Scientifically Speaking | Is there a gene that pushes some men to murder?
A few legal arguments including Yepez’s focused on this combination – low-MAOA activity together with mistreatment as a child. In all, the “warrior gene” argument has been introduced about a dozen times globally with mixed results
Is violent behaviour tied to genetics? Can someone argue that their genes made them commit a crime? Earlier this year, a legal argument centering on these questions made its way to the Supreme Court of the state of New Mexico in the United States.
Is violent behaviour tied to genetics? Can someone argue that their genes made them commit a crime? (Shutterstock)
In October 2012, Anthony Yepez was charged with choking and beating to death a relative of his girlfriend. Yepez and his girlfriend then covered the man in shredded magazines and cooking oil and burned the body. He pleaded guilty to the crime.
In 2015, a lawyer representing Yepez wanted to introduce the argument that he lacked the ability to control his emotions because of a genetic condition. Yepez possessed low levels of an enzyme due to a variant of a gene. The defence wanted to argue that this gene, often dubbed the “warrior gene”, along with abuse in childhood pushed him to act aggressively and impulsively.
This distinction of intent is important because pre-meditation often results in a harsher first-degree murder charge. Without that component, there is often a lighter conviction of second-degree murder.
Yepez was convicted of second-degree murder even in the absence of the “warrior gene” angle in court. The court also ruled that this argument was not admissible. The defence appealed and discussions on the merit of the “warrior gene” argument went on until the state Supreme Court upheld the murder conviction.
The court ruled that “mere genetic susceptibility to a given mental condition is not relevant on the issue of deliberate intent” unless there is “strong predictive value as to be clinically validated as an indicator of the mental condition”.
But what exactly is this “warrior gene”? Does it push men to murder? The scientific research on this topic can be traced back to a paper published in Science in 1993.
Hans Brunner, a Dutch geneticist, was studying an exceptionally violent family. The eight men in the family all attempted or committed violent, criminal acts.
Brunner and his team found that all eight men had a mutation in a gene on the X chromosome. This gene gives rise to an enzyme called monoamine oxidase A (MAOA), which is involved in regulating brain chemistry. Since males have only one copy of the gene (because males have only one X chromosome), those with the variant make little or no functional enzyme.
Humans have different forms of the gene that give rise to different levels of MAOA activity. Low activity has been found in some studies to be correlated with aggression. This has caused the MAOA gene to be dubbed the “warrior gene”.
In 1994, Stephen Mobley who was convicted of murder tried to use “warrior gene” argument to commute his death sentence. Mobley failed and was executed by lethal injection.
About a third of people in Western populations have low-activity MAOA. Not all of them become killers, of course. Conversely, not everyone who kills another human has low-activity MAOA. A study published in 2002 found that low MAOA activity alone wasn’t enough to push someone towards violence. Those who had the low activity enzyme also had to be mistreated as children.
A few legal arguments including Yepez’s focused on this combination – low-MAOA activity together with mistreatment as a child. In all, the “warrior gene” argument has been introduced about a dozen times globally with mixed results.
The widely accepted view in behavioural genetics is that the total “score” of many genes determines people’s preferential behaviours. The extent of the effect of single genes cannot always be parsed out. And scientists are not at a stage where predisposition to a violent act can be definitively determined from looking at one or multiple genes.
This does not mean that the “warrior gene” argument or in fact any form of genetic determinism will go away. There’s a great deal of focus on what modern methods such as DNA fingerprinting can do for criminal investigations that often clouds the limitations of genetics. I’m not sure that all of the biological complexities that make us human have made it into popular knowledge. But as we have learned in the past few decades, genetic effects can be exceedingly small and often very difficult to decipher.
As a science, genetics has advanced from the era of Mendel and his observations on pea plants. Most health-related traits (and this is particularly true of behaviours) don’t result from one change in a gene causing a single trait. There may be hundreds or thousands of genes acting in tandem with each individually contributing less than 1% to the observable outcome. In the past few decades, we have also learned that non-gene sequences of DNA and environmental factors change how genes work. That’s why we haven’t found a single genetic factor for heart disease, obesity, or depression, though we know that heredity plays a role.
In short, there is no gene that acts as a switch for criminal behaviour. Social conditions, emotional state, and lifestyle can also nudge someone to make a choice at a particular moment. That is why criminal investigations also search for motive, means, and opportunity.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction
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