Look down at your right hand. Is your ring finger longer than your index finger? Or vice versa? To be certain, take a ruler and measure from the bottom crease of each finger to the tip.
The measurements tell you something about the environment of your mother's womb just weeks after your conception, a time when your fingers, and more importantly, your brain, were developing. Because of the influx of sex hormones at this prenatal stage, men tend to have ring fingers that are slightly longer than their index fingers. In women, these fingers are usually the same length or the index digit is just a bit longer.
Digits are subtly affected by testosterone and estrogen produced in the womb by the fetus (not by the mother). Between weeks 8 and 14, tiny fetal testes, ovaries and adrenal glands secrete the baby's own supply of sex hormones. 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. 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. He believes digit ratio is an important, if indirect, tool for studying the fetal brain and the womb, an environment that's off-limits to scientists except for analysis by amniocentesis. (And even then, because sex hormones fluctuate hour by hour, amniocentesis is a poor indicator of testosterone exposure.)
"Early sex hormones have an organizing effect on the brain that's permanent," Manning says. But the differences between the sexes aren't all that interesting to biologists. More telling are the variations within each sex. Females with masculine digit ratios have more masculine behaviors, he says. Likewise, males with a typically female ratio exhibit more typically feminine behaviors.
A study of digit ratio in Scottish preschool children between the ages of 2 and 4 found strong relationships between digit ratio and gender-normative behavior. Girls with masculine-type finger ratios tend to have higher hyperactivity scores and more problems relating to their peers than do other girls. The same study, published in Early Human Development, found that boys with female-type finger lengths are on average more emotional than other boys. "They tended to be very sensitive," says Manning.
Except for genitalia, relative finger length is the only physical trait fixed at birth that is sexually dimorphic—meaning males and females show typical gender differences. 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. 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. (Studies indicate they are most common in men with longer than average ring fingers.) A "masculine" finger pattern seems to similarly mark girls predisposed to hyperactivity and autism.
Some scientists believe prenatal sex hormones are also part of the puzzle of homosexuality and that a high level of testosterone may wire the brain for attraction to the same sex. Intriguingly, research shows that a prenatal testosterone level is most strongly linked to homosexuality in women, according to an article in the Archives of Sexual Behavior. Lesbians are more likely than straight women to have a masculine finger ratio, says McFadden.
The data in men, however, are more complicated and contradictory. Some studies have shown hypermasculine finger length in gay men, while other studies show the opposite, a female-like finger pattern. The picture is further muddied by geography. 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. 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. The reigning hypothesis is that testosterone encourages growth in the right side of the brain, while inhibiting growth in the left. Animal models using rats, mice and sheep show that testosterone boosts growth in a part of the hypothalamus involved in sexual behavior and fertility. In sheep, males with hypermasculinized brains are sexually attracted to other males.
You may be tempted to draw conclusions from your own fingers. But it's impossible to do so accurately in a vacuum, cautions Manning. 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. "You can't look at someone's fingers and make a determination about whether they are heterosexual or lesbian, just as you can't decide whether they're neurotic. The [sexuality indicators] are most certainly there, but they're not strong enough to allow us to make predictions."