If you think you might be dating a psychopath, here's some bad news: They may not leave you alone when you break up.According to a new study in Personality and Individual Differences, psychopaths and narcissists are out to benefit from their relationships long after they're over.The researchers first asked people to name reasons why they've stayed friends with their exes. Then, they had a new group of people think back to times they made that decision and rate the importance of those reasons to them. They also gave them tests that measured "dark personality traits," including psychopathy and narcissism.The most common reasons people gave for staying friends with their exes involved the sense of security and sentimental value these friendships gave them, though men were more likely than women to name sex and practicality. But you know who cited the latter reasons most of all? Psychopaths and narcissists. This isn't too surprising given that these personality types are concerned with what they can gain from you above all else."Individuals who score higher on measures of dark personality tend to choose friends for strategic purposes," the authors conclude. "Thus, it is likely that these traits will be associated with valuing friendships for utilitarian or instrumental reasons, such as to maintain sexual access."So, if your ex has psychopathic or narcissistic tendencies , keep that relationship in the past where it belongs. Their desire to "stay friends" may be far from innocent.