A mutated form of two genes is seen in a "substantially higher frequency" in violent offenders compared to normal criminals, said researchers in the United States and Europe.

A study of almost 800 Finnish convicts imprisoned for both non-violent and violent crimes, discovered variants of two genes, called CDH13 and MAOA, to be "associated with extremely violent behavior."

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"No substantial signal was observed for either MAOA or CDH13 among non-violent offenders, indicating that findings were specific for violent offending," the study said.

Researchers also took into consideration environmental factors that may have affected the criminals, such as antisocial personality disorders, childhood maltreatment or substance abuse. These factors didn't change the results of the study.

The study wasn't designed to describe the effect of variants in genes. The study's authors believe there could be a number of other genes that have a role, indirectly or directly, in the molecular cascade.

Researchers also noted that the two mutated genotypes, or gene versions, are "rather common" with as many as one in five people having the genes, The vast majority of these people, however, never commit assault, rape or murder.

"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," study co-author Jari Tiihonen of the neuroscience department at Sweden's Karolinska Institutet told AFP.

The CDH13 gene is believed to play a role in the control of impulses, and has been linked to attention deficit hyperactivity disorder or ADHD.

The MAOA gene has been associated with the metabolism of dopamine, a neurotransmitter that plays a role in the ability to experience pleasure and addiction.

"I think that we have found two genes which have the largest effect on aggressive behavior, and that there are probably tens or hundreds other genes having smaller effects," said Tiihonen.

TagsGenetics, genes, CDH13, MAOA, violence, Rape, Murder, Assault, dopamine, Attention deficit hyperactivity disorder, Aggression