When it comes to violent crimes, the mental state of the perpetrator is a central factor in placing guilt. If a person is considered legally insane, they cannot be held responsible for their actions and are “not guilty.” Diminished capacity bears some resemblance to the insanity defense but is simply pleading for a lesser crime, rather than a full defense to a crime. A genetic explanation for a person’s violent behavior may encourage jurors to support an insanity defense, but a new study shows that jurors may also believe the defendant will be a consistent threat, and likely to commit more crimes in the future.

“Genetic evidence plays an increasingly important role in the criminal justice system, but people often perceive genetic information in biased ways,” said Benjamin Cheung, a doctoral candidate in psychology at the University of British Columbia and lead researcher on the study, in a press release. “If we believe genes lie at the heart of criminal behavior, then we may think the defendant had no control over his actions, even if that isn’t true.”

The study involved over 600 participants, and had them read through different vignettes about a fictional murder. There were two versions of the story — one in which the defendant had a genetic variation associated with violent and aggressive tendencies, and one stated that the defendant had an abusive childhood, and grew up raised by a single mother in a gang-ridden neighborhood. A control group just read about the murder without receiving any extra information about the defendant.

Participants that read about the defendant’s genetic background were more likely than the other group to believe the defendant couldn’t control their actions, and they were more likely to support an insanity or diminished capacity defense. The study’s participants were, however, equally likely to support a guilty verdict for the defendant regardless of the nature or nurture explanation for the crime.

“Defendants should be wary about using a genetic defense because it’s a double-edged sword,” Cheung said. “Judges or jurors may believe the perpetrator couldn’t control his actions, but they may also think he is a danger to society who will strike again.”

Previous studies have identified some genetic variables that may be linked to deviant behavior. A variant of the MAOA gene — also called “the warrior gene” — is associated with aggression, violence, and impulsivity, but there is not yet a scientific consensus about the exact role of genes in violent behavior.

Source: Cheung B, et al. Genetic Defense For Violent Crimes Could Backfire For Defendants. Society for Personality and Social Psychology. 2015.