According to this article, psychopathy affects 3 to 6 percent of the population and is genetically based. That is, only a handful of people will be psychopaths and some of their dangerous behavior is owed to their genes.
"It's biological and one of the most inherited human characteristics," said Dr. Igor Galynker, associate chair of psychiatry and director of the Family Center for Bipolar Disorder at Beth Israel Medical Center in New York City. But of course, environment matters, too.
About 50 percent of the neurological traits that comprise psychopathy are inherited and 50 percent are shaped by other influences. Having the genetic predisposition and growing up in an aggressive environment can be lethal.
Having carefully studied school shootings myself, I can say that not all school shooters are psychopaths; in fact, most are not. But some are clearly scary dangerous cold-blooded killers (think Columbine, for example).
Experts interviewed by ABCNews.com on the 10th anniversary of the Columbine shootings in 2009, said that can't predict which teens will go on a suicide-driven rampage. That is true, although nearly all school shooters studied by the US Secret Service showed warning signs of violence serious enough to cause concern in friends, parents, teachers, and others.
"Not all psychotics or psychopaths are going to kill and most are not dangerous," said veteran FBI behavioral scientist Kenneth V. Lanning. That is the problem that makes prediction so difficult.
Check out the film, We Need to Talk about Kevin.
And here are some related videos:
Can Brain Damage Lead To Murder? Watch Video