On Sept.13, 2006, Kimveer Gill walked into the cafeteria at Dawson College in Montreal and, without apparent motive, shot 22 people, injuring 19 and killing two, including himself.
Also on that day, a court in New York sentenced a man for killing his girlfriend by setting her on fire -- in front of her 10-year-old son.
There was nothing special about that day. From around the world we hear reports of killings and abuse every day. Violence is ubiquitous.
But what drives one person to attack another, sometimes for little or no obvious reason?
In search of the psychological and biological roots of violence, researchers around the world have considered acts ranging from fistfights to murder. They've concluded that behavior results from a combination of risk factors -- among them inherited tendencies, a traumatic childhood and other negative experiences -- that interact and aggravate one another.
The silver lining, however, is that positive influences can offset some of the factors that promote violence and possibly offer hope for prevention.
The Gender Gap in Criminal Behavior
People who exhibit antisocial behavior fall into two distinct groups, according to behavioral scientists Terrie E. Moffitt and Avshalom Caspi, both at King's College London and the University of Wisconsin-Madison. Most are between the ages of 13 and 15, and their delinquency stops just as quickly as it starts. A small minority, however, display antisocial behavior in childhood -- in some cases as early as age 5 -- and this conduct continues into adulthood.
Among this latter group, almost all are boys.
Indeed, male gender is the most important risk factor for violent behavior. Criminal statistics show that boys and young men commit the majority of physical assaults. According to the Federal Bureau of Investigation's statistics on crime in the United States, 90 percent of murderers apprehended in 2004 were male, and men accounted for 82 percent of the arrests for violent crimes.
Yet girls and women are not necessarily less aggressive, as was widely assumed until the 1990s. Instead, women usually engage in more indirect, covert aggression.
Males who are chronically violent from an early age also typically have low tolerance for frustration, attention problems, deficiencies in learning social rules, a decreased capacity for empathy and low intelligence. Their single most characteristic trait, however, is extreme impulsiveness.
Similarly, repeat criminal offenders -- particularly those who have long prison records -- seem unable to keep their aggressive urges in check. The late neuroscientist Ernest S. Barratt and his colleagues at the University of Texas Medical Branch interviewed imprisoned criminals in Texas in 1999 and found that many inmates consistently picked fights, even when they knew that their lives would be made more difficult as a result. Although they frequently resolved to act with greater self-control in the future, they did not trust their own ability to keep their impulses under control.
The Brain Could Be to Blame
Preliminary research indicates that biology may handicap some of these individuals, making it more difficult for them to show restraint.
Looking at violent offenders, neuroscientists have found subtle differences in the limbic system and the prefrontal cortex regions of their brains, both of which help to regulate the emotions.
Possibly these abnormalities interfere with proper communication between these brain centers, leading to a loss of control over fearful and aggressive impulses. Such people might therefore not be entirely able to moderate his or her own emotional reactions.
Researcher Jordan Grafman and his colleagues at the National Institutes of Health have discovered that Vietnam War veterans who suffered damage to the prefrontal cortex tend to be more aggressive. Similarly, adult patients who suffer injuries to their frontal brain areas are generally more uninhibited, inappropriate and impulsive, much like people with antisocial behavior disorders. In these adult groups, however, there is no direct indication that their brain damage predisposes them to actual violence.
For children who suffer frontal brain injury, the behavioral consequences are often more dramatic, as documented by neuroscientist Antonio R. Damasio and fellow researchers at the Medical Center of the University of Iowa.
In one case, surgeons removed a tumor from the right frontal cortex of a 3-month-old infant. By age 9, the boy had become almost impossible to motivate in school, remained socially isolated and spent almost all of his free time in front of the television or listening to music. Occasionally, he would "go wild" and threaten others, sometimes physically.
In another case described by Damasio, a 15-month-old girl suffered head injuries in a serious automobile accident. At age 3, she started to exhibit behavioral disorders. Later she refused to comply with rules, frequently fought with teachers and classmates, both verbally and physically, lied shamelessly, stole and broke into homes.
Additional evidence that frontal brain abnormalities are linked to aggression comes from Adrian Raine and his colleagues at the University of Southern California, who studied convicted murderers. Brain scans revealed that the murderers' frontal brain regions had unusually low metabolic activity. However, this difference existed only among criminals who had killed on impulse. The frontal brain appeared normal in those murderers who had carefully planned their crimes.
These findings and others by Raines suggest that in true psychopaths, who may kill in cold blood and often express little or no empathy for others, the causes of violent behavior are completely different than for those who act in the heat of the moment. Impulse control in these calculating criminals may be normal, but perhaps they have other brain abnormalities that contribute to their behaviors. James Blair of the National Institute of Mental Health believes that malfunction in brain centers such as the amygdala may harm the ability of these individuals to relate to others, reducing their capacity to feel empathy or guilt.
Criminal Behavior Could Have Chemical Link
The biochemistry of the brain may also prime some people for violence. Numerous studies have linked low levels of serotonin, a compound that helps to transmit certain messages within the brain, to antisocial, impulsive acts. This association seems to exist not only among criminals but men in general. Studies have not confirmed the same connection in women, however.
Biochemical differences, along with genetic and structural variations in the anatomy of the brain, do seem to increase a tendency toward violence in some men. Except in the most severe and early cases of damage, however, those factors are not enough to make violent behavior inevitable. Only in combination with psychological and social risk factors does that unfortunate biological mix become explosive. Such risk factors include serious deficiencies in the early mother-child relationship, abuse in childhood, parental neglect or criminality, and poverty and long-term unemployment.
A robust cognitive and emotional makeup, on the other hand, can help some children to overcome the negative influences in their environment. At present, it is still unclear why many people can compensate for terrible childhood experiences or early brain damage while violent offenders cannot. Future research, however, may provide the tools needed to help authorities identify potentially violent offenders sooner and intervene before it is too late.
Adapted in part from "The Violent Brain," by Daniel Strueber, Monika Lueck and Gerhard Roth, in Scientific American Mind (Dec. 2006/Jan. 2007 issue)
Read the article at Scientific American.