Scientists from the Karolinska Institutet in Sweden have genetically analysed 900 violent offenders in Finland and discovered that those with two particular genes are 13 times more likely to have a history of repeated violent behaviour.

The group of criminals had committed a total of 1,154 murders, manslaughters, attempted homicides and batteries. The researchers created a profile for each criminal according to their offences, classifying them as either violent or non-violent.

Their research, Genetic Background of Extreme Violent Behavior, is published in the journal Molecular Psychiatry.

The scientists discovered at least 4-10% of all violent crime in Finland was committed by people who had the monoamine oxidase A (MAOA) gene and a variant of the cadherin 13 (CDH13) gene.

Warrior gene previously linked to crime

The MAOA gene is known as the "warrior gene" and has previously been linked to violent crimes in other studies, although research has been inconclusive.

MAOA breaks down an enzyme known as monoamine oxidase A, which controls the amount of dopamine and serotonin (neurotransmitters) in the brain. In particular, dopamine is released by nerve cells sending signals to other nerve cells and is an important part of reward-motivated behaviour.

In the group of criminals studied, as well as in a replication group of another 114 criminals who had all committed at least one murder each, the researchers discovered these people had very low levels of the MAOA gene.

Without sufficient MAOA to control the amount of dopamine in the brain, a person could then experience "dopamine hyperactivity" from low levels of dopamine recycling, especially if they had taken drugs like amphetamines or drunk alcohol.

CDH13, on the other hand, is a protein-coding gene associated with neural connectivity that has been associated with ADHD and substance abuse. This gene has been linked to the impulse control in extremely violent criminals.

Genes do not make a criminal

Although these genes have been found to be associated with individuals who commit violent crimes, the researchers have stressed it is not possible to screen potential criminals using these genes as many other factors are involved in violent behaviour, such as environment.

In fact, a majority of the people who have these high-risk genes would never actually commit a crime.

"Committing a severe, violent crime is extremely rare in the general population. So even though the relative risk would be increased, the absolute risk is very low," Dr Jari Tiihonen of the Department of Clinical Neuroscience at Karolinska Institutet who led the study told the BBC.

"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."