Without pointing any fingers, a new study suggests a way to take the measure of tough characters.
The research, done at the University of Alberta and announced Wednesday, found a connection between the length of the male index finger relative to the ring finger and the tendency to be aggressive.
No such connection was found in women.
Scientists have known for more than a century that the finger-length ratio differs between men and women. Recently, scientists found a connection between finger lengths and the amount of testosterone that a fetus was exposed to in the womb: the shorter the index finger relative to the ring finger, the higher the amount of prenatal testosterone.
The new study found such a fetus is more likely to be a physically aggressive adult, according to Peter Hurd and his graduate student Allison Bailey.
Hurd says he first thought the idea was "a pile of hooey," but he changed his mind when he saw the data, which is published in the March issue of the journal Biological Psychology.
While the study finds a connection, finger ratios only predict behavior a small percentage of the time, the researchers caution. The research was based on surveys and hand measurements of 300 undergrads at the university.
Other studies have shown that culture and upbringing affect tendencies toward violence. Exposure to violent television has also been linked to violent behavior later in life. Hurd says the new study supports other research that suggests biology plays a role.
"More than anything, I think the findings reinforce and underline that a large part of our personalities and our traits are determined while we're still in the womb," Hurd said. The connection was found only with physically aggressive behavior, not with verbal aggression or other forms of hostility.
A 2003 report in Chemical & Engineering News, a weekly newsmagazine published by the American Chemical Society, said "flawed brain chemistry, brain damage, genetic defects, an unhealthy psychological environment" all contribute to violent behavior.
Another study by Hurd, to be published in the journal Personality and Individual Differences, find that men with more feminine finger ratios are more prone to depression.
"Finger lengths explain about 5 percent of the variation in these personality measures, so research like this won't allow you to draw conclusions about specific people. For example, you wouldn't want to screen people for certain jobs based on their finger lengths," Hurd said. "But finger length can you tell you a little bit about where personality comes from, and that's what we are continuing to explore."
Hurd plans to measure the digits of hockey players next.