Is Our Fate Written in the Lengths of Our Fingers?

Deborah Blum is a Pulitzer Prize-winning science writer and author of "Sex on the Brain: The Biological Differences Between Men and Women."

From my childhood, I remember one particularly goofy joke. It started like this: “What’s the first sign of insanity? Hair growing on your knuckles.” Then, just as the victim checked his or her own knuckles, came the punchline: “What’s the second sign? Looking for it.” The teller and any lurking observers would crack up, and we’d all troop off to try the joke on our siblings.

When the first reports linking finger length to behavior appeared, I had a sudden flashback to those days of checking for hairy knuckles. Scientists have now measured hundreds of people’s hands and linked their finger structure to an extraordinary array of behaviors--musical talent, athletic ability, spatial skills, dyslexia, stuttering, sexual orientation. In March, British researchers added autism to the list.

It sounds like a gotcha joke--but one with potentially troublesome consequences. I can envision the scenarios: couples peering at each other’s hands on the first date; parents checking their children’s hands for signs of trouble; gloves becoming popular again as those of us with the “wrong” fingers (mine are, of course, “normal”) seek to hide them,


Except, of course, that it’s hard to keep a joke going in the face of reasonable science. When you really start exploring the connections between finger length and behavior, they turn out to be less hilarious than we joke lovers might hope. What they provide is a window on the ways scientists try to figure out who we are--and the ways that human biology, beautifully complex, gorgeously convoluted, makes that so hard.

All of this is really about the length difference between two fingers, the index finger (second) and the ring finger (fourth, counting from the thumb). Biologists call this the 2D:4D ratio. It appears that in the first trimester of pregnancy, as hormones are pitching in to help build the body, exposure to testosterone can result in a difference in lengths of these two fingers. Why? Unclear, although biologists have known for a long time that testosterone helps shape some bone growth--high, chiseled cheekbones, for instance. Now it appears that those of us exposed to a little more prenatal androgen tend to have a ring finger that’s longer than the index finger.

It means, not surprisingly, that men--the testosterone heavies in our species--usually have longer ring fingers than index fingers. British researcher John Manning, at the University of Liverpool, sees testosterone as a potent force here. He did the recent autism work and is considering the role of hormones in that disorder. He’s also done studies suggesting that exceptional athletes and math whizzes may have gotten an early high dose of testosterone. Manning has found, for instance, that some of Britain’s best soccer players tend to have extra-long ring fingers compared to the index.

I’m wary of any finding that fully associates the size of a body part with a laundry list of behaviors and abilities. Those mistakes have been made in science before, to our cost, as with the 19th-century belief that because women have slightly smaller skulls than men they are dumber. And, even if there is a statistical correlation between the 2D:4D ratio and male athletes, that still doesn’t make testosterone the sole source of athletic prowess. And it doesn’t say much about female athletes at all. In women, overall, the finger ratio is different. Index and ring tend to be closer to the same length, the index maybe a little longer.

The exception to that, for women, seems to be regarding sexual orientation, which then begs a couple of questions. Is orientation set before birth? If testosterone shapes fingers prenatally, could it shape sexual behavior as well? When scientists at UC Berkeley decided to look into this last year, they were unsure what they would find.

The Berkeley study is one of those lovely examples of scientific reasoning. How do you get a diverse sampling of finger lengths? Researchers went to street fairs in Berkeley with a portable photocopier and copied 720 fairgoers’ hands, while asking them pointed questions about their sex lives. What the Berkeley group found, published in the journal Nature, was that lesbians’ finger lengths tend to resemble the more classic male hands. Do male homosexuals have hands in the so-called female pattern? It’s not that easy, naturally, and those results have been contradictory.

Psychology professor Marc Breedlove is not sure why the results are clear with women. His speculation, though, is that it’s easier for a little extra testosterone to affect females. Males, who tend to have at least seven times as much anyway, are designed to tolerate higher levels of the hormone, whereas females “normally see pretty low levels, so even a modest increase might be registered by the brain.” Breedlove doesn’t believe that all lesbians are merely whipped up by a little extra hormone floating in the amniotic soup. Some women may become lesbian because of that exposure, he says cautiously, but not all. Hormones may influence, but their power varies from person to person.

Over the last year or so, other scientists have tested that lesbian finger result and confirmed it. The most recent study, presented at the Western Psychological Assn. in May, is by Richard Lippa and Michael Cassens of Cal State Fullerton. Lippa has been pulling together a larger test group, including college students, attendees at the Long Beach Gay Pride Festival and so on. He expects to have surveyed about 2,000 people when his results are tallied. He consistently sees the lesbian-versus-straight woman difference, although he emphasizes that it is a small statistical difference. When a scientist raises the “statistical difference” flag, it usually means that these studies tell you nothing about the individual. They are group differences: If we compare hundreds of lesbians to hundreds of straight women, there will be more male pattern hands in the lesbian group. But person by person, there will also be many straight women with longer ring fingers, gay women with the usual “female” hand and so on. Lippa also finds ethnic exceptions. Latinos seem to have, overall, the more “masculine” hand pattern, Caucasians more female. He suspects that this may be another kind of group variation, not necessarily hormonal, in the same way that height varies among ethnic populations.

So the more we look at our fingers, the more complicated this gets. The finger-length ratios are fascinating, says Lippa, because “they provide a possible measure, even if it is a very indirect and ‘noisy’ measure, of prenatal hormone exposure. Human prenatal hormone levels are very difficult to assess in any direct fashion.” So that, for him, the 2D:4D ratios become a “messy proxy” for early hormone exposure, and the ethnic variations are part of the noisy background. Consequently, cautions Lippa, “there’s simply too much variability” to draw conclusions about a person from his or her fingers. “You need large numbers of participants to see these effects,” he says.

Despite such caveats, I suspect that many people will find finger evaluations irresistible. I did. I also speak with the expertise of someone who has mentioned this work to friends, family and fellow science writers--all of whom instantly whipped out a hand for analysis. So far, all have seen the entertainment value. But what about those who might take a more serious view?

We reside in a society still judgmental about sexual orientation. It could be more than risky--downright dangerous--if people become convinced that finger length is a reliable guide to a person’s sexual preference. Critics have suggested that the danger makes the science not worth the risk. On that point, I think they are wrong. Yes, this work can be misinterpreted, despite all the scientific instructions and disclaimers. But these studies may also help correct even bigger mistakes and help counter judgmental attitudes about sexual orientation. The research strengthens the evidence that preference can be set before birth and remain beyond our control.

If finger-length studies are, yet, a messy probe into biology of behavior, then we should support research that refines them, that moves us that slight and critical step closer to a genuinely thoughtful exploration of human behavior, sexual and otherwise.

Until then, the rest of us can at least enjoy the fact that the personal finger check holds up pretty well as a gotcha joke. Made you look, right?