The more similar a person is to a psychopath, the more likely he or she is to assault a romantic partner, a new study suggests.

Researchers at the University of British Columbia’s Okanagan campus pored over data and police reports on 700 psychiatric patients in Pittsburgh, Pennsylvania, and 870 students at the school, according to a statement from the university. “We noted that having higher levels of psychopathic personality traits is an important predictor of how likely someone is to engage in intimate partner violence,” Zach Walsh, an associate professor of psychology and the study's principal investigator, said in the statement.

Psychopathy is a largely untreatable psychological disorder affecting about 1 percent of the total population, yet it accounts for about a quarter of male prison inmates. Psychopaths seem charming and normal but on the inside have no conscience or empathy, “making him manipulative, volatile and often (but by no means always) criminal,” Psychology Today explains. They tend to be violent and disregard laws, social rules and the feelings of others. However, “psychopathic criminals are cool, calm, and meticulous” — those individuals plan every detail of their crimes and can be hard to catch. Because they are skilled manipulators, good at mimicking normal behavior, they often go unsuspected.

People with those tendencies and personality traits were found to be consistently more violent in both groups recently studied, including psychiatric patients (who were not imprisoned) in the United States and Canadian students. The study, published in the journal Law and Human Behaviour, also found that those people tended to drink more alcohol, but “the data tells us it is their personality traits more than substance use that is associated with violence,” Walsh said. “With further investigation, this research may be able to assist policymakers and service providers in their efforts to both predict and reduce violence among couples.”

Famous examples of psychopaths include serial killer and cannibal Jeffrey Dahmer; the Nazi and Auschwitz concentration camp doctor Josef Mengele, also known as the “Angel of Death”; and Hungary’s Countess Elizabeth Bathory, who brutally tortured and murdered dozens of people in the 16th and 17th centuries.

The psychopaths who are not criminals, Scientific American explains, are still “self-centered, dishonest and undependable, and at times they engage in irresponsible behavior for no apparent reason other than the sheer fun of it.” They will have shallow romantic relationships, as they are unable to form emotional attachments, and place blame on others for their actions. But they can be well-educated and hold steady jobs.

There are also people who just have psychopathic tendencies. Criminal psychologist Robert Hare, the inventor of the standard clinical test for psychopathy, with another researcher studied corporate professionals and found that some “scored sufficiently highly ... to be evaluated for psychopathy,” the Telegraph reported. “It’s easy to see how a lack of moral scruples and indifference to other people’s suffering could be beneficial if you want to get ahead in business.”