In 1993, scientists reported on a Dutch family, 14 members of whom were sociopaths, involved in aggressive crimes such as bullying, physical violence, rape, and arson. They all had in common a mutation in a gene that makes an enzyme called MAOA. The function of this enzyme is the breakdown of neurotransmitters such as serotonin and noradrenaline (or norepinephrine, a chemical first cousin of adrenaline). The ready conclusion was defective enzyme caused elevated levels of serotonin and noradrenaline, resulting in overactive brain circuits that serve aggressive behavior.

Case closed? Not so fast…

In a wonderful summary of the topic in Newsweek magazine (April 30, 2007), one of my favorite writers on the subject, Sharon Begley, describes a 2002 study in New Zealand of 442 men who were followed since their birth. Indeed, men with low MAOA were more likely to engage in persistent fighting, bullying, cruelty, and violent crime. But not all of them; only men who had been neglected or abused as children fit the bill. Men who grew up in a normal environment exhibited none of the violent traits.



In previous postings, we waxed scientific about the amygdala, two almond-shaped structures deep inside the brain that are the seat of primitive emotions such as rage and fear; these constitute the emotional basis of the fight or flight reaction, which is mediated by noradrenaline. These waves of seemingly overwhelming emotions are checked and inhibited by another, more modern structure in the brain: the prefrontal cortex. This structure is the seat of judgment, planning, and abstract thinking. It inhibits inappropriate or impulsive behavior and is engaged in constant self-monitoring (could it be the anatomical seat of the Freudian superego?). So in typical Ying/Yang fashion, the outcome of our behavior must then be the product of the amygdala and prefrontal interaction. Remember the then famous case of Kip Kinkel, a 15-year-old who, in 1998, killed his parents and two dozen schoolmates in Springfield, Oregon? His brain scan showed a completely silent prefrontal lobe; he had nothing to check and balance his raging anger emanating from his amygdala.

Is this it? Not quite…



Women love to point out, without much evidence I might add, that men’s aggressive behavior can be traced to their testosterone-addled brain. Only partly true. The level of testosterone is within normal limits between 20% and 200% of the mean; that’s a huge range of normal. However, if the level of testosterone exceeds 400% of the mean, then indeed women are right; men with these levels are more prone to violence. In fact, testosterone is an equal opportunity hormone; in a species of hyenas (I forget which), the first newborn in a litter, be it male or female, will eat the rest of the brood within days of birth. It turns out that this vicious sibling has inordinately high levels of testosterone in its brain, much higher than the other hapless siblings.

But to assume that we are simple automatons, following helplessly the script written by our genes, brain circuits, and hormones, would deny a self-evident fact—we don’t behave automatically, we do have a certain degree of free will.



The interaction of biology and the life one leads turns out to be of paramount importance in shaping the criminal mind. The most important characteristic of the behavior of mass killers is paranoia. They have the sense that the whole world is against them, that everybody but themselves is responsible for their troubles, that the world is unfair. They are usually depressed and socially isolated.

This kind of personality, you might say, could be the product of brain circuitry gone awry. But here is a fascinating finding from animal and human studies: Behavior can change brain circuitry and function, an outstanding example of nature/nurture interaction. So what are the non-biological roots of violent behavior? We finally arrive at the inevitable.


Society and culture

It is the social environment that allows, indeed encourages, psychopathic criminal behavior. Many societies have members with genes gone awry, with malfunctioning brain circuits, with males suffering from raging hormones, with children raised in violent homes. But, sad to say, we have the dubious distinction of being the champions of gun violence in the civilized world. In 2004, there were 29,645 deaths due to gun violence in the U.S., or 10.08 per 100,000. For comparison, France had 4.93, Belgium 3.67, and Spain 0.75 per 100,000.

In 5 years of war in Iraq, about 3,200 of our soldiers got killed. Yet, we tend to see the situation in Iraq as intolerable and dismiss the carnage in our own streets with a helpless shrug: “It’’s the culture…”

We mentioned the case of Kip Kinkel. Yes, his prefrontal lobe did not do its job. But here is rest of the story: A psychotherapist actually suggested that his dad buy him a gun so they could have something to do together.

As Pogo said: We have met the enemy, and it is us.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.