Roots of Violence May Lie in Damaged Brain Cells
A few weeks ago, two Vermont teenagers, Robert Tulloch and James Parker, received long prison terms for the slayings of two Dartmouth College professors. Last month, Andrea Yates, a mentally ill Houston mother, was convicted of drowning her five children, and David Westerfield, a self-employed engineer, was charged with the abduction and murder of 7-year-old Danielle van Dam near San Diego.
These killings, of course, were different in important ways. But every time one human being kills another, the rest of us are left to wonder why. For years, the prevailing assumption was that the roots of violence lie in bad environments and abusive parents, and this view is still scientifically supported--and politically correct.
In the last several years, however, brain scans and other studies of the minds of murderers show that there’s often another factor: damage or poor function of the prefrontal cortex, a part of the brain that lies just behind the forehead and eyes. Though brain scan evidence is not yet widely used in courts, it may soon be. Perhaps more important, understanding the biological roots of violence may enable doctors to intervene with drugs, behavior modification or other techniques to try to offset a person’s violent tendencies.
The current theory is that violent impulses originate in deep regions of the limbic system, or emotional brain, and that it’s then the job of the higher brain regions, specifically the prefrontal cortex, to decide whether to act on these impulses or not.
In particular, violent impulses seem to arise in the amygdala, hypothalamus and periaqueductal gray area, says Allan Siegel, a neuroscientist at the New Jersey Medical School in Newark. In fact, he says, different parts of the hypothalamus seem to be involved in different types of violence--the cold, premeditated type (such as stalking prey) and impulsive acts of rage.
Exciting as the new studies are, several caveats are in order.
First, while damage to the prefrontal cortex may help explain impulsive violence--sudden rage attacks--it can’t truly explain violence that is premeditated, said Dr. Ronald Schouten, a lawyer and psychiatrist who heads the law and psychiatry service at Massachusetts General Hospital.
That’s because the job of prefrontal lobes is “executive function,” that is, planning, integrating information and generally serving as a mechanism to control emotional impulses that originate in deeper brain regions. In other words, if a burglar who meticulously planned and executed a bank robbery tried to argue that he was not responsible because of damage to his prefrontal cortex, he probably would--and certainly should--be laughed out of court.
Second, no matter how compelling brain scan data are, the fact of brain injury or dysfunction by itself cannot explain violence, says Dr. Dorothy Otnow Lewis, who teaches psychiatry at New York University and Yale University.
Lewis, who has studied hundreds of murderers, stresses that “most brain-damaged people are not violent, and most people with serious mental illness are not violent either.” What does create a “cocktail of violence” occurs when a child with brain damage is raised in an abusive environment and is also prone to psychosis (loss of contact with reality). Indeed, childhood abuse itself can lead to brain damage, both from direct head injury and from the brain’s emotional response to abuse, researchers say. Exposure to trauma early in life can result in a surge of the stress hormone cortisol, which can affect the structure and function of nerve cells in the brain.
Third, none of the new studies probes the most obvious feature of violence: that, across cultures, men are far more likely to commit violent crimes. Citing FBI data from 1998, Lewis notes that in the U.S., men are eight times more likely than women to commit murder, nine times more likely to commit armed robbery and four to five times more likely to commit aggravated assault.
A leading theory for the striking gender difference involves the hormone testosterone, which is more abundant in men than in women. Precisely how testosterone may trigger violence in the brain is a mystery. In animals, considerable data show that aggression is linked to high testosterone and that castration (surgical removal of the testicles, which produce the male hormone) decreases aggression.
Research shows that male sex offenders who are castrated are less likely to repeat their crimes and that men who take body-building steroids, which are chemically close to testosterone, can become aggressive. Studies of prisoners--both male and female--also suggest that aggression is linked to high testosterone levels. So far, perhaps the most compelling visual evidence for the link between brain damage and violence is the work of Adrian Raine, a clinical neuroscientist at USC.
Raine performed PET scans on 41 convicted murderers and 41 other people of similar age who had no history of violence. In each group, 39 of the 41 people were male. (PET scans, which measure glucose uptake by brain cells, show which brain regions are most active.) The murderers had lower glucose metabolism in the prefrontal cortex, a sign that this region was not functioning as it should to inhibit aggressive impulses.
The results support previous work by researchers at the University of Iowa showing that healthy people who suffer damage to the prefrontal cortex can become impulsive and antisocial. It is also consistent with years of research by Lewis and Dr. Jonathan Pincus, chief of neurology at the Veterans Affairs Medical Center in Washington, D.C., showing that prefrontal damage is linked to violent behavior. Significantly, when Raine divided the murderers into those who killed with premeditation and those who killed impulsively, it was the impulsive killers who showed the poorest functioning in the prefrontal cortex.
In addition, in murderers’ brains, the corpus callosum--a band of tissue that links the right and left hemispheres--also functioned poorly. This makes sense, Raine says, because it may mean that the left hemisphere cannot communicate with the more emotional right side, thus allowing aggressive impulses to go uncontrolled. Also, the deep brain regions where scientists believe primitive emotions such as fear and aggression originate were more active in the brains of murderers than controls.
In other studies, Raine used MRI technology to scan the brain--a technique that looks at the structure, as opposed to the functioning, of different brain regions--to compare people with antisocial personality disorders with healthy people, others with substance abuse problems and yet another group with other psychiatric problems.
He found that brain cells within the prefrontal cortex regions of people with antisocial personality disorder--who show a psychopathic lack of remorse and a penchant for breaking rules and violent crime--were on average 11% smaller than those in the other groups, yet another clue that damage or dysfunction in this area may predispose people to aggression.
People with antisocial personalities also show other classic signs of a predisposition to aggression. Research suggests that a low resting heart rate at age 3 predicts aggression by age 11. In general, men have lower resting heart rates than women, another possible clue to the gender differences in aggression.
In fact, Raine’s studies show that violent people, in general, exhibit less physiological response to stimuli, including more sluggish sweating and skin changes, than nonviolent people. This raises the intriguing possibility that violence may serve as a kind of rush that jolts the brain toward more normal functioning.
At the National Institute of Neurological Disorders and Stroke, Dr. Jordan Grafman, chief of the cognitive neuroscience section, has studied wounded Vietnam veterans. He found that those with penetrating head injuries that caused damage to parts of the prefrontal cortex, as shown on CT scans, were at increased risk for violent behavior.
After compiling recent studies of violence and the brain in more than 500 people, University of Wisconsin psychologist Richard Davidson concluded that violent people have diminished brain activity in the prefrontal region, while activity in the amygdala is increased. His findings were published last July in the journal Science. Moreover, Davidson’s compilation confirmed that violent people may have disrupted functioning of a key brain chemical, serotonin. The data on violence and serotonin, and especially on the role of serotonin-boosting antidepressant drugs such as Prozac, is complicated.
Serotonin is clearly important for impulse control, says Yale University’s Lewis, and animal studies suggest that when serotonin goes down, vicious behavior increases. The bottom line? Violence is a complex behavior. The more researchers learn about the neurobiology of this impulse to harm, the more human beings, with their huge brains, may find ways to control it.
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Judy Foreman is a lecturer on medicine at Harvard Medical School. Her column appears occasionally.
