You see them on the movie screen; you read about them in the news. They’re calculating, charismatic, cold-hearted. But are they psychopaths or sociopaths?

Look to pop psychology for your answer and you’ll get a lot of conflicting opinions. Some folks believe that psychopaths are born, while sociopaths are made, the products of difficult childhoods and traumatic home environments. Others say “sociopath” is just the latest buzzword for “psychopath.” There’s no real consensus.

But there may be a reason for that: neither “psychopath” nor “sociopath” is a clinical diagnosis. They're common terms for people who exhibit "pathological" personality traits. In the U.S., such traits fall under the diagnosis of antisocial personality disorder, or APD, according to the American Psychiatric Association, which issues the Diagnostic and Statistical Manual of Mental Disorders, now in its 5th edition (DSM-5). The World Health Organization calls this dissocial personality disorder, or DPD.

APD and DPD are essentially the same thing. In order to be diagnosed with either, a person must exhibit “disregard for and violation of others’ rights.” The DSM-5 lists 6 major criteria [PDF]:

  1. Impairments in personality including a lack of remorse, egocentrism, "goal-setting based on personal gratification," and the inability to form mutually intimate relationships;

  2. Pathological personality traits, including manipulativeness, deceitfulness, callousness, hostility, irresponsibility, impulsivity, and risk-taking;

  3. These personality traits and impairments must be stable and consistent over time;

  4. These personality traits and impairments are not normal for the person's developmental stage (many toddlers could easily be described as psychopaths) or cultural environment;

  5. The person's personality and behavior are not explained by a medical condition or substance abuse; and

  6. The person has to be at least 18 years old—a contentious criterion, since many psychologists feel that children can begin displaying signs of APD at a very young age

Another frequently used "psychopath test" is the Hare Psychopathy Checklist-Revised, or the PCL-R, a 20-question checklist used by researchers, clinicians, and the courts to measure antisocial tendencies. (You can see the list to check yourself, or your boss, here.)

It's worth mentioning that there is a big difference between psychopathy and psychosis. The two words sound similar and are both used as slurs, but that's where the similarity ends. Unlike psychopathy, psychosis describes the condition of losing touch with reality, undergoing rapid personality changes, and having trouble functioning. The terms are typically mutually exclusive; most people with APD will never experience psychosis, and vice versa.

Scientists are still not sure what causes APD. Some recognize two forms of psychopathy, primary and secondary, each with its own set of causes [PDF] and manifestations. Traumatic childhoods and difficult home environments can definitely contribute, but there’s also a clear physiological component. A genetic variant called MAOA-L has been linked with an increased risk of violent and aggressive behavior, and brain scans of people with APD have shown low activity in areas related to empathy, morality, and self-control. 

This does not mean that all people with APD are violent, nor does it mean that they’re bad people. Many cases of APD go undiagnosed because the people in question are living successful, ordinary lives. 

For proof, just look at neurologist James Fallon: Fallon has spent decades researching the anatomical side of so-called psychopathy. His research has helped identify areas of difference in the brains of people with APD. One day in 2005, Fallon was looking at the brain scans of people with APD, as well as those of people with depression and schizophrenia. Sitting on his desk at the same time were a stack of scans from Fallon’s family members taken as part of a study on Alzheimer’s disease. 

“I got to the bottom of the stack, and saw this scan that was obviously pathological,” he told Smithsonian. The brain in the image appeared to belong to a psychopath—but the scans at the bottom of the pile belonged to his family members. Shaken, he decided to look up the code on the scan to determine whose brain he was looking at.

It was his own brain.

Fallon couldn’t quite believe it. His first thought was that his research had been wrong, and that low activity in those brain areas had nothing to do with APD. Then he talked to his family. Duh, they told him. Of course you’re a psychopath. His mother, wife, and children had recognized and been living with his personality issues all along.

Fallon underwent more tests, which confirmed the diagnosis. Over time, he realized that he’d known all along. Throughout his life, he wrote in an op-ed in The Guardian, strangers had commented that he seemed “evil,” and while he was never violent, he did have an icy streak. He had put other people in danger. He had also just generally been “kind of an a**hole,” he admitted to Smithsonian.

“I’m obnoxiously competitive,” he told the magazine. "I won’t let my grandchildren win games … I’m aggressive, but my aggression is sublimated. I’d rather beat someone in an argument than beat them up.”

Fallon believes his relative success may be the result of growing up in a healthy, stable environment with a lot of support. He was raised in a loving home, which, he says, may have helped him overcome some of his ugliest impulses.

APD currently has no cure. Finding successful treatment methods has been tricky, in part because people with APD tend to feel pretty comfortable with their personalities and have little motivation to change. Still, some, like Fallon, are determined to do at least a little bit better.

“Since finding all this out and looking into it, I’ve made an effort to try to change my behavior,” Fallon told Smithsonian. “I’ve more consciously been doing things that are considered ‘the right thing to do,’ and thinking more about other people’s feelings."