Serial killers and other people who repeatedly commit violent crimes, such as assault and battery, may be hardwired to hurt others, suggests a new study that identifies two genetic variants tied to extreme violent behavior.
The findings, published in the latest issue of the journal Molecular Psychiatry, could help to explain why the majority of violent crimes are committed by a small group of antisocial, repeat offenders. The extensive study represents the first effort to investigate the genetic background of people exhibiting such repetitive, brutal behavior.
"I think that we have found two genes that have the largest effect in aggressive behavior, and that there are probably tens or hundreds of other genes having smaller effects," lead author Jari Tiihonen told Discovery News.
Tiihonen, a professor in the Department of Clinical Neuroscience at the Karolinska Institute in Sweden, and his colleagues screened 1004 prisoners in Finland. Of that group, 184 refused to participate in the study and 26 were excluded because of "a psychosis diagnosis." Blood samples for DNA extraction were taken from the remaining prisoners, whose crimes and backgrounds were also investigated.
The scientists determined that the prisoners who had repeatedly committed, or attempted to commit, violent crimes tended to have one or both of the following genetic variants: CDH13 and MAOA, a.k.a. the "warrior gene."
Tiihonen explained that MAOA metabolizes an important neurotransmitter called dopamine. The presence of this genetic variant and substance abuse helps to create a perfect storm.
"If this activity is decreased, it might lead to a larger dopamine burst in the brain when alcohol, cocaine or amphetamine is used," he explained. "It is known that these substances induce dopamine burst and aggression."
CDH13, on the other hand, is a gene that codes for neuronal adhesion protein, so it contributes to the development of neuronal connections in the brain. It is one of the most important genes associated with Attention Deficit Hyperactivity Disorder (ADHD).
People with both of these genetic variants have a 13-fold increase of committing a violent crime versus those who do not have the mutations, according to the study. Add substance abuse again to the mix and there is a likely recipe for disaster.
The study could also help to explain why most mass murderers are men.
"Since MAOA is located in the X-chromosome, men have only one copy of the gene and women have two copies," Tiihonen said, explaining that women have two X-chromosomes. "Therefore, females can have one low activity allele (alternative form of a gene that arises by mutation) and one high activity allele, but if males have a low activity allele, they cannot have another allele functioning more efficiently because they have only one copy of the gene."
Yet another recent study, conducted on a sample of about 100 inmates from a correctional institution in the Southern United States, also determined that MAOA is associated with higher rates of crime when the individual additionally experienced childhood adversity.
"These findings (published in the journal Psychiatric Genetics) indicate that gene-by-environment interactions are important for understanding variation in crime amongst populations with high base rates of criminal activity," said principal investigator Todd Armstrong of Sam Houston State University.
Tiihonen agrees that environmental factors, such as childhood abuse, are important to consider. Also of relevance is, as Tiihonen said, "the mental capacity to understand the nature and consequences of one's deeds, and ability to control one's behavior."
He added that worldwide, about 20 percent of many populations carry at least one of the two genetic variants. Clearly not all of these people are murderers. Genetics is turning out to be a very important risk factor for committing crime, but it's not the only one.