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"Others have pointed out that such research is dangerous: Would it result in attempts to prevent children from being born gay? 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. 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.
More telling are the variations within each sex. "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. Animal models using rats, mice and sheep show that testosterone boosts growth in a part of the hypothalamus involved in sexual behavior and fertility.
The same study, published in Early Human Development, found that boys with female-type finger lengths are on average more emotional than other boys. Girls with masculine-type finger ratios tend to have higher hyperactivity scores and more problems relating to their peers than do other girls. Many scientists believe relative finger length—or digit ratio—is a marker for brain differences molded by hormones.
The reigning hypothesis is that testosterone encourages growth in the right side of the brain, while inhibiting growth in the left. They thought this might prove that it’s not a 'choice,'" he said. Some studies have shown hypermasculine finger length in gay men, while other studies show the opposite, a female-like finger pattern.
But it's impossible to do so accurately in a vacuum, cautions Manning.
Finger length 'could provide a clue' to one's sexual orientation, study finds
The length of one’s index and ring fingers could be linked to sexual orientation, according to a new study published in the Archives of Sexual Behavior.
The relationship between finger length and sexuality was only evident, however, in females, according to the report by researchers from the University of Essex, who examined 32 sets of identical twins with different sexual orientations.
Typically in women, the index and ring fingers are similar in length, while in men there is a greater difference between the two fingers.
The picture is further muddied by geography. 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.
Females with masculine digit ratios have more masculine behaviors, he says. Or vice versa? In sheep, males with hypermasculinized brains are sexually attracted to other males.
You may be tempted to draw conclusions from your own fingers. But the differences between the sexes aren't all that interesting to biologists.
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. "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.
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. Looking at someone's hands "could provide a clue to their sexuality," according to Watts.
Dr. 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.
Will people misinterpret the study and go around looking at each other’s hands in an attempt to out people?"
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Length of ring and index fingers 'linked to sexuality'
Both men and women were exposed to the "male" hormone, testosterone, in the womb - but some may be exposed more than others, the scientists said.
Study author Dr Tuesday Watts, from the psychology department at Essex University, said: "Because identical twins, who share 100% of their genes, can differ in their sexual orientations, factors other than genetics must account for the differences.
"Research suggests that our sexuality is determined in the womb and is dependent on the amount of male hormone we are exposed to or the way our individual bodies react to that hormone, with those exposed to higher levels of testosterone being more likely to be bisexual or homosexual.
"Because of the link between hormone levels and difference in finger lengths, looking at someone's hands could provide a clue to their sexuality."
The findings are published in Archives of Sexual Behaviour.
Sexuality: Your Telltale Fingertips
Look down at your right hand.