A short piece published in BBC Magazine explored the science of whether murderers are born or made. A British neurocriminologist named Adrian Raine has made a career out of studying the brains of violent criminals. Raine was the first person to conduct a brain imaging study on murderers, and has since scanned the brains of numerous homicidal individuals, looking for similarities. Raine’s brain scanning studies found two similarities in the brains of nearly all his participants: 1) reduced activity in the pre-frontal cortex, which means less emotional impulse control, and 2) over activation of the part of the brain that controls our emotions, called the amygdala. According to the BBC, Raine’s study suggests that ” that murderers have brains that make them more prone to rage and anger, while at the same time making them less able to control themselves.” Childhood abuse could be a factor because of the damage it can cause to the brain, particularly to the pre-frontal cortex. But, as the BBC put it, “only a small proportion of those who have a terrible childhood grow up to become murderers,” which brings us to the next possibility: genetics. Are there genetic factors that predispose us to crime?

A breakthrough came in 1993 with a family in the Netherlands where all the men had a history of violence. Fifteen years of painstaking research revealed that they all lacked the same gene.

This gene produces an enzyme called MAOA, which regulates the levels of neurotransmitters involved in impulse control. It turns out that if you lack the MAOA gene or have the low-activity variant you are predisposed to violence. This variant became known as the warrior gene.

About 30% of men have this so-called warrior gene, but whether the gene is triggered or not depends crucially on what happens to you in childhood.

The research in question was conducted by Han Brunner, a Dutch geneticist working out of a teaching hospital in the Netherlands’ oldest city. Brunner’s research was first published in Science in October 1993, and that same month Sarah Richardson wrote about it for Discover magazine in an article entitled “Violence in the Blood.” Richardson’s piece is fascinating, especially in its explanation of how the geneticists used different clues to determine the origin of the aggressive behavior. This is how it begins:

One day in 1978 a woman walked into University Hospital in Nijmegen, the Netherlands, with a problem: the men in her family. Many of them–including several of her brothers and a son–seemed to have some sort of mental debility. Gradually, as the clinical geneticists who counseled the woman got to know her and her family, the details of the strange behavior of the woman’s male kin emerged. One had tried to rape his sister; another had tried to run his boss down with a car; a third had forced his sisters to undress at knife point. Furthermore, the violent streak had a long history. In 1962 the woman’s granduncle had prepared a family tree that identified nine other males with the same disorder, tracing it as far back as 1870. The granduncle, who was not violent himself–he worked in an institution for the learning disabled–had apparently come to suspect that something was terribly wrong with his family.

Three decades later, and 15 years after the woman’s first office visit, geneticist Han Brunner and his colleagues at the Nijmegen hospital think they’ve figured out what that something is. Some of the men in the woman’s family, they say, suffer from a genetic defect on the X chromosome- -a defect that cripples an enzyme that may help regulate aggressive behavior. If Brunner and his colleagues are right, it would be the first time a specific gene has been linked to aggression. That means their finding cannot fail to be controversial.

See the stories:

1. “Are Murderers Born or Made?” (BBC Magazine)

2. “Violence in the Blood.” (Sarah Richardson, Discover)