The study findings are likely to accelerate the use of brain science in legal proceedings, experts said, and to intensify a long-running debate about its relevance.

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Judges who learned that a convicted assailant was genetically predisposed to violence imposed lighter sentences in a hypothetical case than they otherwise would have, researchers reported Thursday, in the most rigorous study to date of how behavioral biology can sway judicial decisions.

The findings, published in the journal Science, are likely to accelerate the use of brain science in legal proceedings, experts said, and to intensify a long-running debate about its relevance. Courts have increasingly admitted such evidence — brain scans, mostly, and genetic analyses — although many experts say the science remains too primitive to inform legal decisions.

Defense lawyers commonly introduce brain scans of clients as mitigating evidence in appeals of death sentences, experts said.

Previous studies of how such evidence affects legal decisions are scarce, and their results mixed. Some reports suggest basic neurobiology affects juries’ verdicts, others that it does not.

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In the few real-life criminal cases in which brain science seems to have had an effect, it has usually gone in favor of the defendant, reducing punishments, legal researchers said.

Judges, not juries

The new experiment focused on sentencing by judges, not jury verdicts. It found that neurobiological evidence reduced judges’ sentences by an average of 7 percent for a fictional defendant convicted of battery and identified as a psychopath.

“What’s pathbreaking about this paper is that it both isolates, as well as one can, the effects of biological testimony on outcomes and it also does this within a sample of the real-world decision makers, the judges,” said Owen Jones, a professor of law at Vanderbilt University and director of the MacArthur Foundation Research Network on Law and Neuroscience. “This moves our understanding forward considerably.”

Jones and other experts cautioned that the effect might not apply broadly to other kinds of criminal defendants. It may also play out differently in jury trials. But they said the findings were convincing and plausible.

In the study, three researchers at the University of Utah tracked down 181 state judges from 19 states who agreed to read a fictional case file and assign a sentence to an offender, “Jonathan Donahue,” convicted of beating a restaurant manager senseless with the butt of a gun.

All of the judges learned in their files that Donahue had been identified as a psychopath based on a standard interview — that is, he had a history of aggressive acts without showing empathy.

The case files distributed to the judges were identical, except that half included testimony from a scientist described as “a neurobiologist and renowned expert on the causes of psychopathy,” who said the defendant had inherited a gene linked to violent, aggressive behavior. This testimony described how the gene variant altered the development of brain areas that generate and manage emotion.

The account is an accurate description of one theory of how brain development may underlie aggressive behavior. Its applicability to any individual is unknown, however, given that many other factors could increase the likelihood of violence, researchers said.

The judges who read this testimony gave Donahue sentences that ranged from one to 41 years in prison, a number that varied with state guidelines. But the average was 13 years, a full year less than the average sentence issued by the judges who had not seen the testimony about genetics and the brain.

In interviews about their decisions, the judges said a crime of aggravated battery such as this one normally carried a sentence of nine years, on average, and 15 years if the defendant was identified as a psychopath, the researchers found.

“But then those who read about the biological mechanism subtracted a year, as if to say, ‘This guy is really dangerous and scary, and we should treat him as such, but the biological evidence suggests that we can’t hold him as responsible for the behavior,”‘ said James Tabery, an assistant professor of philosophy at Utah.

He wrote the study with Lisa Aspinwall, a psychologist, and Teneille Brown, an associate professor in the university’s school of law. A research assistant, Gabriela Cash, recruited the judges by calling court administrators in every state.

Double-edged sword

This mixed result — added punishment for the defendant’s being identified as a psychopath, tempered by empathy for his having a possible genetic predisposition — provides a good illustration of what legal researchers call the double-edged sword of biobehavioral evidence.

On one hand, a biological predisposition suggests a person is likely to be dangerous in the future and should get a longer sentence; on the other, it implies a lower threshold of responsibility. The evidence could cut either way, depending on the judge.

“One of the things we judges do is project to the public this idea that what we do at sentencing is an entirely rational act, when it isn’t,” said Judge Morris Hoffman, a Colorado District Court judge in Denver who was not involved in the study. “I think these results show that, just like everyone else, we can be tricked into this psycho-legal error that a plausible biological cause becomes an excuse at some point.”

Hoffman, who has taught and written about biology and the law, said that in theory, sentencing should be based on the crime, the criminal, the value of deterrence and the possibility of rehabilitation, without need of biological theories that try to explain behavior that, after all, must be rooted in brain processes.

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