Recently, scientists in North America and Europe have looked to the relative lengths of index and ring fingers for clues about a variety of characteristics, including musical ability, athletic prowess and, in a study just released, osteoarthritis risk.

The researchers believe that the difference between the two fingers' lengths signifies the level of testosterone exposure in the womb. The longer the ring finger compared to the index finger, the thinking goes, the higher the exposure.

Scientists express the fingers' relative lengths as a ratio, computed by dividing index finger length by ring finger length. Men tend to have longer ring fingers than index fingers, or ratios less than 1, and women tend to have index and ring fingers of equal length, or ratios of 1.

Don't worry if your finger ratio looks to be more like that of the opposite sex, says Marc Breedlove, professor of neuroscience at Michigan State University. There's less of a sex difference in finger ratios than there is in height, he says.

"I wish it was a better marker … of prenatal testosterone," he says. "It's not a very good correlation. It's easy to find women who have more masculine ratios than some men."

Still, Breedlove says, short of a time machine, he doesn't know of a better tool with which to assess prenatal testosterone exposure.

Just made the connection

Giacomo Casanova, the famous womanizer who died in 1798, observed in his memoirs that the ring finger is longer than the index finger.

But it wasn't until 1998 that British psychologist John Manning first linked the index-ring finger ratio to prenatal hormone levels.

"It's been known for about a hundred years that there's this tiny sex difference in the ratio, but it's so small that one wouldn't think it's particularly important," says Manning, who recently retired from the University of Central Lancashire and is now associated with Southampton University.

Manning had been studying whether body asymmetry — in which, say, a finger on one hand is longer than the same finger on the other hand — is linked to such traits as fertility. He noticed that in young boys, but not young girls, ring fingers tended to be longer than index fingers. He speculated that prenatal hormone exposure played a role.

"The sex difference almost certainly arises before birth," Manning says, adding that it can be seen in fetuses at nine weeks' gestation, "and it doesn't change at puberty."

Since 1998, Manning has published studies suggesting that male symphony orchestra musicians have lower finger ratios than less-musical men, that heterosexual men have lower ratios than homosexual men and that people with lower ratios tend to do better on certain tests of spatial ability.

But "the links with sports are the strongest I've found," Manning says. "They're particularly strong with endurance running." He theorizes that prenatal testosterone benefits the cardiovascular system.

"I think the goal is to see whether you can find any evidence that prenatal testosterone makes any difference at all," Breedlove says. "If you do see a relationship between the digit ratios and whatever symptom you're looking at, then you have to wonder."

For example, he says, "how might prenatal testosterone influence how your joints feel when you're 55 years old? Ten years ago, no one would have even asked the question."

The link to osteoarthritis

British rheumatologist Michael Doherty and his collaborators at the University of Nottingham did just that in a study in the January issue of Arthritis & Rheumatism.

Osteoarthritis is more common in men, Doherty says, and, he and his co-authors write, increased activity and physically demanding sports could contribute to the condition through repetitive joint trauma. So it makes sense that a lower finger ratio, thought to be more common in men and in athletic individuals, would be linked to a higher osteoarthritis risk.

By comparing about 2,000 osteoarthritis patients with about 1,000 people without osteoarthritis, Doherty's team found that is indeed the case. The strongest link: osteoarthritis of the knee in women whose ring fingers were longer than their index fingers.

Even after accounting for such osteoarthritis risk factors as physical activity and higher current testosterone levels, Doherty and his co-authors found that a relatively long ring finger was itself a risk factor. If they had studied elite athletes, though, perhaps they would have seen a link between physical activity and osteoarthritis risk, Doherty says, noting, "we're just one study."

Although finger ratio is easily measured, says Michael Peters, a psychology professor at Ontario's University of Guelph, "I don't see it becoming a powerful diagnostic predictor anytime soon."

But, Manning says, one country hopes the tool will help identify future athletes. He is working with Qatar's Aspire Sports Academy, whose vision, according to its website, "is to discover the best young sporting talent … and transform them into world-renowned champions."

Manning's goal: to prove that finger ratio at age 10 predicts athletic ability at age 18.