Why we can’t blame “warrior genes” for violent crime

It’s tempting – but wrong – to assume that our fates are entirely written in our genes, says Adam Rutherford.

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On 16 October 2006, Davis Bradley Waldroup waited at his trailer home in the mountains in Tennessee for his estranged wife Penny to arrive home with their four kids. Their relationship had been strained for months, and he had been drinking heavily. When she arrived, with her friend, Leslie Bradshaw, he appeared carrying a .22 rifle and they began to fight. Penny said she was leaving. Minutes later, Waldroup shot Bradshaw eight times with a rifle. She died. Penny attempted to escape up the mountain, but he shot her in the back as she ran and cut her with a pocketknife when he caught up with her. He then bludgeoned her with a shovel, and then a machete, slicing her dozens of times, and chopping off one of her fingers. Waldroup dragged her to the trailer wanting sex, and became angry that she was unresponsive and too bloody. He told his children, ‘Come tell your mama goodbye.’ But she managed to escape.

25 March 2009 

After eleven hours of deliberation, the Polk County Grand Jury handed down a verdict of aggravated kidnapping and voluntary manslaughter of Leslie Bradshaw, and aggravated kidnapping and attempted second-degree murder of Penny Waldroup. Bradley Waldroup had avoided execution. The defendant’s experts had claimed that he could not have committed first-degree murder because he was ‘unable to engage in the reflection and judgment necessary to premeditate the crimes’. The reason why, according to Waldroup’s legal team, was genetics.

At the root of their defence was monoamine oxidase A, known as MAOA. The gene MAOA encodes an enzyme whose role is to destroy molecules called neurotransmitters. MAOA is essential for a normal life. When it’s not working at full capacity, or when unusual genetic variants are at play in a person’s neurons, all manner of problems can transpire.

Towards the end of the twentieth century, reports began seeping out of labs that particular versions of MAOA were turning up more often in people with aggressive, impulsive or criminal behaviour. It all started in Nijmegen with the men in a large nefarious Dutch family. Five generations of this dynasty, all the way back to 1870, were packed with criminals, including arsonists, attempted murderers and a rapist. Han Brunner, a doctor in Nijmegen, examined the family histories, and narrowed down a shared anomaly on the X chromosome. When Brunner tested the urine of the Dutch men, he found significantly low levels of the normal molecular leftovers following the action of MAOA. It appeared that it was not working as it should do.

MAOA acquired the nickname the ‘warrior gene’ around 2004. Blaming the tabloids for bastardizing or distorting science is frequently not unreasonable, as they are prone to magicking away nuance, or conjuring up meaning. But in this case it was the august academic journal Science that instigated the frothing media frenzy that would characterize the debates about genes and crime for a decade to come. A study in monkeys had uncovered variants of MAOA, and postulated that the aggression that fell out of a defective gene might be an advantage in battling other monkey troops. There, in the headline was the label: ‘warrior gene’.

A chunky paper in 2003 suggested that violence associated with the defective variant was significantly worsened if the perpetrator had been sexually abused as a child, whereas abused children with a normal MAOA were less likely to be criminals. This result was affirmed in 2012 in a meta-analysis – a super-powerful way of aggregating multiple studies to ramp up the analytical depth. And the studies keep coming, with subtly specific conditions, with the law trailing behind them. In 2009, an Algerian living in Italy called Abdelmalek Bayout had his sentence for the murder of a Colombian man reduced by three years, after his defence also identified him as a carrier of the defective MAOA gene.

All these studies point to possible factors in the aetiology of violent crime, that may under the right (or more precisely, wrong) circumstances agglomerate into a tinderbox combination. Behaviour is complex. Genetics is complex. Real world narratives mostly fail to recognize that cumulative complexity. Bear in mind that according to one of the studies, one third of white men carry the same allele as those who murder or fight. Statistically speaking, none of them will murder.

Bradley Waldroup carries that version, the so-called ‘warrior gene’. He was beaten and abused as a child, horrifically. He had been drinking heavily on that night when he killed and maimed. He had ready access to firearms and other weapons. I do not support capital punishment. But I have no doubt that the route by which he and Abdelmalek Bayout were at least partially exonerated from heinous crimes was wrong. One juror in Waldroup’s trial was quoted as having said:

A diagnosis is a diagnosis, a bad gene is a bad gene.

Despite all the studies, we simply do not know well enough how this gene works, how it participates in the biological melee of a life, how life experiences and chance coordinate with the external world of people. Even if we did, the legal ramifications would be equivocal, and subject to political leaning. Are we slaves or masters of our genes? We are neither, and it’s a dumb, simplistic question. To say otherwise is a biological determinism with profound legal consequences.

Of course, this is not new to the justice system, it is merely that now genetics is coming of age, it is now part of the pantheon of legal defences and excuses for predestination. A gene that has a measurable association with violence if the bearer was beaten as a child is not irrelevant. But perhaps exoneration via the complex and poorly understood root of genetics is missing the broader point that maybe we shouldn’t abuse children.

This is an edited extract from Adam Rutherford’s A Brief History of Everyone Who Ever Lived: The Stories In Our Genes, out now.

Dr Adam Rutherford is a geneticist, writer and broadcaster. He presents BBC Radio 4's weekly programme Inside Science and his documentaries include the award-winning series The Cell (BBC4), The Gene Code (BBC4), Horizon: “Playing God” (BBC2) as well as numerous other programmes for BBC Radio 4. Creation is his first book. It is shortlisted for the Wellcome Book Prize 2014.