Common variants of two genes linked to behaviour can significantly raise the likelihood of violent criminal behavior, a study - which has received a mixed reaction from expert commentators - suggests.
The research involved DNA analysis of almost 900 convicted criminals in Finland, making the study possibly the largest of its kind. Each criminal was categorised according to the violence of the offences they committed.
Violent offenders were more likely than non-violent criminals to carry two gene variants in particular. This association became even stronger with the 78 offenders in the 'extremely violent' category.
In total, the people in this group had committed a total of 1,154 murders, manslaughters, attempted homicides or batteries. When the researchers looked to replicate their findings a similar pattern was found in a group of 114 convicted murderers.
'When compared to the control population, non-violent offenders were not observed to exhibit either variant to a greater degree, indicating that these genetic variants may be specific to extremely violent behaviour,' the Daily Mail quotes Professor Jari Tiihonen of the Karolinska Institutet in Sweden, the lead author of the study, as saying. In fact, both variants are widespread in the general population.
The first variant is a 'low-activity' version of the Monoamine Oxidase A (MAOA) gene. The gene helps control the activity of important neurotransmitters like dopamine and serotonin in the brain. The MAOA variant has already been linked to behavior and even appears in popular science articles as the 'warrior gene'.
The study authors suggest that a combination of the MAOA variant and alcohol intoxication or substance abuse can heighten levels of aggression, thereby increasing the risk of violent criminal behaviour.
Dr William Davies, a senior lecturer at the Centre for Neuropsychiatric Genetics and Genomics, Cardiff University, who was not involved in the study, said that the findings 'apparently substantiate previous controversial suggestions that this version of the gene is in some way associated with violent criminality'.
However, other scientists were less impressed. Professor Jan Schnupp, professor of neuroscience, University of Oxford, disputed the authors' conclusions and noted that half the general population carries the low-activity MAOA variant.
'Half the people in your office will carry these genes. Odds are 50/50 that you do. How violent has your day been?' Professor Schnupp asked.
'The overwhelming majority of carriers of these genes, more than 99.5 percent or so, manage to make it through each day without giving in to impulses to bash other people's heads in.'
To link these variants specifically to violent behaviour 'would therefore be a massive exaggeration', Professor Schnupp argued.
The second variant identified in the study was for the Cadherin 13 (CDH13) gene, which is important in regulating how brain cells grow and connect. Just over half of the repeatedly violent offenders carried at least one copy of the variant, compared with just under a third of the non-violent subjects.
Dr Davies called the finding 'intriguing', but added: 'It is important to note that this second finding did not quite meet accepted statistical criteria for significance, and will need replication in larger sample sizes.'
In the paper, the authors say their findings 'imply that at least about five to ten percent of all severe violent crime in Finland is attributable to the […] MAOA and CDH13 genotypes'.
John Stein, emeritus professor of physiology at the University of Oxford, questioned this. He said that, instead, the results suggested the variants 'may contribute five to ten percent to the chance of an individual being very violent'.
'In fact their Genome-Wide Association Study (GWAS) did not give any significant results,' Professor Stein added. 'So, if a single genetic factor alone could significantly explain violent behaviour they would have seen it in their GWAS results.'
The research was published in the journal Molecular Psychiatry.