On the "nature" side of nature/nurture, two genes were recently discovered that increase a person's risk for violent behavior 13-fold: MAOA and CDH13. The research was led by Jari Tiihonen, a professor in the Department of Clinical Neuroscience at the Karolinska Institute in Sweden. He points out that men are more likely than women to carry the two genes.
"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."
It's then possible that the genes are passed down in families, mostly affecting males. A forthcoming study on twins, which will be published in the journal Biological Psychology, found that some males, even as children, have a measurable "state of low arousal in the brain" that may require riskier, more impulsive and higher excitement activities "to achieve the arousal levels that a normally aroused brain typically experiences."
On the "nurture" side of nature/nurture, several studies have found that the environment in which children are raised can have a profound impact on the way their brains process information.
For example, Michaela Chraid, of the University of Bucharest's Faculty of Psychology and Educational Sciences, found that children who are raised in unstable families, including with abusive individuals, can become desensitized to violence at a subconscious level. They often are unaffected emotionally when witnessing horrific acts, such as beheadings, which otherwise disturb children who empathize with the victims.
A family's religious beliefs, although often mentioned in the media as playing a role in sibling terrorist acts, have been falsely implicated, Hanlon said.
"Many terror-oriented murderers are predisposed to violence as a result of their personality (i.e., antisocial psychopaths). Because of their antisocial personalities, they seek out terror organizations with which they can identify. Then, their antisocial and violent tendencies are reinforced by the leaders of terror organizations and they are motivated to kill for the cause of the organization," he said.
Hanlon added, "Religion and religious beliefs, in my opinion, play a very small role in terror-related murders. Religion is used as an excuse by murderers to kill."