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