Finger Ratio: Science or Palm Readings?

Testosterone and Your Ring Finger

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

Studies about finger length ratio abound, but researchers are divided about what the difference can mean for mental traits.

(Getty/ABC News)

"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.

Related

WATCH: Testosterone Levels Affect Overall Health

WATCH: Gender Math

Why Do Women Leave Science Careers?

"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.