Brothers Who Are Murderers: Why So Many?

From genetics to life at home, there appear to be a number of factors that predispose some brothers to violent behavior.

So it may not be a coincidence that two sets of brothers have been identified as terrorist suspects -- Dzhokhar Tsarnaev and his slain brother, Tamerlan Tsarnaev, in the Boston Marathon bombing and, now, brothers Saïd Kouachi and Cherif Kouachi for the killing of 12 people at the offices of the Charlie Hebdo newspaper in Paris.

The Kouachi brothers were killed by police today as the pair held a hostage outside of Paris.

"There are numerous factors" that contribute to terror-related murders, "including neurobiological, personality, social and cultural," neuropsychologist Robert Hanlon of the Northwestern University Feinberg School of Medicine told Discovery News.

He added, "One brother is usually a stronger character and the leader of the two. The other brother is a follower and is influenced by the leader-brother. The follower wants to please the leader and obtain the respect of the leader."

Hanlon added that such siblings tend to be male, with men committing 90 percent of murders annually in the United States alone.

"Due to biological, psychological and socio-developmental differences between the genders, men are far more violent than women," he said.

In addition to terrorist duos, there are also a number of serial killers who are brothers, such as Pete and Pat Bondurant, Larry and Danny Ranes and Reginald and Jonathan Carr, just to name a few.

Terrorism is a subtype of violence that, as Hanlon indicates, can have multiple driving influences.

Neuroscientist Jeremy Richman of The Avielle Foundation added, "All of our behavior has both a genetic and environmental component -- they cannot be separated."

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.

Read more at Discovery News