This could make sense, the team at Ohio State University said. Factors such as smoking or drinking during a pregnancy could stress a fetus in various ways, causing not only slight physical imperfections but also poorer impulse control.

"Stressors during pregnancy may lead to asymmetrical body parts. The same stressors will also affect development of the central nervous system, which involves impulse control and aggression," said Zeynep Benderlioglu, who led the study.

"So while asymmetry doesn't cause aggression, they both seem to be correlated to similar factors during pregnancy."

Benderlioglu, Randy Nelson, a professor of psychology and neuroscience, and Paul Sciulli, professor of anthropology, reported their findings in the American Journal of Human Biology.

They told 100 college students they were taking part in a study of persuasive ability by asking them to call people to raise money for charity.

But their calls went to two experimenters who followed a careful script, either apologetically saying they did not have money to donate, or becoming confrontational and challenging the caller and the charity.

The researchers had rigged the phones so they could measure how hard participants slammed the receiver down after the call.

The more asymmetrical their ears, fingers and feet, the more force the volunteers tended to use when hanging up, they found.

Women were more likely to slam the phone when challenged, while men seemed angrier when politely turned down, they found.

"Research has shown that men are quicker to anger than are women," Benderlioglu said. "But while unprovoked men are generally more aggressive than women, the gender differences either disappear under provocation, or women may actually become more aggressive than men."

It could be men just dropped the aggressive call before it escalated, she added.

http://edition.cnn.com/2004/TECH/science/08/24/science.aggression.final.reut/index.html