If divining personality from finger length sounds like nonsense, Peter L. Hurd understands. An assistant professor of psychology at the University of Alberta, Hurd thought that such efforts “seemed like palmistry.” But now he is a believer.

Research had shown that the shorter a male's index finger is relative to his ring finger, the more testosterone he was exposed to as a fetus. Hurd has since found that men with a greater disparity are more prone to be physically aggressive throughout life. (There is no correlation for females.)

Although the association isn’t strong enough to predict the trait, it is stronger than the relation between adult testosterone levels and aggression, a sign that “the causal effect of testosterone seems to be in the womb,” says Hurd, co-author of the just released study of 300 volunteers. “The take-home message,” he adds, “is that hormones during development explain far more variation in human behavior than hormones during adulthood.” Still skeptical? Bet you’ll find it hard not to compare hands and personalities at your next party.