Unless you stay away from all social media, you've encountered internet trolls. They're the ones who drove Robin Williams' grief-stricken daughter off of Twitter, who caused one female journalist to call the police in the middle of the night for fear that death threats might be carried out, and who turned Pepe the frog into an official hate symbol, over the objections of the cartoonist who created him.

But have you ever wondered why trolls are so effective? How do they know just the thing to say to upset you, no matter how hard you try to stay above the fray?

Now there's an answer. In a new study by Natalie Sest and Evita March of Federation University in Australia, researchers gave 415 internet users questionnaires that tested for various personality traits and for propensity to trolling behavior, looking for the specific traits that would make someone likely to become a troll. In the process they got some clues as to why people become trolls, and why trolls are so very, very good at making their victims feel very, very bad.

They began with a simple observation you probably have made yourself: Although both genders become trolls, men are likelier to do so than women. Beyond that, things get more interesting:

1. Trolls exhibit psychopath traits.

Now, before you start thinking that the average internet troll has dismembered bodies hidden in the basement, let me note that there are different degrees of psychopathy and many people have some amount of it without turning into serial killers. Psychopathy is broadly defined as a lack of empathy or feeling for others, an inability to feel guilt, and an ability to manipulate through charm. Other than in the movies, most psychopaths are not violent.

On the other hand, thrill-seeking and acting on impulse are associated with psychopathy. That may explain why psychopaths find trolling so enjoyable. It could be that "creating mayhem online is a central motivator to troll," the researchers write. That's why most experts agree that the best way to deal with trolls is to ignore them and not respond in any way (except perhaps to block their messages). Any sense that they've caused disruption or upset serves as a reward for their behavior.

2. Trolls are sadistic.

Admittedly, you might not need a personality questionnaire to know that much. To say that trolls take pleasure in causing upset and disruption is to state the obvious. But, as with psychopathy, there are degrees of sadism and most of them stop well short of the Marquis for whom the term is named.

For years, I was part of an online community where a single member caused endless unhappiness with her constant criticisms and insults. Then one day, a mutual friend invited her to a party and I met her in person. She was a cheery elderly lady who would likely remind you of your favorite grandmother. "I enjoy causing trouble on our online forum," she told me blithely.

Many of us have at least a little bit of sadist in us. Don't believe me? Think about this: Have you ever played "keepaway," tossing a ball or Frisbee to another older child, over the head of your frustrated younger sibling? Internet trolls may have that kind of sadism, only more so. That's another reason it's smarter not to let them know they've upset you.

3. They have cognitive empathy, but not affective empathy.

There are two kinds of empathy, it turns out. What most people mean by "empathy" is actually affective empathy--the ability to experience and respond to the feelings of others. Rats, for example, have so much of it that if you hurt one of them, others nearby will also squirm in apparent pain.

There's also cognitive empathy, which is the ability to understand and analyze the feelings of others but not feel them yourself. Internet trolls have lots of cognitive empathy and it helps them figure out exactly what to say that will upset you the most. Because they have little affective empathy, upsetting you won't upset them. Because they have psychopathy, they won't feel guilty about it either.

"Results indicate that when high on trait psychopathy, trolls employ an empathic strategy of predicting and recognizing the emotional suffering of their victims, while abstaining from the experience of these negative emotions," the researchers wrote.

They didn't offer concrete suggestions for stopping trolling, but said their goal was to "explore and extend the personality profile of internet trolls." And, they wrote, "Results have implications for establishing education and prevention programs." Prevention--wouldn't that be nice?