The findings might help explain why these men tend to have more children. The study, showing a link between a biological event in fetal life and adult behaviour, was published in the journal Personality and Individual Differences.

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 as a fetus: the smaller the ratio, the more male hormones. The McGill study suggests that this has an impact on how adult men behave, especially with women.

“It is fascinating to see that moderate variations of hormones before birth can actually influence adult behaviour in a selective way,” says Simon Young, a McGill Emeritus Professor in Psychiatry and coauthor of the study.

Smiles and compliments

Several studies have been conducted previously to try to assess the impact of digit ratio on adult behaviour. This one is the first to highlight how finger lengths affect behaviour differently depending on the sex of the person you are interacting with. “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 at McGill. 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 though, digit ratio variation did not seem to predict how they behaved, the researchers report.

Digit ratio and children

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 behaviours they engaged in. Based on prior work, the scientists classified the behaviours as agreeable or quarrelsome. Men with small digit ratios reported approximately a third more agreeable behaviours and approximately a third fewer quarrelsome behaviours than men with large digit ratios.

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 behaviours 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.