Fallon, a professor of psychiatry and human behavior at the University of California Irvine, accidentally discovered what friends and family have suspected for years -- he has all the genetic traits and brain scan patterns of a psychopath.
"When somebody gets mad at me, I never show it -- they can't read it on my face," Fallon, 66, told ABCNews.com. "I never get even immediately, but four years down the road, I get them with revenge."
"I don't have special emotional bonds with those who are close to me -- I treat everyone the same," he said. "I am involved in a lot of charities and good works, and my intentions are good for the world. But I don't have the sense of romance or love I am supposed to have for my wife. It's not there."
But Fallon is not a mass murderer and in his new book, "The Psychopath Inside: A Neuroscientist's Personal Journey Into the Dark Side of the Brain," he tries to understand why.
For years Fallon has worked with criminologists and other legal experts to evaluate the brain for abnormalities. But while volunteering with his own family for a study of Alzheimer's disease, Fallon learned on his PET scan that he has all the features of a psychopath.
"The last scan in the pile was strikingly odd," he writes about the 2005 discovery. "In fact it looked exactly like the most abnormal of the scans I had just been writing about, suggesting that the poor individual it belonged to was a psychopath -- or at least shared an uncomfortable amount of traits with one. ... When I found out who the scan belonged to, I had to believe there was a mistake. ... But there had been no mistake. The scan was mine."
Fallon, who has three children and five grandchildren, analyzes why he is a law-abiding, though impulse-driven, citizen, and yet other psychopaths with the same genetic predisposition, go on to kill.
Two of his distant relatives were notorious: One, Lizzie Borden, was acquitted of murdering her father and stepmother with a hatchet in 1892. Another, Thomas Cornell, was the first in the American colonies hanged for killing his mother in 1672.
Fallon said he escaped the same fate because of the interplay between nature and nurture. He was raised in a loving family. Still, he had some other telltale signs, such as panic attacks, obsessive-compulsive tendencies and social anxieties.
"Looking at my genetics, I had lethal combination, but I just had the happiest childhood growing up," he said. Fallon's mother had four miscarriages before his birth and, as a result, he said he was, "treated well because they didn't think I would be born."
"There were dark periods I went through, but they didn't bring me to a psychiatrist, but they told my sisters and teachers to watch out for me," he said. "My mother instinctively knew there was a problem."
Conscience and a sense of morality and impulse control lie in the limbic system and in the orbital cortex in the brain, according to Fallon.
"They connect and inhibit each other not unlike the super-ego controlling the id," he said. "It's the interface between the intellectual mind and the emotions attending to them."
Fallon's brain scans show low activity in both regions of the brain.
"No behavior is really evil or bad -- it's all contextual," he said. "There is a time for sex and a time for killing, when someone attacks the family. But it's done in context. The orbital cortex adjudicates the idea of morality and interacts with the amygdala's drive to eat, drink and screw. There would be mayhem if it didn't exist."
As a neuroscientist, Fallon said he always believed humans were ruled solely by their genes and not their environment in the nature versus nurture debate.
"I never took it seriously," he said. "I was the poster boy for genes causing everything. But I had to eat crow and say I was wrong."
His personal story was the subject of a TED talk that went viral on YouTube in 2007 and he even had a guest role on the television show, "Criminal Minds." Fallon was contacted by literary agents last year to write a book about his experience.
He blames abuse in the first three years of life, combined with biological features that turn off serotonin in the brain, leading to psychopathic violence.
"It's a loaded gun," he said, but not necessarily a "death sentence."
Fallon suggests that a child born with biological tendencies to be a psychopath can be pushed over the edge by early abuse and by bullies.
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."
All serial killers seem to share a "feeling of profound powerlessness," said Levin.
One of the earliest signs can be animal cruelty. "If you see a 6-year-old who sadistically abuses a dog or cat that is the family pet in an up-close and personal way, in order to maximize the suffering of the animal, clearly that's a red flag and you have a problem on your hands."
Levin suggests that psychopathic killers have difficulty transitioning from adolescence into adulthood.
"If the triggers occurred in early childhood, they would start killing people when they were 9 or 12," he said. "There is some environmental factor beyond how they were raised."
"It's impossible to predict [who will be a killer] under the Fallon model," said Levin. "A lot of people have the symptoms, but don't get the disease. They have been brutalized under terrible circumstances, been sexually stimulated by their parents and yet grow up to be healthy, decent people."
Fallon said his bad biology didn't stop his professional success, even if has taken a personal toll. Throughout life, he said he has had a larger-than-life personality that attracts people, but puts those he loves at risk.
"I wouldn't want to marry me," he said. "I am a pain in the ass and competitive. I can be so manipulative and I am always on the make, but I am not going to kill anyone or rape anyone."