Gay finger

“Having said that, it is not clear whether there is any real difference in the level of testosterone exposure between these pairs of male twins with different sexual orientations.”

Watts said she and the study’s co-authors were interested in “investigating the factors that could contribute to such a fundamental difference” — like sexual orientation — in two genetically identical people.

“These include prenatal factors,” she added, including the level of testosterone exposure in the womb.

Watts said the study’s findings were not particularly surprising, as they were in line with previous research on the topic of finger length and sexuality.

“I guess what is most interesting is that particularly for the female twins, results suggest that they might have been exposed to different prenatal environments, even though they were developing at the same time in utero,” Watts said.

The bottom line?

Manning and others have linked finger length ratios to aggression, left-handedness, heart disease, autism and attention deficit disorder, all traits that are more common in men. Like a bit of prenatal graffiti, a longer ring finger says, "Testosterone was here."

John Manning, a biologist at the University of Liverpool, first identified digit length as a sign of prenatal hormones eight years ago.

In sheep, males with hypermasculinized brains are sexually attracted to other males.

You may be tempted to draw conclusions from your own fingers. Fingers are an indication of the environment that molded the brain, but only if you know how you measure up to others.

"You have to be careful," he says. Jack Turban, a resident physician in psychiatry at Massachusetts General Hospital who researches gender and sexuality and was not involved in the study, said there have been a number of studies over the years that have explored whether prenatal hormone exposure plays a role in determining sexuality.

Girls with masculine-type finger ratios tend to have higher hyperactivity scores and more problems relating to their peers than do other girls.

Studies indicate genes wield much influence.

Even as digit ratio research flourishes and more behavioral links are established, the relationships will remain mere statistical correlations until researchers fully understand how sex hormones physically affect the brain. But the differences between the sexes aren't all that interesting to biologists.

The same study, published in Early Human Development, found that boys with female-type finger lengths are on average more emotional than other boys. But it's impossible to do so accurately in a vacuum, cautions Manning. These chemical messengers, particularly testosterone, cause chain reactions in the body, spurring the growth of the genitals, encouraging and inhibiting growth in brain regions and causing changes in the fingers.

Many scientists believe relative finger length—or digit ratio—is a marker for brain differences molded by hormones. Is your ring finger longer than your index finger? Race and ethnic differences seem to affect digit ratio, although scientists don't yet understand how.

Still, even if prenatal testosterone is a factor in homosexuality, it's unlikely to be the only element.

The reigning hypothesis is that testosterone encourages growth in the right side of the brain, while inhibiting growth in the left. Females with masculine digit ratios have more masculine behaviors, he says. Some studies have shown hypermasculine finger length in gay men, while other studies show the opposite, a female-like finger pattern.

Other sexually dimorphic traits, such as height and waist-to-hip ratio, don't appear until puberty.

"Everything you see as far as sex differences in the behavior of toddlers is an aftereffect of prenatal testosterone," says Dennis McFadden, a psychology professor at the University of Texas at Austin. Will people misinterpret the study and go around looking at each other’s hands in an attempt to out people?"

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