of
prenatal testosterone in men as well as women, he says. But they also show that
fetal hormones alone don鈥檛 determine sexuality. First-born males have
indistinguishable 2D:4D ratios, whether they are gay or straight, yet some
first-born males are gay. So other factors clearly come into play, says
Breedlove.

To some researchers, the idea that gay men are 鈥渉ypermasculinised鈥� seems
counterintuitive. John Manning of the University of Liverpool has found the
opposite in gay men: that their finger ratios veer more towards the feminine. He
suspects that both very low and very high levels of testosterone in the womb
could produce homosexuality. 鈥淭here may be more than one phenotype of male
gays,鈥� he says.

Manning also wonders if Breedlove鈥檚 data may have been slightly muddied
because the research did not take account of ethnicity. He has found big
population variations in 2D:4D ratios. 鈥淭he geographical differences swamp the
sex differences,鈥� says Manning. 鈥淭here鈥檚 more difference between a Pole and a
Finn than between a man and a woman.鈥