Can the length of our fingers really determine promiscuity?

Eyes may be the window to the soul, but our hands may offer a window into romantic faithfulness.

In a new study published in Biology Letters, scientists at Oxford University set out to discover whether the ratio between a person’s ring and index finger could reveal the likelihood that he or she would cheat on a romantic partner. In other words: Could promiscuity be genetic?

Turns out there’s a whole body of (controversial) research exploring the index-to-ring-finger ratio and the role it plays in our lives. The current theory is that the ratio is a biological marker that can reveal how much testosterone a person was exposed to in the womb. A low ratio—meaning the ring finger is longer than the index finger—means more testosterone exposure. Men’s ratio, for example, tends to be lower than women’s, signifying their greater exposure.

Previous studies have found correlations between a person’s digit ratio and fertility, athletic ability, and social behaviors—though, again, the findings are debated. The latest study suggests that ratio could predict a person’s mating habits as well.

“The length of your fourth finger is affected by some of the same genes which code for some of the differences between the sexes,” Rafael Wlodarski, a post-doctoral researcher in experimental psychology at Oxford and co-author of the study, told Fusion. He’d heard that having a long ring finger may be correlated with higher promiscuity and set out to explore the connection.

To study a possible link, Wlodarski and his team compared two different data sets. First, they gave 595 people from Britain and North America a questionnaire about sexual attitudes and habits, known as the Sociosexual Orientation Inventory. Then they measured the digit ratios of 1,314 Brits. They saw a similar distribution of promiscuity rates and digit ratios across the two sets, leading them to believe there might be a correlation.

By comparing the two sets, the researchers found that people of both genders may fall into two distinct mating categories, with “one set tending towards greater promiscuity,” said Wlodarski. His research also suggested men were more likely to “stray” than women. Fifty-seven percent of men fell into the more promiscuous category, compared to 47 percent of women.

Perhaps most notably, the findings suggest that promiscuity could, in fact, be genetic.

Most mammals are either monogamous or not, explained Wlordarski, but not humans. We fall on a spectrum that can’t be explained by biology. “If human males and females actually consist of two distinct subtypes”—promiscuous versus not— “then this would go some way to explaining this interesting conundrum,” he said.

So should everyone go measure his or her partner’s ring finger immediately?

“There are dozens of factors which, all combined, can affect how promiscuous an individual may be, and finger length is just one potential, and very subtle, factor,” said Wlordarski. “At the individual level, it is impossible to predict anything.”

Duly noted. Now where’s my tape measure?

Taryn Hillin is Fusion’s love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.