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.