This answer is in the ring finger. How ironic?
Stop snooping through his phone. Sign out of his Facebook account. End your search for suspicious hair ties in his car or the smell of perfume on his collar. Science found the easiest way to determine if your partner is a lying, cheating a-hole: Look at their ring finger.
In a new study from Oxford University, called "Stay or stray? Evidence for alternative mating strategy phenotypes in both men and women", researchers assessed 1,314 hands, including 572 men and 742 women. (Fun stuff, huh?) After looking at the length of the index finger compared to the ring finger, they discovered that both men and women who have a right ring finger that's much longer than their index finger on the same hand are more likely to be unfaithful.
Scratching your head (or looking at your own hand)? The idea behind it is that the length of your ring finger reveals of the amount of testosterone you were exposed to in the womb, which is linked with promiscuity.
The study also found that men and women are inclined to either stay or stray. According to News.Com.Au, it's the first study to show different mating approaches are used by both men and women, though the study did find that males were slightly more likely to be promiscuous.
So, what do you do if your partner's ring finger is much longer than their index finger? Aside from screaming, "WHO IS SHE?!" at the top of your lungs, remember that finger lengths are determined before birth, and that there are plenty of other factors that influence whether your partner will hook up with the neighbor or not.
"There's really almost no one that exists who's purely promiscuous or purely monogamous," Researcher Ralph Wlodarski told Bloomberg. "Everything we are is a combination of both our genetics and our environment. There’s massive room for variation, and that variation will depend on your upbringing, on your early relationship experience, on your development, and on aspects of free will and aspects of making decisions based on your experiences."
And, if all else fails, add "even finger lengths" to your list of non-negotiables—science says people with even digits are likely to be in it for the long haul.