We’ve all gone through the same song and dance — we meet someone we’re compatible with, we date, we fight, we kiss, we make up, and then we break up — unless we’re married to our first love. In between the sad songs, the tissues, and moving boxes, our ex says the unimaginable: “we should stay friends.” This phrase is uttered to soften the wound in our broken heart, but does an ex really want to be friends?

A recent study published in the journal Personality and Individual Differences showed that the real reason our “psycho ex” wants to stay friends is for personal gain.

Lisa Welling and Justin Mogilski, study authors and psychologists at Oakland University in Michigan, believe some couples are able to maintain a friendship after breaking up because doing so serves as a tradeoff of “desirable resources,” such as money, sex, or information.

"This research builds upon literature examining [cross-sex friendships] CSFs and suggests that [post-relationship friendships] are functionally similar insofar as they permit continued exchange of desirable resources,” wrote the researchers.

The researchers polled 860 people about their reasons for staying friends with an ex. The respondents were then asked to fill out questionnaires designed so researchers could determine what they called manipulative personality traits, like narcissism, psychopathy, or Machiavellian instincts to deceive and exploit others to achieve goals. Previous research suggests those with such traits often make friends purely for “strategic” reasons.

The findings revealed people who had "measures of dark personality" were more likely to maintain relationships with their exes for "practical and sexual reasons." Specifically, those who scored high for narcissism were more likely to choose "practicality and the chance of hooking up" as reasons for prolonging a former romantic relationship.

This isn't surprising since people in romantic relationships tend to have better psychological health. A 2010 study found those in committed relationships experience significantly fewer mental health problems than single people. Meanwhile, people not in serious relationships who had more sexual partners, were more prone to physical and mental health problems.

Moreover, healthy and intimate relationships can also lead to a longer life, decreasing the probability of death. In a 2011 study, researchers found that being continuously married as opposed to being continuously single or transitioning between separation and divorce, led to longer lives among men with low income and socioeconomic inequalities. Despite those who have undergone divorces, married people have more supportive social groups and children that encourage continuing family contact, therefore, extending their life expectancy.

With a list of scientifically-proven benefits, it's no surprise some exes may want to keep a former partner close or rekindle what they once shared. Narcissists long to stay connected to valuable resources because dark personality types are more interested in how relationships can be useful to them and what benefits they can continue to receive. Also, researchers theorize narcissists also have inside information about their ex’s vulnerabilities and weaknesses, which they can exploit and manipulate to get power and control.

However, this doesn’t mean all exes who are friends are narcissists. Previous research has found the existence of a friendship before the onset of a romance can also determine if you stay friends after a breakup. Regardless of whether you did the dumping or were the one who was dumped, if you were friends with your ex before your relationship, you’re more likely to remain friends after the relationship is over.

Although narcissists may have their ulterior motives for being friends with their exes, not all exes are narcissists, and not all relationships need to end in heartbreak.

So, although a number circumstances determine the likelihood we’ll stay friends with an ex after a breakup, we have to ask ourselves why we want to be friends with them in the first place.

Our reason for rekindling a friendship will help answer the age-old question, “Can you be friends with your ex?”

Source: Mogilski JK and Welling LLM. Staying friends with an ex: Sex and dark personality traits predict motivations for post-relationship friendship. Personality and Individual Differences. 2016.