Researchers have linked violent crimes to two genes, one which affects feelings, mood and behaviour, and the other, impulsivity.

Two genes have been identified that may be partly responsible for extremely violent crimes.

Scientists who studied 794 prison inmates in Finland found that at least 5 to 10 per cent of the country's severe violent crimes could be attributed to the genes.

One of the pair is a low-activity version of the monoamine oxidase A (MAOA) gene, which affects feelings, mood and behaviour by altering levels of the brain chemicals dopamine and serotonin.

The other, CDH13, is believed to influence impulsivity.

Low-activity MAOA showed an association with violent crime that increased for extremely violent offenders, while certain CDH13 variants were linked to repeat offending and homicides.

Previous research from Sweden has associated the criminal records of biological parents with violent and non-violent criminality in their adopted children. This suggests that at least some elements of criminality may be genetically inherited.

A rare mutation leading to a complete lack of the MAOA enzyme has also been linked to aggressive behaviour.

The research, led by Dr Jari Tiihonen, from the Karolinska Institute in Sweden, is published online in the journal Molecular Psychiatry.

Summarising their findings, the authors hint at a possible link with alcohol.

They wrote: "A majority of all severe violent crimes in Finland are committed under the influence of alcohol or amphetamine, both of which induce a transient increase in the dopamine levels in the brain.

"Therefore, it is logical to assume that a low dopamine metabolism (break down) rate due to low-activity MAOA ... may result in higher level of aggression during alcohol or stimulant intoxication."

Dr Seena Fazel, at Oxford University's Department of Psychiatry, said: "This is a potentially interesting study that needs replication in larger cohorts and should also be interpreted cautiously as recent reviews of MAOA genes have not found clear associations with violence."

Dr Malcolm von Schantz, from the University of Surrey, said: "Does this paper bring us closer to a situation where violent criminals can claim diminished responsibility because of the genes that they were born with?

"I think we have to remember that it becoming increasingly clear that there is not one single genetic variant that has a large effect on this, or indeed any complex behaviour. The pattern that is emerging is one of many genetic factors where each one has a small predisposing effect."

Copyright AAP 2014.