Editor's note: Saul Kassin is distinguished professor of psychology at the John Jay College of Criminal Justice and Massachusetts professor of psychology at Williams College. He is the author of several textbooks and has written extensively in the areas of social and forensic psychology.
(CNN) -- Twenty-two-year-old Jared Lee Loughner is the latest in a long line of young men to capture infamy. Mercifully, his name, though never the deed, will soon be forgotten.
From the University of Texas massacre of 1966 to Columbine, Virginia Tech, Fort Hood and countless others of more recent vintage, the names, dates and places may change, but the underlying problems are very much the same.
Nineteen people were shot in the heinous Tucson, Arizona, massacre, six fatally. The most critically wounded survivor was Democratic U.S. Rep. Gabrielle Giffords. Each victim's story is its own tragedy. Each leads Americans to wonder why it happened and what can be done to prevent it from happening again.
This past week, news commentators and bloggers have scrambled to lay blame on America's culture of violence, its failing schools, a lack of family values, hate-filled political rhetoric, financial cuts to mental health services, a lack of security for lawmakers, and easy access to assault weapons, to name just a few culprits.
As with any complex event in human behavior, this one cannot be explained by any simple univariate theory. There is a kernel of truth in much of what's been written, which is precisely why none of it provides a correct or complete explanation.
Over the past 100 years, research psychologists have studied the root causes of aggression. The list is so long that it begs two sobering conclusions: First, some degree of aggression is inevitable to the human condition. Second, certain types of individuals and situations increase that risk -- sometimes with perilous consequences.
Let's start with the types of individuals prone to violence, which might stem from a genetic predisposition or past experiences.
Research shows that men who are diagnosed with psychopathy betrayed signs of antisocial behavior as children by cutting school, setting fires, harming animals and the like. As adults, they lack empathy or anything that resembles a conscience. They remain calm in the face of electric shock or images of crying children, an attribute that makes them fearless.
This is not meant as a diagnosis of Loughner -- a young man who had become isolated from friends. In high school, we are told, Loughner drank excessive amounts of alcohol, a lubricant of disinhibition, sometimes to the point of passing out. He also used other illegal drugs. His behavior further indicates paranoia and a deep-seated mistrust in government -- in his mind, the perpetrators of 9/11. When he tried to enlist in the military, he was rejected.
It is important to realize what everyday observation tells us as a matter of common sense: Historically and universally, men are more violent than women and responsible for the vast majority of homicides. In the United States, all but two in the rash of school shootings were perpetrated by young men.
This gender difference is stable over time and place and has led many researchers to examine the link to testosterone. Studies with animals and humans have shown a positive correlation between testosterone levels and aggression, hence, the "roid rage" too often seen in men who take anabolic steroids.
Still, most people who are prone to aggression never lash out in a mindless determination to annihilate everyone in sight. A predisposition needs a trigger to spur action.
One classic theory states that frustration increases the drive to violence, either directed against the source of that frustration or at innocent scapegoats if the source is not available.
Frustration can stem from personal, social or economic failure. Road rage is a prime example. In someone who is predisposed, other noxious triggers include physical pain, excessive heat, insult, jealousy, loss of self-esteem and social rejection.
Social rejection is a particularly major risk factor for violence. Being bullied in school, excluded, neglected or otherwise ostracized are significant sources of frustration. In school shootings, many of the perpetrators were social outcasts, rejected by the classmates they targeted.
A U.S. Secret Service study of 37 such shootings showed they were not impulsive acts but carefully planned out. As in this case, almost every attacker had done something before the shooting that concerned at least one adult. Many talked of their plan to classmates who, tragically, did not alert parents or teachers.
It is often said that life imitates art. When it comes to exposure to depictions of graphic violence in movies, video games and YouTube videos, people, especially children, are suggestible. The constant exposure not only makes acts of violence seem more acceptable but has a numbing effect on our tolerance levels.
People learn by example, which is why seeing someone preach or model violence, often drawing praise from others, can set off a contagion of copycat crimes. It is why certain high-profile types of violence, such as school shootings, tend to happen in clusters, with each event serving as a catalyst for next.
Then there is the matter of guns. The finger pulls the trigger, we are told. Guns don't kill people, people do. Yet in laboratory experiments, subjects administer more painful electric shocks to an innocent stranger when a weapon is visually present than when it is not. This is an effect that has led social psychologists to conclude that "the trigger may also be pulling the finger."
To complicate matters further, recent studies show that merely handling a gun can increase a man's testosterone level.
These findings are reinforced by the correlation that shooting death rates are substantially higher in states that boast the highest per capita rates of gun ownership than in states with the lowest rates.
The rampage in Arizona is a simple act demanding a complex explanation. Loughner was troubled and therefore vulnerable. He had a motive, possibly incited by the tone of the politics of our time. He had opportunity by virtue of his access to an assault weapon.
None of this by itself can be blamed in a postmortem rush to judgment. Rather, this worst aspect of human nature happened in a complex web of known forces yet to be uncovered.
The opinions expressed in this commentary are solely those of Saul Kassin.