A man’s hand may be all you need to find out how nice he is toward women. Researchers from McGill University found a link between a baby boy’s exposure to hormones in the womb, their hand development, and their behavior toward women as adults. They published their findings in the journal Personality and Individual Differences.

“It is fascinating to see that moderate variations of hormones before birth can actually influence adult behavior in a selective way,” said the study’s co-author Simon Young, a psychiatry professor at McGill University, in a press release. The study may also provide a new research perspective on why certain men tend to have more children. 

The smaller the ratio is between a man’s index finger (pointer) and his ring finger, the nicer his behavior is toward women, according to the findings. Finger length is determined by the levels of male hormones, particularly testosterone, a baby is exposed to while in their mother’s belly. The lower the male hormone levels are, the kinder the man is toward a woman. 

“When with women, men with smaller ratios were more likely to listen attentively, smile and laugh, compromise, or compliment the other person,” said the study’s lead author Debbie Moskowitz, psychology professor at McGill University, in the press release."Our research suggests they have more harmonious relationships with women; these behaviors support the formation and maintenance of relationships with women. This might explain why they have more children on average."

Researchers studied the behavior of 155 participants over 20 days, evaluating their social interactions, levels of agreeableness, and frequency of quarrels. They found men with smaller differences between their ring and index fingers' length were a third more agreeable and a third less quarrelsome. This isn’t the first time fingers have been able to predict behavior. Hands have now become clues to what happened to a person while they were in the womb, and have also been linked to fertility, female assertiveness, happiness, and even a person's susceptibility to certain diseases. 

Researchers from Northumbria and Oxford Universities found that the length of the ring finger when compared to the index finger could also reveal infidelity. The study analyzed finger length of both men and women and found that the shorter someone’s index finger is in relation to their ring finger, the more testosterone they were exposed to in the womb. Sexual promiscuity is linked to higher levels of testosterone, which means a simple measurement of the fingers could clue you in to your partner's likelihood of having a higher amount of sexual partners, or their tendency to wander.

If you grab your partner’s hand, however, and discover their ring finger is significantly longer than their index finger, it’s not a guarantee they’ll cheat or that they’ll be less nice to women — it's just a risk factor after all. 

Source: Young SN, Moskowitz DS, Sutton R, and Zuroff DC. Fetal exposure to androgens, as indicated by digit ratios, increases men’s agreeableness with women. Personality and Individual Differences. 2015.