Can Your Genes Make You Kill?

Science's search for the roots of violence
Killer genes
Killer genes Javier Jaén

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).

Bradley Waldroup
Jurors ruled out the death penalty for killer Bradley Waldroup after lawyers argued a gene variant helped predispose him to violence. Courtesy WRCB Chattanooga

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 Adam 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.

Psychopaths suffer from severe emotional detachment. They lack both empathy and remorse. Kiehl has found that they make up about 16 percent of the U.S. prison population. Such people also comprise about 1 percent of the general population. To put that in perspective, it is about as common as bulimia but much more difficult to diagnose. That’s troublesome because psychopaths are inclined to violence. On average, a criminal psychopath will be convicted of four violent offenses before age 40. Twin studies have pointed out a genetic component to psychopathic traits, but few experts agree on an exact cause for the disorder. Kiehl believes it can be traced to defects in the limbic and paralimbic cortex, used in generating emotions, controlling impulses, and paying attention.

During an exam, an inmate lays his head beneath a coil that sends and receives magnetic signals. Kiehl displays phrases, like “stealing from your job site,” or images, like that of a car crash, on a screen. He asks the inmate to rate the moral offensiveness of each.

As the inmate makes decisions, his or her neurons fire, and the computer records the response time and the area of the brain activated. A nonpsychopath will show activity in regions, such as the almond-shaped amygdala, related to empathy and emotion. Psychopaths will not. Depending on which region is active, Kiehl can determine how the inmate processes the material. A psychopath might show little activity in the amygdala and instead be processing the material in a logical portion of the brain, in some cases trying to trick Kiehl or give Kiehl answers he thinks are appropriate or that he thinks Kiehl wants to hear.

Altogether, Kiehl has collected brain-imaging data from more than 4,000 criminals at eight prisons in two states, building what amounts to the largest forensic neuroscience library in the world. He has found that psychopaths tend to have less gray matter in the region that he’s targeting, as well as smaller amygdalas. In short, he says, “they have different brains.” And those differences are “at least 50 percent caused by genetics,” he says, adding, “That shouldn’t surprise people with neuroscience knowledge.”

Kiehl’s work has become so well-known that parents of troubled kids frequently seek his advice. It’s a situation he finds depressing because he doesn’t yet have answers for them. “I get an email at least once a week from a parent whose child is struggling. And it’s heartbreaking,” Kiehl says. “‘Is my child a psychopath?’ I’m the last guy they want to call.”

brain scans
Brain scans have shown that psychopaths might have defects in regions related to impulse control and emotional processing. Callista Images/Getty Images

The modern search for the genetic roots of violence began when a woman walked into a university hospital in Nijmegen, Netherlands, in 1978. She had come to seek help for the men in her family—several brothers and her own son—who she suspected were suffering from the same mental disability. Two had committed arson. One had tried to rape his sister. Another tried to kill his boss by ramming him with a car. Another had drawn a knife against his sisters and forced them to undress. In fact, the family violence appeared to stretch as far back as the 1870s, according to a detailed family tree of violent offenders that a concerned uncle drew up in 1962.

More than a decade after the woman appeared at the Nijmegen hospital, researchers there finally figured out what was wrong. The violent men possessed a mutation on their X chromosome. That defect turned out to be a flaw in the MAOA gene. Since the gene is on the X chromosome, men—who have only one X chromosome—are more likely to suffer its effects than women, in whom a second, normally functioning X chromosome can compensate for the problems of the defect. Women can, however, pass the defect to their sons. Soon the women in the family were coming in to be tested to figure out if they were carriers.

Since then, several projects have found other genetic risk factors for violent behavior. In 2011, a German researcher canvassing the field, found a connection between homicidal behavior and a variant in a gene that codes for a protein called atechol-O-methlytransferase (COMT). Like MAOA, it regulates dopamine. Four years later, Finnish researchers studying prison inmates found that violent offenders often possessed MAOA variants or variants of genes that code for CDH13—a protein that assists in brain-cell signaling. Previous studies linked those same variants to autism, schizophrenia, and attention-deficit/hyperactivity disorder. The inmate study, which appeared in the journal Molecular Psychiatry, concluded that the CDH13 variant, and the cellular dysfunction that it contributes to, was a “plausible factor” for violent criminal behavior.

The notion that a biological basis to aggression might exist troubles many scientists and ethicists. They are quick to note that environmental factors play a huge role in how genes are expressed. Having a gene that increases the risk for breast cancer doesn’t mean a woman will get breast cancer—and having a gene linked to schizophrenia doesn’t mean you will develop it. “Genes are programs that run every activity of every cell in your body every second you are alive,” says Daniel Weinberger, director of the Lieber Institute for Brain Development at Johns Hopkins University. “If you inherit small glitches, little pieces of noise, this sets you on a path. But it doesn’t determine you will end up with mental illness. These glitches aren’t fate. They are for risk. Environmental factors are at play too.”

Plenty of people carry the same gene variant as the man who killed his wife’s friend, and they’ll never kill anybody.

