Can Your Genes Make You Kill?
Popular Science|May - June 2016

 Science’s search for the roots of violence.

Lois Parshley

The killer read his Bible. He drank. Heavily.

It was a fall night in 2006, when Bradley Waldroup walked out of his rural trailer in southeastern Tennessee, carrying his .22 caliber hunting rifle. His estranged wife and her friend, Leslie Bradshaw, had just pulled up to drop off the Waldroups’ four children. Waldroup began arguing with his wife and Bradshaw, who was unloading the car. Drawing his gun, Waldroup shot Bradshaw eight times, killing her. He used a knife to cut her head open. He then chased his wife with the knife and a machete, managing to slice off one of her pinkies before dragging her into the trailer. There, he told their frightened children, “Come tell your mama goodbye,” because it was the last time they’d ever see her. Miraculously, his wife managed to slip his grasp and escape.

Three years later, in a county court, Waldroup admitted the whole thing. He said he had “snapped.” “I’m not proud of none of it,” he told the judge. Convicted of felony murder, he faced the death penalty.

To save his life, his legal team took an unusual approach, never before admitted in a capital-murder case. They sent a sample of Waldroup’s blood to the molecular genetics lab at Vanderbilt University in Nashville. Lab techs there were told to look at a specific gene. Sure enough, they found Waldroup had a genetic variant on his X chromosome, one that coded the enzyme monoamine oxidase-A (MAOA).

MAOA’s job is to break down crucial neurotransmitters, such as dopamine and serotonin. If left unchecked, these potent chemicals can build up in the brain and cause a loss of impulse control and an increase in violence and rage. In part, Waldroup’s lawyers were claiming, his genes made him do it. 

It’s been more than two decades since geneticists linked MAOA deficiency to violent behavior. And it’s been a decade since the media dubbed one of the genes that causes the deficiency “the warrior gene.” It is among the most controversial of several genes linked to violence and psychopathic behaviors. 

Mental illnesses have also been linked to genetic causes. In January, Harvard scientists jolted the mental-health field when they identified a gene that might lie at the root of schizophrenia: During adolescence and early adulthood, a variant  of the gene causes the overpruning of synapses in the brain’s decision-making frontal lobe, impairing things like attention and impulse control. While only a fraction of the 2 .2 million Americans suffering from schizophrenia turn violent—a point that mental-health workers are careful to point out—people with serious mental illnesses are two to three times more likely to become violent than those who are not. 

As each mass shooting and road-rage murder fills our daily newsfeeds, scientists, law-enforcement officials, politicians, mental- health experts, and the public ask what we can do to stop the next one.

Can we identify violent people before they hurt someone? Is there a genetic link among serial killers like Ted Bundy, mass murderers like dam Lanza, and roadside shooters like the Uber driver, Jason Dalton, who police charged with killing six people in a random rampage in Michigan this past February?

These are uncomfortable questions, ones that conjure the quackery of phrenology and the eugenics of the Nazis. But as geneticists come closer to unlocking the doors of personality traits and pathologies, we seem to be stepping beyond behaviorism to embrace genetic determinism. We accept science has found a gene that increases the risk for alcohol­ism, a condition once associated with weakness of character. We accept that genes can alter brain function and may trigger anxiety behaviors. There is evidence that the same could be said for violence.

Kent Kiehl works inside a portable trailer on the grounds of the Western New Mexico Correctional Facility, home to 440 inmates in the small town of Grants. He sits at a cramped desk in front of computer screens that monitor activity in a nearby and loudly humming cylindrical tube. It is a $2.2 million functional magnetic-resonance-imaging scanner (fMRI). One by one, Kiehl slots in murderers, rapists, arsonists, and other violent criminals, and then peers into their brains. He has become a top expert on the neuroscience of schizophrenia and psychopathy.

Kiehl has a unique and personal perspective on the subject he studies. His family had lived down the street from Ted Bundy in a quiet Tacoma neighborhood. When Bundy was arrested in 1975 and later accused of killing more than 36 women over nearly two decades, it sent a collective shiver through his neighborhood. Kiehl wondered, “How could someone like that grow up in our sleepy little suburb?” As a neuroscientist at the University of New Mexico, he has spent the past 25 years looking for an answer.

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