The Science of Sex: Fingers and Fetuses

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Fingers and fetuses

In an earlier column, “How Oprah Screwed Me,” I reported on recent research suggesting that there are differences between the fingerprints of gay and straight people — even between gay and straight identical twins. Yet that’s not all that fingers have to tell us about sexual orientation, it seems. If neuroscientist Marc Breedlove of U.C. Berkeley is to be believed, there’s also something to be learned from finger length — specifically, the relative lengths of the index and ring fingers. The findings are published in the current issue of the British journal Nature (and misreported in a number of newspapers, as well).


Breedlove’s research team included his daughter Tessa (age nine), son Nick (sixteen) and Berkeley undergrads Terrance Williams and Michelle Pepitone. They set up booths at several Bay Area street fairs, including the Castro fair (which is mostly gay) and the Solano Stroll (mostly straight). During the fairs, they lured 720 passers-by into the booth with the promise of free “Scratchers” lottery tickets. They asked the volunteers about their sexual orientation, handedness and family details, and made images of their left and right hands with a photocopier. (Nick kept track of the images, Tessa handed out the lottery tickets.) Then Breedlove and his students returned to their lab and began number-crunching.


Breedlove’s research follows an earlier study that had reported on a hitherto unnoticed sex difference in finger length: in the “average” woman, the index finger is very nearly as long as the ring finger, but in the “average” man the index finger is about five percent shorter than the ring finger. When they compared the data for heterosexual men and women, Breedlove’s group confirmed this basic sex difference, but found that it was more marked for the fingers of the right hand than the left hand. Breedlove believes that the sex difference is a consequence of higher levels of testosterone and similar hormones (collectively termed androgens) in males during fetal life.


Among women, those who identified themselves as lesbian had index fingers that were (on average) shorter than their ring fingers. The data for lesbians fell about halfway between the data for the heterosexual men and women. The difference between the measurements for the lesbians and the heterosexual women was statistically significant, although only for the fingers of the right hand.


Breedlove’s group interprets this finding to mean that lesbians are (on average) exposed to higher levels of androgens during fetal life than are heterosexual women. The conclusion is in line with a couple of other previous findings. First, several studies have reported that girls with congenital adrenal hyperplasia, a genetic condition that causes high fetal androgen levels, are more likely to experience same-sex attraction during adulthood than are comparison groups of women, such as their unaffected sisters. Second, researchers at the University of Texas, Austin, have reported on subtle differences between the hearing of heterosexual and lesbian or bisexual women — differences that seem to result from variations in fetal androgen exposure. Collectively, these studies bolster the notion that prenatal sex hormones play a significant role in setting up a woman’s ultimate sexual orientation.


The results for the gay men were murkier. The data suggested an opposite effect from that seen in lesbians, i.e. a tendency for length of the index finger to be longer in gay men than in straight men. By itself, this might fit with a long-popular theory that gay men experience lower-than-average androgen levels during fetal life. However, the difference was seen only in the left hand and it did not reach statistical significance.


Breedlove’s group actually pursued a different interpretation, on account of a curious additional finding. They noticed that the men in their sample who had more than one older brother were “hypermasculine” with respect to finger length: that is, their index fingers were even shorter (compared with the ring finger) than the overall average for men. It is known from previous studies that having older brothers slightly increases a man’s chances of being gay. Thus, the researchers argued, it may be higher rather than lower fetal androgen levels that predispose males to homosexuality. Their hypothesis is rather indirect and speculative, but it may be clarified by future studies.


It’s important to stress that, even for the women, the results only emerged after averaging measurements from many people. You can’t “diagnose” an individual’s sexual orientation, or predict a child’s future orientation, by examining her or his fingers or fingerprints. Genes and prenatal factors have only so much to say about our sexuality: there still could be some role for parenting, for life experiences or even for that bugaboo of many gay activists — choice.