When choosing a boyfriend, size matters.

We're talking fingers.

A new study shows that men whose ring fingers are longer than their index fingers are typically nicer to women and are less prone to aggressiveness toward other men. A bigger difference in length also indicates men who will have more children, according to PsyBlog. The study points out that this correlation is not noted in women.

Men's ring fingers (which are referred to by the study as 4D) are generally longer than their index fingers (which the study refers to as 2D). The variances in length are a result of fetal exposure to hormones called androgens. A smaller ratio (difference between the length of 2D and 4D) indicates higher androgen exposure, according to the study lead by D.S. Moskowitz, a psychologist at McGill University in Quebec. "The hormonal environment in which the male fetal brain develops may influence adult social behavior in specific contexts," the authors wrote.

The study had 155 men and women keep notes on their social interactions over the course of three weeks. After speaking to a person for longer than five minutes, the subjects had to check off behaviors they exhibited from a list provided by the study authors. The behaviors listed related to how quarrelsome or agreeable the subjects were during their exchanges.

The study, published in the March 2015 issue of the journal Personality and Individual Differences, showed that men with longer ring fingers (4D) than index fingers (2D) were 30 percent more agreeable when interacting with women.

The results showed that men with longer ring fingers in comparison to their index fingers were around 30% more agreeable - but only when interacting with women.

(HNGN does not know if the men in the study were heterosexual or homosexual and if sexual preference would be a factor in agreeableness to the opposite sex).

"When with women, men with smaller ratios were more likely to listen attentively, smile and laugh, compromise or compliment the other person," Moskowitz said, according to PsyBlog.

"It is fascinating to see that moderate variations of hormones before birth can actually influence adult behavior in a selective way," said study author Simon Young, according to PsyBlog.

How is it that men with smaller 2D:4D 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," said Moskowitz, according to PsyBlog. "This might explain why they have more children on average."

If you're wondering how you measure your (or slyly, your boyfriend's) ratio: divide the length of your index finger (2D) by the length of your ring finger (4D). The average digit ratio for men is 0.947 and 0.965 for women.