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October 22, 2015

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In court, genetic defense may win sympathy—and no shorter sentence

Oct. 22, 2015
Courtesy of the Society for 
Personality and Social Psychology
and World Science staff

Ge­net­ic ex­plana­t­ions for vi­o­lent crimes may en­cour­age ju­ries and judges to sup­port an in­san­ity de­fense, yet al­so per­suade them to lock up the de­fend­ant, ac­cord­ing to new re­search.

Al­though some pre­dic­tions have held that ge­net­ic de­fenses might per­suade so­ciety to go easy on de­fend­ants, a new group of ex­pe­ri­ments finds that’s not nec­es­sarily so.

In­stead, the de­fense could be a dou­ble-edged sword, prompt­ing ju­rors to view the de­fend­ant as a per­sist­ent threat who will com­mit more crimes. The find­ings are pub­lished in the jour­nal Per­son­al­ity and So­cial Psy­chol­o­gy Bul­le­tin.

“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 re­search in­clud­ed three ex­pe­ri­ments with a to­tal of more than 600 par­ti­ci­pants 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.

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, the in­ves­ti­ga­tors said.

Some pre­vi­ous re­search has found that a var­i­ant a gene named MAOA, 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.

“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.”


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Genetic explanations for violent crimes may encourage jurors to support an insanity defense, yet also persuade them to lock up the defendant, according to new research. Although some predictions have held that genetic defenses could persuade jurors to let defendants go, a new group of experiments finds that’s not necessarily so. Instead, the defense could be a double-edged sword, prompting jurors to view the defendant as a persistent threat who will commit more crimes. The findings are published in the journal Personality and Social Psychology Bulletin. “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 research included three experiments with a total of more than 600 participants 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. 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, the investigators said. Some previous research has found that a variant a gene named MAOA, 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. “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.”