Jan. 13, 2008 -- Men who have a shorter index finger relative to their ring finger proved to be better at high-stakes, fast-paced stock trading than men with relatively longer index fingers, according to a new study.

John Coates, the lead author on the stock trading study, published yesterday in the journal Proceedings of the National Academy of Sciences, was initially uncertain that digit ratios would tell him anything useful.

"I didn't put much stock in it to tell you the truth, until I saw the results -- I almost fell out of my chair," said Coates, of the Judge Business School and department of physiology, development, and neuroscience at University of Cambridge, England.

What Your Fingers Can Really Measure

In the study, researchers measured the right hands of 44 male stock traders who were engaged in a type of trading that involved large sums of money, rapid decision-making and quick physical reactions.

Over 20 months, those with longer ring fingers compared to their index fingers made 11 times more money than those with the shortest ring fingers. Over the same time, the most experienced traders made about nine times more than the least experienced ones.

Looking only at experienced traders, the long-ring-finger folks earned five times more than those with short ring fingers. Coates said he turned to the digit ratio because he was searching for a convenient marker for a person's sensitivity to testosterone.

More accurate physical markers of testosterone sensitivity include a measurement in the inner ear, and the "ano-genital" distance, but Coates thought stock traders were more likely to offer up their hands to measurements than their rear ends.

Exactly how or why is unclear, but studies have shown the relative length of the ring finger is related to the amount of androgens (testosterones) present in a baby's body during the 8th to 19th week of pregnancy.

Testosterone and Your Ring Finger

Girls may have some testosterone during that time, but the hormones surge through boy's bodies.

"When they do so, they basically create the male body and brain," said Coates. "It affects your body, your metabolism, the structure of your bones."

The higher the testosterone level during that crucial time, the more likely the ring finger will grow, and the more sensitive the brain is to testosterone for life, said Coates.

To find your digit ratio, divide the length of the index finger (called the 2D or second digit) by the length of the ring finger (called the 4D). It's easiest to measure from the crease of the finger at the palm to the tip.

Large surveys have shown that men tend to have lower digit ratios (short 2D and long 4D) and women tend to have a ratio of 1 or greater.

Coates, who used to run a derivative trader desk on Wall Street in the dot-com boom era, is in the midst of a long investigation into how testosterone affects market behavior. The digit ratio study may be only one in a series of investigations.

"During the dot-com bubble I was convinced there was some chemical in people's bodies that were making them go nuts. They were manic, delusional; they had diminished need for sleep," said Coates. "It got me thinking that there was some chemical affecting these people."

Coates has done previous studies (without digit ratios) measuring blood testosterone and trading performance. He hypothesizes a cycle of testosterone-fueled winning streaks and subsequent over-confidence contributes to stock market bubbles.

"I thought about why people lose their wits when they were caught up in a bull market," said Coates. "There was another thought that occurred to me: that women were not as affected by the bubble."

Scientists have looked at finger ratios for years, coming up with a variety of conclusions, some controversial.

Homosexuality, aggression, elementary school math scores, and even skiing ability have all been linked to finger measurement before.

In 2000, psychologist S. Marc Breedlove, now at Michigan State University, photocopied more than 700 people's hands at street fairs in San Francisco and anonymously asked them about their sexuality.

Breedlove found the lower the digit ratios (a shorter index finger and longer ring finger) the more likely the woman would be a lesbian. But he found no difference between low digit ratios and men's sexuality.

Goodfinger, Badfinger

Last year, researchers in the U.K. studied 2,000 people's hands, hips, knees and risk factors for arthritis. All things being equal, they found a person with a low digit ratio is more likely to develop osteoarthritis of the knee.

Scientists have even started to use the digit ratio to study student aptitude.

In 2007, U.K. researchers found a link between 7-year-olds' standardized test scores and their digit ratios; the lower the digit ratio, the higher the math scores.

But, digit ratios are not that clear cut. Plenty of women have the stereotypically male digit ratios, and plenty of men have the stereotypically female ratios. Moreover, digit ratio can vary by ancestry.

That difference has some researchers skeptical of every digit ratio study to hit the news.

"It's easy to administer some questionnaires and measure finger length -- the literature is probably full of those," said David Andrew Puts, an assistant professor in the department of anthropology at Penn State University in University Park, Pa.

"I think that it's a really intriguing marker but I tend to be really skeptical of these results until I see it replicated by a bunch of different studies," he said.

Puts has published studies using digit ratios and says he may use them in the future. However, he's followed up on many of the so-called digit ratio traits. For example, Puts reviewed studies of spatial relation skills. While one study found better spatial relation skills with a low digit ratio, comparing the findings of many other studies did not.

The connection "was essentially zero," said Puts.

Simon LeVay, a former neuroscientist who researches the brain and sexuality, has a similar reaction to digit length studies.

"It's a serious field of study," said LeVay, noting there is evidence linking exposure to testosterone to finger length.

But LeVay cautions against personal digit reading for various traits. Across hundreds or thousands of people, a reliable statistic correlation may be found.

"It's a kind of weak label, if you'd like," he said. "You can't say anything about any one individual by looking at finger length ratios."

The AP contributed to this report.