Should murderous psychopaths be punished less severely if their behavior can be blamed on brain differences or genes? Or, conversely, should their sentence be longer precisely because their biology makes them even more intractable and dangerous than other criminals?

A new study published in Science explored these questions by asking judges to impose a prison term on a hypothetical convict. When the judges were initially told that the offender was a psychopath, they tended to consider it an aggravating factor in sentencing, but when they heard additional expert testimony that biological factors could explain the guilty man’s behavior, they saw that information as mitigating and handed down a shorter sentence.

The impact of such expert testimony depended in part on whether the biological arguments came from the defense or the prosecution — it influenced judges’ reasoning more when it was delivered by the defense. But, overall, judges still levied lengthy sentences for the crime and viewed the convict as morally and legally responsible for his behavior: they reduced prison time only by a year, from 13.93 years on average to 12.83, when considering brain or genetic explanations for the convict’s behavior.

“The judges did not let the defendant off,” said lead author Lisa Aspinwall of the University of Utah in a statement. “They just reduced the sentence and showed major changes in the quality of their reasoning.” The researchers noted that they were surprised the judges reduced their sentencing at all, considering that they were dealing with psychopaths who are in general a highly unsympathetic bunch.

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The hypothetical case used in the new study was loosely based on the 1994 trial of Mobley v. State. In 1991, Stephen Mobley robbed a

Georgia Dept. of Corrections via AP

Domino’s pizza shop in Georgia, during the course of which he shot the restaurant’s manager to death; at trial, his attorney attempted to present evidence showing that Mobley had a variant of a gene linked to violent behavior: the MAO-A or so-called warrior gene.

Because the scientific data on MAO-A was so new at the time, however, the judge rejected its use in court and Mobley was executed in 2005. But since then, research has supported the link between the gene and violence, and studies have found that men who have the gene and are abused as children are significantly more likely to display antisocial behavior.

In the new study, researchers tweaked the hypothetical case to eliminate the murder; instead, the defendant was convicted of aggravated battery for savagely beating a fast-food restaurant manager with a gun during a robbery attempt and causing permanent brain damage. By taking murder off the table — and therefore the death penalty or a life sentence — the researchers compelled the judges to consider the future dangerousness of a criminal who could eventually be set free.

Researchers presented one of four versions of the hypothetical case to 181 judges in 19 states. In all versions, judges read scientific evidence that the convicted criminal was a psychopath and what that meant, namely that psychopathy is incurable. Half of the judges also received expert testimony on the genetic and neurobiological causes of the criminal behavior, presented either by the defense as a mitigating factor, or by the prosecution, which argued that it should increase the convict’s sentence. The other judges got no mention of the idea that biological differences in the convict’s brain could have caused his behavior. Researchers controlled for the fact that different states have different sentencing laws.

The judges who were given a biological explanation for the convict’s psychopathy issued shorter sentences, but notably, all judges committed the criminal to significantly more prison time than their average nine years for aggravated battery. And while all judges viewed psychopathy as an aggravating factor in sentencing, the judges who heard evidence about the genetic and neurobiological causes of the condition from the defense reported viewing it as less aggravating. Nearly 9 in 10 judges listed at least one aggravating factor in their reasoning for their sentence, but when they heard the expert testimony from the defense, the percentage of judges who also listed mitigating factors rose from 30% to 66%. And judges who received this evidence were 2.5 times more likely than other judges to report actually having weighed aggravating versus mitigating factors in deciding their sentence.

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The expert testimony offered in the study described how the MAO-A gene affects the amydgala, a part of the brain involved in emotion and learning. The amygdala is the seat of the so-called violence-inhibition mechanism, which is what triggers anxiety in normal people when they recognize that others are in pain or distress. People with low MAO-A activity, like the convicted psychopath, don’t experience normal brain development, however; that may explain why psychopaths are incapable of responding to the fear and pain of others with normal distress. Ultimately, the testimony argued, because of their genetic and brain-related differences, psychopaths don’t undergo functional moral development and fail to learn right from wrong.

Interestingly, however, even though the judges handed out reduced sentences when presented with this expert testimony, they did not report viewing the convict as having less free will or as being any less responsible, legally or morally, for his crime. “What this tells me is that the effect of neuroscience evidence may operate at a non-conscious level. People think it does not affect their judgment of responsibility, but in fact it does,” says Barry Schwartz, professor of psychology at Swarthmore, who has researched this issue, but was not involved in the study.

It is this basic question of responsibility that many psychologists find crucial — and that so many people misunderstand. “There is a lot of interest these days in the implications of neuroscience for justice and the legal system. Some of this interest focuses on the radical notion that neuroscience undermines the very idea of personal responsibility,” says Martha Farah, director of the Center for Neuroscience and Society at the University of Pennsylvania, who was also not associated with the new study. “The idea is that, since everything I do results from my brain, and my brain is the product of my genes and my life experiences, then how can you hold me responsible for anything? Isn’t it always true that ‘my brain made me do it?'”

Indeed, earlier studies have shown that when participants are presented with neuroscientific evidence in cases involving people who have caused harm or behaved violently, they see it as far more mitigating than psychological factors like child abuse — even though research now shows that brain differences themselves can actually be caused by such abuse and that child abuse is more strongly linked with violence than most neurobiological factors.

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Schwartz and a colleague described their findings on such research in a recent New York Times op-ed:

The pattern of results was striking. A brain characteristic that was even weakly associated with violence led people to exonerate the protagonist more than a psychological factor that was strongly associated with violent acts. … In contrast, while psychologically damaging experiences like childhood abuse often elicited sympathy for the protagonist and sometimes even prompted considerable mitigation of blame, the participants still saw the protagonist’s behavior as intentional. The protagonist himself was twisted by his history of trauma; it wasn’t just his brain.

The problem here, however, is that all of our psychology and behavior has a biological cause, even if we don’t understand exactly how it works. As Schwartz put it, “’Was the cause psychological or biological?’ is the wrong question when assigning responsibility for an action. All psychological states are also biological ones.”

Schwartz called the new study “terrific,” noting in particular that hearing evidence of biological causes of behavior had a larger impact on how mitigating the judges considered the convict’s psychopathy than on the actual sentences they handed down. Among the mitigating factors that judges cited after hearing the neurobiological evidence was the idea that mental illness made the perpetrator less responsible for his behavior.

As one judge in the study explained: “The evidence that psychopaths do not have the necessary neural connections to feel empathy is significant. It makes possible an argument that psychopaths are, in a sense, morally disabled, just as other people are physically disabled.”

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Consequently, as Schwartz says, “If you sentence to punish, it will reduce sentencing. But if you sentence to protect society, it may well increase sentencing, by implying that the perpetrator is incorrigible.”

“This is not the grand, metaphysical, ‘We are all helpless to override the inevitable workings of our brains’ idea that neuroscience is incompatible with moral or legal responsibility,” says Farah. “It is a more subtle, but still important, finding that judges are influenced by neurobiological evidence.”

Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.