Tuesday, October 17, 2006
You've checked out your lover's zodiac sign, and you know what her sleep number is. Before taking the next step, though, you might want to have a close look at her fingers.
A new study in a British medical journal finds a link between the relative length of a woman's index and ring fingers and her athletic prowess. The research takes its place among dozens of other studies tying that ratio -- known in finger-measurement circles as 2D:4D (the relationship between the length of the second digit, 2D, and the fourth) -- to all manner of physical and psychological traits, from breast cancer risk to schizophrenia.
Digit ratio, as the measurement is called, has been found to relate to left-handedness and autism, to hyperactivity and bullying in children, eating disorders in women and depression in men.
You're staring at your own fingers this very minute, right? Well, before you put too much stock in what you see there, read on.
It has long been known that men's ring fingers are usually longer than their pointer fingers (giving them a low digit ratio, calculated by dividing the length of 2D by the length of 4D). Women's second and fourth fingers are generally equal in length.
But nobody was sure why the difference exists or what to do with the information until, in 1998, British researcher John Thomas Manning suggested that the difference between male and female digit ratios stemmed from prenatal exposure to the hormones testosterone and estrogen. If the digit ratio -- which is established by the time a fetus is 9 weeks old and remains constant throughout a person's life -- reflects the level of that exposure, Manning reasoned, then it might serve as a marker for other conditions -- including predisposition to many diseases -- thought to be affected by prenatal hormone exposure.
That gave scientists a good reason for examining digit ratio. Since he offered his hypothesis, Manning reports, about 150 digit-ratio studies have been published. Manning, a professor of evolutionary psychology at the University of Central Lancashire, published a compendium of 2D:4D findings in his 2002 book "Digit Ratio: A Pointer to Fertility, Behavior, and Health" (Rutgers University Press).
The current study, conducted by researchers at St. Thomas' Hospital in London and published in the British Journal of Sports Medicine, examined hand X-rays from 607 women, all of them participants in a larger study of twins. While a strong correlation was found between low (or more manlike) digit ratio and self-reported athletic achievement, researcher Timothy Spector reports that the study actually lent added weight to the influence of genes, rather than prenatal hormone exposure.
Studying twins allowed his team "to look at the relative influences of genes and environment on finger ratios," Spector wrote via e-mail. "We found that 66 percent of the differences between people were due to genes -- i.e., heritable -- with no real influence of common or womb environment."
According to Spector, his study confirms that digit ratio is important in reflecting females' athletic prowess -- a finding already established among males. But, he wrote, "The twin study casts some doubt on the original testosterone in utero theory, as we would have expected to see an effect of the fetal environment influencing our twin studies. Our results suggest genes and not hormone levels are the predominant force in shaping sporting potential, and finger length is just a marker."
Whether a key influencing factor or a just a marker, digit ratio and its potential implications are likely to attract continued scientific interest. But what distinguishes 2D:4D from other attempts -- such as phrenology (the practice of "reading" skull shape as an indicator of character and psychology) and palm reading -- to divine information about people by measuring their body parts?
"Science is about theories and data," Manning said in an e-mail. "As far as I can see, phrenology, palmistry, etcetera has plenty of the first and very little of the latter."
Marc Breedlove, professor of neuroscience at Michigan State University, concurred. "I would say a big difference is that today's scientists are gathering data," he said in an e-mail. "If phrenologists had actually gotten measures from hundreds of people and tried to correlate with various traits, they might have found something (although I doubt it). Their notions of which head bump related to which behavior was all based on armchair conjecture. Having come up with an elaborate, and basically arbitrary, framework, they were careful to see what would confirm their pre-judgement."
Despite its grounding in science, digit ratio has its shortcomings. First, accurate measurement of finger length is no task for the layperson; it's a lot trickier than it might seem. "Digit ratio is not easy to measure because one is measuring very small differences between the index and ring finger," Manning explained. "Some [researchers] measure directly from the fingers (which is good), others from photocopies and scans of the fingers (not so good), a few from tracings around the fingers (very bad)." And, Manning added, digit ratio varies among ethnic groups: Caucasians tend to have high 2D:4D, while black and East Asian people tend to have low 2D:4D, he notes.
In any case, as Breedlove observes, digit ratio is not a great tool for making predictions about individuals; it's more useful at assessing probabilities for groups of people.
"I think some are looking for markers to help identify people at risk, and I think John Manning is one of these," Breedlove wrote. "He may be right -- it's too soon to say -- but I'm a bit skeptical that they will ever be useful for identifying probabilities of the outcome of individuals. I just don't think they reflect prenatal testosterone accurately enough for that to work."
So, is there anything parents or physicians can do to manipulate or control their babies' prenatal exposure to testosterone and estrogen?
"There probably is," Manning allowed. "But we shouldn't even think of trying." ·
Jennifer Huget is a regular contributor to the Health section. Comments:firstname.lastname@example.org.