By Christian Jarrett

Around the world, neuroscience evidence is being introduced into courtrooms at an increasing rate, including findings from behavioural genetics. Specifically, some legal teams for the defence have been allowed to argue that the defendant has a low activity version of the MAOA gene, which codes for an enzyme that regulates the levels of several neurotransmitters. In combination with experiencing child abuse or maltreatment, having this low activity gene has been linked with increased impulsivity, including aggression. Defense lawyers presumably hope that jurors will interpret this as meaning the defendant was less culpable for their violent crime. However, before now, little research has examined how jurors will treat this evidence.

For a new study in Behavioral Sciences and the Law, Natalie Gordon and Edie Greene presented 600 mock jurors (half were students, half were from the wider community) with a detailed trial summary based on a real US murder trial in which the defendant, already in jail for an earlier crime, had murdered his cell-mate. The jurors’ task was to decide whether he should face the death penalty.

The jurors were presented with different versions of the trial – for example, some read that a medical geneticist had described the gene-environment interaction (that the defendant had the low activity MAOA gene and had suffered maltreatment as a child, which can lead to increased impulsivity); others read only about genetic evidence; others only heard about the environmental factor (maltreated as a child). The jurors also read about evidence from a clinical psychologist about whether the defendant was high or low risk for being dangerously violent in the future.

Overall the results suggested that “evidence of a genetic x environment interaction did little to reduce the likelihood of a death sentence”, the researchers said.

The clinical psychologists’ evidence was the most powerful mitigating factor. When the psychologist said the defendant was low risk, the jurors were less likely to choose the death penalty regardless of what other evidence they heard about genes, childhood, or the interaction between the two. Preference for death penalty (at 13 per cent) was lowest among jurors who heard the defendant was low risk and who heard the gene-environment evidence.

When the defendant was classified as high risk, the gene-environment interaction evidence didn’t have much of a mitigating impact – a similar proportion of jurors still proposed the death penalty compared with those who heard only about his maltreatment as a child (30 per cent vs. 34 per cent). Meanwhile, genetic evidence on its own actually led to more recommendations for the death penalty (41 per cent when he was high risk; 35 per cent when low risk).

“While the defence might continue to use [gene-environment evidence] – wishfully according to our data – to argue that genetic and environmental factors interact to impair a defendant’s ability to control behavior, the prosecution could argue that these same factors show that the defendant has a stable, criminal disposition and poses an ongoing threat to society,” the researchers concluded. “If presented by the prosecution in this way, evidence of a G x E interaction could result in an increased likelihood of a death sentence.”

—Nature, nurture, and capital punishment: How evidence of a genetic-environment interaction, future dangerousness, and deliberation affect sentencing decisions.

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest