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. 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.
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. In men, however, the index finger is usually the shorter of the two digits. But the ratio of finger sizes in men was more complicated. Comparisons between all men showed no differences. Only gay men with several older brothers had an unusually "masculine" finger ratio - in other words, they had significantly shorter index fingers.
Having a large number of older brothers had previously been established as a factor predisposing men to homosexuality, and like finger length reflects prenatal androgen exposure. Homosexual men without older brothers had finger length ratios indistinguishable from heterosexual men, indicating that factors other than hormones - such as genetic influences - also contribute to sexual orientation. This is because they suggest younger brothers are being exposed to higher levels of androgen in the womb than their elders.
Womb 'memory' "We think it is inescapable that the mother's body is remembering how many sons she has carried before, and somehow she is then increasing the amount of androgen that each subsequent son sees before birth. So the fascinating questions are: Scientists in the UK have given it a cautious reception.