Northeastern University criminologist Jack Levin agrees environmental influences determine whether a psychopath will go on to be violent, but discredits Fallon’s theory

The author of a book about mass murderers, “Extreme Killing,” Levin said most serial killers are in their 30s and 40s.

“You can determine the biological roots of psychopathy, including the lack of empathy and remorse and manipulative disposition, but the problem is, that does not necessarily translate into violent behavior,” he told ABCNews.com. “There are literally millions of psychopaths.”

The American Psychological Association claims that as many as 3 percent of all Americans have antisocial personalities, according to Levin, “meaning they are crafty and shrewd and masters at presentation of self.”

“They might sell you a bad used car, or might be womanizers or pathological liars or cheaters, but that doesn’t mean they will kill anyone,” he said. “Not unless you become an obstacle to their success — and then you better watch out.”