Violence
Fourteen men in an extended family of Dutch sociopaths committed impulsive, aggressive crimes including arson and attempted rape. In 1993, scientists reported that all 14 had the identical form of a gene on the X chromosome. The gene makes an enzyme called MAOA, which breaks down such brain chemicals as serotonin and noradrenaline. The normal version of the gene produces lots of MAOA; the aberrant form produces low amounts. Studies in animals had linked low enzyme levels to aggression, perhaps because when MAOA is in short supply the brain remains jacked up on neurochemicals in a way that induces aggression.
In 2002, scientists who had followed 442 New Zealand men since their birth found that the MAOA link was not nearly as straightforward as the Dutch study suggested. Yes, men with the low-activity form of the MAOA gene were more likely to engage in persistent fighting, bullying, cruelty and violent crime than were men with the high-activity version. But that was so only if they had been neglected or abused as children. If they had not been mistreated, men with the low-activity MAOA gene were not much likelier to be violent.
In his research on killers, Adrian Raine classifies them as either reactive, those who murder in response to an insult or slight (real or imagined), or proactive, who kill to achieve a thought-out goal such as robbery. Proactive killers show brain-activity patterns no different from that of normal, nonviolent volunteers, Raine reported in 1998. But the brains of reactive killers have clearly reduced activity in the prefrontal cortex, the site of such “executive” functions as judgment, planning, abstract reasoning, inhibiting inappropriate or impulsive behavior and self-monitoring.
In the brains of reactive killers the eerie quiet in the prefrontal regions is paired with increased activity in the limbic regions, site of emotions. Also overactive is a region involved in shifting attention, called the cingulate gyrus. “You become obsessive,” Daniel Amen says. “Someone with violent thoughts can’t let them go. Stalking is one sign of that.”
Life experiences and even introspection can alter patterns of brain activity. When people suffering from obsessive-compulsive disorder learn to think about their thoughts differently, for instance, they can quiet activity in the cingulate. That raises the possibility that killers’ aberrant brain activity is itself the result of experiences they had or thoughts they thought, rather than something that was wired in at birth. Individual differences in testosterone levels (as long as they are within 20% to 200% of normal) do not cause differences in levels of aggression, nor do changes in a man’s testosterone levels over time predict changes in aggression. Only levels at least four times the norm (as can occur with “roid rage”) spell trouble. And just as experiences can alter brain circuitry, so behavior can alter biology: aggression can raise testosterone levels even more strongly than testosterone raises aggression.
More than 90% of killers, and even more mass murderers, are male. Mass killers are usually 25 to 35, though school shooters are younger.
Studies find that up to 45% of boys who commit serious violent crimes by the age of 17, and up to 69% of girls, were inappropriately aggressive in childhood, picking fights with other kids. It is very rare for violence to show itself for the first time in a person’s 20s.
On a national scale, the countries of Western Europe that Pope Benedict laments are turning their backs on their Christian heritage have the world’s lowest rates of homicide. At the individual level, though, there is some evidence that regular church attendance “promotes moral integration,” says Jack Levin.