After all, plenty of people carry the same gene variant as Bradley Waldroup, the man who killed his wife’s friend, and they’ll never kill anybody. But courts have proved ready grounds for the genes-made-me-do-it ethical and science debate. Criminal defense cases have cited genetics nearly 80 times in the U.S. between 1994 and 2011. “Attorneys are getting more sophisticated in looking for explanations for behavior,” says Deborah Denno, director of the Fordham University Neuroscience and Law Center.

In the case of Waldroup, the jury spared him the death penalty and found instead that he should spend his life in prison. The killer-gene defense worked. After, when asked if Waldroup’s genetics informed her decision, one juror said, “Oh, I’m sure.”

But Denno says the role of gene variants, and links to increased risks of violence, has been misunderstood in the courts and by the media. Behavioral genetics seeks to study genetic as well as environmental sources for clues to behavior. It is interdisciplinary, incorporating psychology, sociology, statistics, and other fields. “While genes influence behavior,” Denno has noted, “they do not govern nor determine it.”

In fact, environmental factors—as varied as malnutrition, social and economic strife, and poor education—remain some of the strongest predictors of behavioral pathologies in adulthood. Psychologists have long known that childhood abuse alone is a risk factor for violence. Boys exposed to erratic, coercive, and punitive parenting are at risk of developing antisocial personalities and becoming violent offenders, according to a 2002 study in Science. Of course, not all abused boys become violent. The idea that gene variants—like those leading to neural disruption or hyperactivity in the brain—might put them at increased risk for violence is an intriguing one. But it is by no means the sole cause or even a root cause.

One afternoon last fall, the University of Connecticut Health Center campus in Farmington stood nearly flooded after a late-season downpour. Julian Ford, a clinical psychologist who specializes in children and adolescents with PTSD, sat in his book-lined fourth-floor office. Ford helped write the official 114-page investigative report on Adam Lanza and the 2012 Sandy Hook Elementary School shooting.

After Lanza killed 20 schoolchildren, six staffers, his mother, and himself, the state’s medical examiner sent a piece of his brain to UConn geneticists and asked them to analyze his DNA. It was the first time a mass murderer’s genome had ever been studied.

Despite formal requests from Popular Science, neither the medical examiner, UConn, nor its geneticists would release the report’s findings or even discuss what they were looking for. But they most likely searched for gene variants linked to mental illnesses.

During his early life, Lanza suffered from insomnia and struggled with speech. Shy, quiet, and a social outsider, he wrote a story for a fifth-grade project called “The Big Book of Granny.” In it, an old woman shoots children and talks about preserving a boy for her mantle. Lanza was eventually diagnosed with Asperger’s syndrome, anxiety, and obsessive-compulsive disorder. While Asperger’s is not associated with violence, it might have masked his violent thoughts and behaviors. On the basis of a recommendation by a psychiatrist, his mother took him out of school to teach him at home. While various people had noted Lanza’s challenges, “What was apparently missed,” says Ford, “was his emotional turmoil.”

Adolescence is a vulnerable period, and not just because of mood swings caused by surging hormones. It is a time when mental illness is most likely to manifest. With schizophrenia, for example, symptoms often appear suddenly during this period and in early adulthood. In their landmark study at Harvard this past January, scientists identified a gene potentially responsible for this timing: The natural process of synaptic pruning, during which the brain deletes ineffective connections between neurons, occurs as the brain matures. This takes place in the prefrontal cortex, where thinking and planning are based. People who carry a gene variant that accelerates the pruning have a higher risk of developing schizophrenia.

That’s why it’s critical that adolescents get care, says Steven McCarroll, a Harvard geneticist and senior author of the study. “Often when teenagers manifest symptoms, they’re seen by pediatricians without psychiatric specialization,” he says. One success story he cites is an ongoing Australian mental-health program for adolescents, started in 2006, called headspace, which runs more than 80 clinics in some unusual and convenient places. “Some are in shopping malls,” he says. “They have warm colors and welcoming furniture to avoid a clinical feel. It’d be wonderful to have something like that here.”

But what about kids like Adam Lanza who fell through the healthcare-system cracks? Would genetic screening have helped? As of now, no. And researchers are skeptical that it ever could in the future. “We don’t know enough about genetics yet to use genetics as part of diagnosis,” says McCarroll.

There are plenty of reservations about what we would look for and what we’d find—worries over privacy and stigmatization, the question of what to do once you know someone has a genetic risk of violence. But learning about genetic markers, even if it doesn’t entail screening for criminals, still helps us better understand violence and its origins. The more we understand, the more we can do to prevent it.

So it’s hard to stop looking for genetic clues. Daniel Weinberger, the Johns Hopkins neuroscientist, has the world’s largest collection of brains from deceased PTSD sufferers. He studies them for molecular clues to mental illness. “For a century, we knew what mental illness looked like, sounded like, what it felt like,” he says. “But we didn’t know the basic underlying cause. Today, thanks to genetics, we’re able to explore things that were science fiction 10 years ago.”

But even he worries about what science will find and how society will act on it. After all, he says, “everyone’s genome has a different level of risk for different disorders. Everyone’s got something.”