Genetic defense for violent crimes could backfire for defendants

October 21, 2015

Genetic explanations for violent crimes may encourage jurors to support an insanity defense, but jurors may also believe the defendant is a persistent threat who will commit more crimes in the future, according to new research published by the Society for Personality and Social Psychology.

"Genetic evidence plays an increasingly important role in the criminal justice system, but people often perceive genetic information in biased ways," said lead researcher Benjamin Cheung, a doctoral candidate in psychology at the University of British Columbia. "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 included three experiments with a total of more than 600 participants who were recruited online or at the University of British Columbia in Vancouver. Participants read different nature-vs.-nurture vignettes about a fictional murder committed by a college student. In one version, the defendant had a genetic variation associated with aggression and violent tendencies, while another version stated the defendant was beaten as a child by his single mother and grew up in a gang-ridden neighborhood. Participants were told that either the genetic or environmental background would cause the same fourfold increase in the likelihood of violent behavior. A control group just read about the murder without any additional information about the defendant.

Participants who read about the defendant's genetic background were more likely to support an insanity or diminished capacity defense, and they were more likely to believe the defendant couldn't control his actions or didn't intend to kill the victim. A diminished capacity defense claims that a defendant is only partially responsible for his actions because some condition hindered his ability to plan to kill someone or understand the consequences. However, the study participants were just as likely to support a guilty verdict for the defendant regardless of the genetic or environmental explanation for the crime.

Some previous research has found that a variant of the MAOA gene, nicknamed "the warrior gene," is associated with impulsivity, aggression and violence, but there is no clear scientific consensus about the role of genes in violent behavior. In 2009, an Italian judge reduced a convicted murderer's sentence by one year based on genetic evidence, and the same defense was used in a brutal murder case in Tennessee where the killer was convicted of a lesser offense and avoided the death penalty.

In the current study, participants who read the genetic explanation for the murder supported a shorter sentence for the defendant in only one of the three experiments, even though they believed the defendant had less control over his actions.

"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 also may think he is a danger to society who will strike again."

Society for Personality and Social Psychology

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