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People tend to have strong opinions about whether you can stay friends with an ex or not. You may be optimistic about the situation, and are happy to become friends soon after a break up, or you may think that when you cut the cord it means goodbye forever.

It's not an exact science either way, but according to one study, a tendency to keep in contact with exes could be a sign of something sinister.

The research, published in the journal Personality and Individual Differences, found that how people relate to their ex partners could be a new test for psychopathy.

Generally, a psychopath is someone with dark triad personality traits — these encompass psychopathy, narcissism, sadism, and Machiavellianism.

Most of us fall on the spectrum somewhere, but what sets a psychopath apart is a complete lack of empathy. They don't have any sympathy for others, and everything they do is for their own gain or amusement.

The study, performed by researchers at Oakland University, looked at the personality traits of 861 subjects, as well as their relationship histories. They were asked about their current partners and whether they were friends with exes, then given a questionnaire to determine narcissistic and psychopathic qualities.

Overall, those with the most dark personality traits were more likely to report staying in contact with former lovers. This wouldn't be too alarming on its own, but it was the reasons they gave for the prolonged contact that was most worrying.

Having exes around provides opportunity for the psychopaths to still get access to certain resources, such as information, money, or sex.

Psychopaths are very charming, and hook their victims with great intensity. However, this also means it's hard to let them go. Psychopaths and narcissists tend to target very empathetic people, because they can mistreat them as much as they want without them leaving. By sporadically giving them hope, they can successfully string along multiple people at once.

By collecting bonds with ex partners that can't stay away, psychopaths reported their "strategic" motivations, such as the "practicality and the chance of hooking up."

"The thing about a [psychopaths] is they treat you or anybody as meat," Perpetua Neo, a doctor of psychology and expert in dark personality traits, told Business Insider.

"It's like how crocodiles store meat under a rock. Whenever they feel like it, they just take bites of this meat. Psychopaths and narcissists use people just like that, so if you're going to keep getting connected with them, that's when it exacerbates this trauma bond, and you find it very difficult to break away."

The study found the psychopaths reported picking friends with a similar agendas. This in turn helps them with keeping their victims around. Neo told Business Insider that friends help psychopaths spin their lies, and make their partners feel like the behaviour is normal.

"They say 'you've met all my friends, this is how we act,'" she said. This normalises the situation and can make partners hang around for even longer, because they start to think that's what a relationship is like.

This doesn't mean that if you're friends with an ex or two you are a psychopath. But if your ex does have a habit of collecting ex partners as "friends," you might want to re-examine their motivations, and distance yourself if anything makes you feel uneasy.