And if he's learned anything from being involved in such cases, it's that evil can be found almost anywhere, even in the office next door.
"We can identify which people, (who) without appropriate interventions, are going to become employees you wish you didn't have," Dietz says.
Dietz is a forensic psychiatrist who advises Fortune 500 companies on how to spot potential psychopaths on the payroll. That makes him the perfect candidate to understand why someone would go on the rampage in Wakefield Mass., that left seven people dead just after Christmas.
Co-worker Michael McDermott has been charged with those killings. Friends say he had been complaining about money problems recently and acting troubled. Dietz teaches that management needs to learn how to spot potential troublemakers and get them appropriate counseling or face the consequences, Correspondent Jim Stewart reports.
"If you deal with them in the best of ways, everyone walks away safe," Dietz says. "If you deal with them in the worst of ways, you may get killed along with your buddies."Is there a profile of the guy who's going to shoot up the office the next day?
"Well there is, but it's also the profile that fits thousands of other people," Dietz explains. That profile basically fits people who could be trouble.
"Some are going to be going to the union with grievances," Dietz explains. "Some are going to be going out on disability. Some are going to fake a workers' comp injury. Some are going to be lousy performers."
"Some are going to stick things in a machine to sabotage it," he continues. "Some are going to threaten to kill the boss. And one might even shoot people up. But we can tell which ones are going be a pain in the neck if you don't intervene."
Spotting psychopaths is a skill that Dietz not only teaches large companies but one that has made him into one of the most sought-after expert witnesses in the country, able to distinguish the insane.
When psychiatrist Dietz makes a house call, he's been hired either by the defense or the prosecution to examine a defendant and then testify to whether a mental illness made him unable to control his actions or tell right from wrong. "The legal test of insanity requires first that there be sickness, and also that that sickness have particular effects at the time of the crime," Dietz says.
One of his first subjects was John Hinckley Jr. who shot President Reagan.
Dietz had little doubt that Hinckley was sane in the eyes of the law. Hinckley may have been troubled but Dietz believes those troubles did not relieve him of his responsibility.
"He knew that the world thought what he'd done was terrible. And he knew perfetly well that what he'd done was wrong," Dietz says. "That's what makes him sane."
The jury eventually disagreed with Dietz, finding that Hinckley had to have been crazy to shoot the U.S. president. But the case brought Dietz a notoriety that has never faded. To this day, he remains the lead consultant to the FBI on serial killers.
"I've never thought mankind was essentially good," he says. "Mankind is essentially instinctual and riddled with bad impulses." Dietz says his pessimistic outlook has been shaped by seeing the worst of the worst.
And he is most adept at explaining what makes the criminal mind tick. His preparation for trial is legendary. Where other psychiatrists often testify in terms that confuse, Dietz instructs jurors with the simple details of a case. He insists on seeing every piece of evidence. He interviews witnesses, goes to crime scenes and inspects weapons, he says.
"I want a chance to find the truth. And you can't find the truth with both eyes closed," he says.
Dietz always expected he'd follow in the family tradition and become a physician. But as a premed student, he was drawn to cases of people dying in unusual ways. Then he discovered a book about forensic medicine, filled with ghastly pictures, he says. He saw people, now lifeless, to whom terrible things had been done, he adds.
This was sort of an epiphany for him, leading him to his current line of work. "It was the chance to combine medicine with criminology," he explains. The author of more than 100 works, Dietz has taught at several major universities, including Johns Hopkins and Harvard.
Dietz hasn't spent much time treating patients, though. "A lot of it is very boring," he says. "Treating private psychiatric patients...means listening endlessly...to people with fairly normal lives whine about why their lives aren't as great as they wished."
Instead he has devoted his career to understanding the true nature of crime and explaining it to others. When he was hired by the state in its case against Jeffrey Dahmer, the notorious Milwaukee cannibal who cofessed to killing 17 men and boys, Dietz reviewed every scintilla of evidence so he knew what to ask.
"I've never had anyone so willing to disclose it all," Dietz recalls. "Basically he saw that I was very curious, knew something special and that he might get some insight out of this."
Dahmer spent hours telling Dietz how he had lured victims to his apartment with promises of money and sex and how he slept with some - after killing them. He revealed how he had dismembered others and ate some.
"What he had done for the few that he'd eaten some tissue from - he had saved what amounts to a filet of biceps," Dietz says.
Dietz even asked Dahmer what he ate with the flesh.
"And he hesitated," Dietz recalls. "And I asked him, 'Was it onions and potatoes?' And he was surprised that I knew and said, 'How did you know that?' I said,...'That's all you had in your kitchen. You had them under the sink.'"
Dietz says he had to see what interested Dahmer. "Why did he pick these victims? Why did he save the biceps from this one and not the rest?"
It would be easy to think Dahmer was legally crazy. But Dietz testified that Dahmer knew that the killings were wrong, took steps to avoid detection and even had a fair amount of self control with his victims.
"If Jeffrey Dahmer can take the time to wear a condom when he's having sex with his victims, he's got a little bit more self-control than the average teen-ager," Dietz concludes.
The jury agreed, and Dahmer got life in prison, where he was later killed by another inmate. The doctor's stock soared even higher. Today his opinion alone can dictate the course of a case. If Dietz finds someone insane, there may be no point in even going to trial, prosecutors have found.
He can't even think of a case where he's been stumped. "If you have the facts, it's always obvious," he says. "The most common stupid motive for murder is: He was looking at my girl. Is that crazy? That happens every day."
Every day Americans work and live among people on the verge of committing violence, he says.
"It used to be easy to spot the psychopath," he says. "If you saw someone with dice hanging from the rearview mirror, smoking a cigarette, with tatoos, odds were 90 percent you had one there."
Today it's a lot harder to spot the psychopath, but there are still some telltale signs, he says.
"Some of them will just fail to pay child support, drive recklessly, have affairs, and cheat on their spouses," he says.
"If somebody was a delinquent as a kid, and in adulthood is reckless, anti-social, and doesn't honor their obligations, behaves badly, that's a psychopath."
The mystery to Dietz is why the public puts up with them as much as it does. There are 2 million people in prison in the United States today. There are 5 million psychopaths, Dietz says, with 2 million in prison and 3 million not behind bars.
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