How long a man’s second finger is relative to his fourth finger appears to predict whether he is prone to be physically aggressive towards others, according to a new study by researchers at the University of Alberta, Edmonton, Canada.

But it’s not finger length that causes aggression, study author Allison A. Bailey warned in an interview.

She explained that the important factor is the male hormone testosterone.

Fetuses are exposed to various levels of this hormone in the womb, and research shows that men who were exposed to higher levels tend to have shorter second fingers, relative to their fourth fingers. “More testosterone, relatively longer ring finger,” co-author Dr. Peter L. Hurd told AMN Health.

Men with shorter second fingers were probably exposed to more testosterone in the womb, and this may cause them to be more prone to physical aggression later in life, Bailey explained. “More testosterone in the womb predicts more physical aggression in men,” she told Reuters Health.

Alternatively, both testosterone exposure and finger length may be influenced by a “deeper third variable,” Hurd noted. Regardless, the study shows that “events in the womb can have subtle effects on children’s personality,” Hurd added.

This is not the first study to link the ratio between a man’s second and fourth fingers to his personality, Bailey noted. Previous reports have found that men with smaller ratios - meaning, their second finger is much smaller than their fourth - tend to do better in sports, and are perceived as more dominant and masculine by women.

However, other research has shown that men with smaller second-to-fourth finger ratios are at higher risk of autism and immune deficiency.

To investigate how finger ratios match up with physical aggression, Bailey and Hurd measured the finger ratios in 298 psychology students, and asked them to complete a questionnaire measuring aggression.

The questionnaire measured four types of aggression, Bailey noted: physical (“if someone hits me, I hit back”), anger (“I flare up quickly”), hostility (“I am often eaten up by jealousy”), and verbal (“I tell my friends when I disagree with them”).

The researchers found that shorter second-to-fourth finger ratios predicted proneness to physical aggression, but not other types of aggression, and only in men, not in women.

In general, men had smaller finger ratios than women, the authors report in the journal Biological Psychology.

Bailey cautioned that these findings only link a shorter finger ratio to a tendency towards physical aggression, and do not show that men with shorter ratios are actually more aggressive. “Somebody might never have acted on anything,” she said.

Moreover, behavior is influenced by many other factors, Bailey added. Finger ratios are “just one piece of the puzzle,” she said.

SOURCE: Biological Psychology, March 2005.