Life isn’t always black and white. But a new study suggests that when it comes to the likelihood of staying faithful to your partner, things may be more cut and dry than you think. The authors suggest that humans belong to one of two groups — promiscuous or loyal to one partner — and this can be discovered by the length of your fingers.

The study, which was published in the journal Biology Letters, was carried out as a joint collaboration between Professor Robin Dunbar’s lab at Oxford University and Professor John Manning at Northumbria University.

Typically, species are either considered promiscuous or faithful: swans, wolves, penguins, and even critters like cockroaches either mate for life or remain monogamous to one partner for a long period of time. While there’s biological motivation for sowing one’s seed among plenty of partners, evidence also suggests that staying with one love can be beneficial to both survival and offspring. For the most part, all animals are either faithful or not; interestingly, humans are a mix of both promiscuous and faithful — we’re something of a “mid-way” species.

“This research suggests that there may be two distinct types of individuals within each sex pursuing different mating strategies,” Dr. Rafael Wlodarski of Oxford University’s Department of Experimental Psychology said in the press release. “We observed what appears to be a cluster of males and a cluster of females who are more inclined to ‘stay,’ with a separate cluster of males and females being more inclined to ‘stray,’ when it comes to sexual relationships.”

The Telling Length Of Fingers

In the study, the researchers analyzed a survey from 575 North American and British participants about their desires and opinions on sex with no strings attached, or “non-committal” sex. They also examined photocopies of the right hands of 1,314 British men and women, using them to measure the length of the index and ring fingers.

Past research has shown that the lengths of your fingers can tell a lot more about your genetic predisposition, personality, and sexuality than you might imagine. The ratio between the lengths of your index and ring finger, known as the 2D:4D ratio, can be quite telling when it comes to hormone levels and physical qualities.

For example, the shorter someone’s index finger is in relation to the ring finger, the more testosterone they were exposed to in the womb. Higher levels of testosterone exposure in the womb are typically linked to greater sexual promiscuity as an adult. In studying the participants’ hands, the researchers found that people were “bimodal,” meaning they were able to fit into two distinct groups: one more likely to be licentious, and the other more loyal; and the length of the person’s ring finger helped point to who was more promiscuous. People with longer ring fingers were more likely to seek out higher numbers of sexual partners, and both men and women were bimodal.

But the authors warn that it’s quite difficult to decide, on an individual level, who will stay or stray: even if you look at their fingers. So just because your ring finger is significantly longer than your index finger doesn’t mean you’re one to go out on the town; perhaps you’re in a loving monogamous relationship. Besides, there are plenty of other factors that goes into a person’s sexual life, including environment and past experiences; which in and of itself proves the point that again, nothing is black and white.

“It is important to note that these differences are very subtle, and are only visible when we look at large groups of people: we cannot really predict who is going to be more or less faithful,” Dunbar said in the press release. “Human behavior is influenced by many factors, such as the environment and life experience, and what happens in the womb might only have a modest effect on something as complex as sexual relationships.”

Source: Wlodarski R, Manning J, Dunbar R. Stay or stray? Evidence for alternative mating strategy phenotypes in both men and women. Biology Letters. 2015.