In 2009, an Italian court reduced a murderer's sentence by one year because doctors had identified a gene in the defendant's DNA, called MAOA, that had been linked to violent behavior. The ruling was controversial and some scientists objected to the sentence reduction. "MAOA findings have been generally used in murder trials, sometimes to suggest diminished capacity of the defendant to premeditate his criminal behavior," but most often to reduce a sentence, writes Paul Appelbaum, a psychiatrist at Columbia University, in an essay published today in Neuron. In the essay, Appelbaum explains that genetic evidence demonstrating a defendant's predisposition for antisocial behavior or mental illness is showing up in courtrooms at an ever-quickening pace. And that pace, he warns, might be outrunning the legal system's ability to interpret it.

misinterpreted by judges and jurors

"Premature introduction of genetic evidence in court carries a number of risks," said Appelbaum in an email to The Verge. "The most obvious is that the purported associations [between genetics and behavior] are not real and will be disproved over time." But even the most replicated and widely accepted findings, he said, can be misinterpreted by judges and jurors.

A bargaining tool

In the case of the Italian court, the gene that was used has indeed been associated with impulsiveness and criminal behavior among men in a number of studies. Moreover, childhood maltreatment has been linked to lower MAOA activity — providing a great example of how a genetic predisposition can be triggered by environmental factors. Most experts agree, however, that even evidence as strong as that surrounding MAOA shouldn't be used to absolve someone of responsibility for their actions. "The major mistake the people make is to think that if you've identified a cause, it must mean that people are excused or mitigated," says Stephen Morse, a professor of law at the University of Pennsylvania who has written on the subject. But that isn't how the law works. Only a limited number of impairments — such as failing to appreciate the wrongfulness of one's acts — are considered exculpatory. So "if someone is a rational agent," Morse says, "I don't care if they have bad genetics."

It "doesn't necessarily mean a lessened responsibility."

Jorim Tielbeek, a neuroscientist and criminologist at VU Medical Center Amsterdam who has studied the effects of genetics on antisocial behavior, agrees with Morse, stating in an email to The Verge that a "higher genetic liability towards committing a crime doesn't necessarily mean a lessened responsibility." Although scientists have made links between certain genes and antisocial behavior, Tielbeek says, there is "no clear predictive relationship between a single gene and a criminal act — especially since hundreds of genes are involved in criminal behavior, and that each have a very small effect."

Yet that doesn't mean that either Tielbeek and Morse disagree with using genetics in the courtroom. "In the future, it may inform a judge of the type of justice interventions that is most suitable for the defendant," Tielbeek says. Morse also thinks genetic evidence can be valuable, but he clarifies that "it should only be used where the behavioral information is unclear."

But what exactly might genetic information help clarify? Deborah Denno, a law professor at Fordham University in New York City whose work was cited in Appelbaum's essay, says that genetics may be especially valuable when trying to figure out what distinguishes one person who grew up in an underprivileged neighborhood from their neighbor — or even their sibling. "There have been cases where a judge has said, ‘Well your brother and sister didn't commit a crime and they grew up on the same household,'" Denno says, "but genetics might explain that difference, because even brothers and sisters don't have the exact same genes."

a win for those who want a legal system free of bias

That's why some legal scholars think the introduction of genetic evidence in criminal cases is a huge win for those who want a legal system free of bias. As Denno puts it, it's a lot harder to argue with a defendant's traumatic family history when you have genetic evidence backing it up. "Genetics cuts through biases," she says, "so judges are not going to be bringing in their own personal garbage into a case."

Judging the evidence

And despite a handful of controversial examples, the use of genetic evidence doesn't appear to be making that big of an impact on the world's legal proceedings — at least not yet. One study that looked into the effect of genetics on the sentencing of psychopaths found that the average criminal case sentence is 13 years when genetic evidence is introduced. When it isn't, most defendants get 14 years. "It's no different than anything that is already being used to reduce sentences," says Kevin Beaver, a criminologist at Florida State University who has studied how people who are predisposed to antisocial behavior — people whose parents have been arrested, for instance — are more at risk for being processed through the criminal justice system than the general population. "So if we believe in mitigating factors, such as peer influence or abuse by parents, then I think those biological explanations would fall directly in line with those. I don't really see it as significant."

when a child's bloody t-shirt carries more weight

Appelbaum himself has looked into the impact of genetic evidence in homicide cases. In a 2014 study involving 250 survey participants, he found that people are disinclined to accept claims of diminished responsibility or pleas for reduced sentences based on genetic claims when that evidence is presented uncritically. "You can introduce all the genetics evidence you want in court, but in a case where a child was beaten to death by their parents, what do you think will have more weight?" asks Denno, "The fact that a parent has a history of violence going back four generations, or the moment when the prosecution shows the child's bloody T-shirt?"

Interestingly, some experts argue that we already use manifestations of our genetic makeup to reduce a person's prison sentence — it just isn't conceptualized that way. "Criminal defendants have long pleaded for mercy on the basis of problematic family histories," said Appelbaum, "such as substance abuse or criminality that may have genetic foundations, or because of experiences like child abuse," which may impact the way genes are expressed. When observed through that lens, the introduction of genetic evidence doesn't appear all that novel.

lawyers will have to wait for the evidence to pile up

That may be why experts are more worried about misuse of genetic evidence than genetic evidence itself. "As genetics becomes a more frequent visitor to the courtroom ... the risk that genetic information will be misinterpreted will be real," Appelbaum writes in the essay. But regardless of the fear that this information might be badly employed, science marches onward, so the answer to this dilemma might simply be that legal professionals need to learn to wait for evidence to pile up before making use of new genetic findings.

Of course, determining when a study has been replicated "enough" won't be easy. But the waiting part — where we ignore evidence that doesn't seem all that relevant — is something we already know how to do. After all, Morse jokes, society has known the identity of the number one predisposing risk factor for violent crime for far longer than it has known about the double-helix, and that hasn't stopped anyone from ignoring it in a court of law. "The number one risk factor for violent crime is the Y chromosome," Morse says. If we allowed "maleness" to be used as an excusing condition, he says, "most violent criminals would be getting off."