Whether criminals are born with an innate tendency to hurt others, are slaves to mental disorders, or are molded by factors such as childhood trauma, a history of abuse or too many violent video games is a persistent and complicated question.
Now, new research suggests that genetics may in fact contribute to a propensity for violent criminal behavior.
In a study published Tuesday in the journal Molecular Psychiatry, researchers in Finland examined the genes of 895 people found guilty of crimes ranging from non-violent offenses such as drug or property crimes to severely violent offenses such as homicide and battery. They found that variants of two particular genes -- called MAOA and CDH13 -- were linked to "extremely violent behavior," defined as having committed at least 10 homicides, attempted homicides or batteries.
A strong relationship between behavior and either of the two genes was not present among non-violent offenders. Even when the researchers accounted for factors such as personality disorders, childhood maltreatment or substance abuse, the effects were still specific to violent offenders.
The investigators estimate that between 5 and 10 percent of all violent crimes in Finland can be attributed to those two genes. But they also stress that the results should not be considered when sentencing criminals.
"It is equally important to realize that, according to the basic principles of forensic psychiatry, only the actual mental capability (phenotype) of the offender matters when punishment or legal responsibility is considered, and the putative risk factors per se (such as genotype) have no legal role in the resulting judgment," the authors wrote in the study.
The relationship between genetics and violent behavior was strongest for the 78 people in the study who were classified as "extremely violent offenders." The people in this group committed a total of f 1154 murders, manslaughters, attempted homicides and batteries.
The MAOA gene plays a role in the metabolism of the neurotransmitter of dopamine that helps regulate emotions and reactions to pleasure and rewards. The other gene, CDH13, has been linked to impulse control.
Previous research has also suggested a link between hereditary factors and violent behavior, particularly when it comes to the MAOA gene variant, which has even been dubbed a "warrior gene." In a more general study that did not focus on a particular gene, published in 2013 in the journal Labor of Economics, scientists examined a large Swedish adoption database and found that criminal records of biological parents predicted criminality among their children who had been adopted by other people. In another study, published in 2012 in the journal Science, investigators found a relationship between convictions of violent crimes and a combination of low-activity MAOA plus childhood maltreatment. A large analysis of previous studies published this year in the journal Biological Psychiatry that included 11,000 participants also pointed to a relationship between low-activity MAOA, childhood adversities and antisocial behavior.
In an interview with the BBC, Jan Schnupp, a neuroscientist the University of Oxford commented on the new study with skepticism, saying that up to half the population could have one of the genes that the study linked to violent behavior.
"To call these alleles 'genes for violence' would therefore be a massive exaggeration," Schnupp said. "In combination with many other factors these genes may make it a little harder for you to control violent urges, but they most emphatically do not predetermine you for a life of crime."