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.
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.
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.