A new genetic study linking two genes to extreme violent behavior is raising serious questions as to what makes a criminal and whether or not these genes could be used to screen — or even let off — potential offenders.
A Karolinska Institute research team led by Jari Tihonen found that prisoners in Finland with mutated forms of the two genes — the MAOA gene (sometimes called the "warrior gene") and a variant of cadherin 13 (CDH13) — were 13 times more likely to have a history of repeated violent behavior. According to the study, which now appears in the journal Molecular Psychiatry, at least 5 to 10% of all violent crime in Finland is attributable to individuals with this particular genetic profile. That's obviously quite significant, but there's more to it than that.
For the study — the largest of its kind to date — nearly 900 Finns imprisoned for both violent and non-violent crimes underwent a genetic analysis. Each criminal was assigned a profile based on the nature of their offences. In all, the group committed a total of 1,154 murders, manslaughters, attempted homicides, or batteries. A replication group consisting of 114 criminals was also studied. After comparing the genetic profiles of these offenders to those in the general population, the researches found a "substantially higher frequency" of the implicated genes in violent offenders.
What's more, "No substantial signal was observed for either MAOA or CDH13 among non-violent offenders, indicating that findings were specific for violent offending," noted the researchers in the study.
The scientists also considered environmental factors, like whether or not people had a history of substance abuse, antisocial personality disorders, or childhood maltreatment. But these co-factors, say the researchers, did not alter the outcome.
Importantly, the study wasn't designed to explain the association. In fact, the scientists involved in the study are quite certain many other genetic variants play a role — either directly or indirectly — in the cognitive molecular cascade that's known to influence behavior.
Moreover, the two mutated gene versions are "rather common." In fact, as many as one in five people have them — the vast majority of whom never commit extreme violent crimes such as rape, assault, or murder. At the same time, people without the variants were also found in the ultra-violent trial group.
"Although the high-risk genotype combination of MAOA and CDH13 has a risk of about 13-fold compared with the 'usual' genotype combination, still the vast majority (of) high-risk genotype individuals do not commit severe violent crimes," noted Tihonen in an AFP article.
As noted, other genes may be responsible for violent behavior, and despite screening for environmental factors, it's a safe bet that social and economic factors play a non-trivial role in facilitating violent behavior.
That said, given the association, it's important to look at the two genetic variants to see how a person's brain could be primed for such behavior. The MAOA gene is linked to the metabolism of dopamine, an important neurotransmitter that's known to play a role in addiction and the ability to experience pleasure. A deficiency in this enzyme results in "dopamine hyperactivity", which is pronounced when a person drinks alcohol or takes drugs. And in fact, the majority of all individuals who commit severe violent crime in Finland were under the influence of alcohol or drugs.
As for CDH13, it's thought to be involved impulse control. It may also play a role in ADHD.
Tihonen agrees that a person's genetic information should not have any bearing on conviction outcomes in criminal courts. Speaking to the BBC he added:
There are many things which can contribute to a person's mental capacity. The only thing that matters is the mental capacity of the individual to understand the consequences of what he or she is doing and whether or not the individual can control his or her own behaviour.
Indeed, these two genes aren't hindering an individual's capacity to function as a responsible individual in the real world, or to impair their ability to discern right from wrong.
That said, some courts are reducing sentences of criminals with genes linked to bad behavior. For better or worse, studies like these are poised to change the way criminals are sentenced and how we assess accountability.