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Email You are free to share this article under the Attribution 4.0 International license. University McGill University

New research links men’s finger length ratios with their behavior, especially while interacting with women.

Men’s index fingers are generally shorter than their ring fingers. The difference is less pronounced in women. Previous research has found that digit ratio—defined as the second digit length divided by the fourth digit length—is an indication of the amount of male hormones, chiefly testosterone, someone has been exposed to in the womb: the smaller the ratio, the more male hormones.

“It is fascinating to see that moderate variations of hormones before birth can actually influence adult behavior in a selective way,” says Simon Young, an emeritus professor in psychiatry at McGill University and coauthor of the study.

Several past studies have tried to assess the impact of digit ratio on adult behavior. This one is the first to highlight how finger lengths affect behavior differently depending on the sex of the other person in an interaction.

Lovers and fighters

“When with women, men with smaller ratios were more likely to listen attentively, smile and laugh, compromise, or compliment the other person,” says Debbie Moskowitz, lead author and professor of psychology.

They acted that way in sexual relationships, but also with female friends or colleagues. These men were also less quarrelsome with women than with men, whereas the men with larger ratios were equally quarrelsome with both. For women, however, digit ratio variation did not seem to predict behavior, the researchers report.

For 20 days, 155 participants in the study filled out forms for every social interaction that lasted 5 minutes or more, and checked off a list of behaviors they engaged in. Based on prior work, the scientists classified the behaviors as agreeable or quarrelsome. Men with small digit ratios reported approximately a third more agreeable behaviors and approximately a third fewer quarrelsome behaviors than men with large digit ratios.

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A previous study had found that men with smaller digit ratios have more children.

“Our research suggests they have more harmonious relationships with women; these behaviors support the formation and maintenance of relationships with women,” Moskowitz says. “This might explain why they have more children on average.”

The researchers were surprised to find no statistically relevant link between dominant behaviors and digit ratios. They suggest future research could study specific situations where male dominance varies—such as competitive situations with other men—to see whether a correlation can be established.

The study appears in the journal Personality and Individual Differences.

Source: McGill University