How do individuals become radicalized to the point where they choose to massacre innocents?

From left: The California State University–Fullerton student identification of San Bernardino mass murder suspect Syed Farook sits on a desk inside Farook's home in Redlands, California. (Photo: Robyn Beck/AFP/Getty Images); Colorado Springs shooting suspect Robert Lewis Dear appears before Judge Gilbert Martinez via video feed. (Photo: Mark Reis-Pool/Getty Images)

While much remains unknown about the motives of our two most recent alleged mass murderers—the man who shot up a Planned Parenthood clinic in Colorado last month, and the husband-and-wife couple who killed 14 people in San Bernardino, California, on Wednesday—all appear to have become self-radicalized. Something happened within the mind of each individual that led them to make the tragic leap from strong belief in an ideology to violent action on its behalf.

This is, fortunately, a relatively rare occurrence. The overwhelming majority of people who believe abortion is murder, or that Islam is being unfairly attacked by the West, do not feel compelled to kill innocents in the name of their cause. But as we have witnessed over the past week, a few do, and this lends increasing urgency to the question of why.

Terrorism researchers emphasize that there is no single path that leads to mass murder, and the motivations behind such attacks are often a mix of the political and personal. In a 2010 paper, Australian sociologist Ramón Spaaij reported that "lone-wolf terrorists tend to create their own ideologies that combine personal frustrations and aversion with broader political, social, or religious aims."

If terrorists were not able to get their hands on guns, ammunition, bomb-making equipment, and the like, they would not be able to act on their violent impulses.

In a 2014 paper that asks "What moves an individual from radical opinion to radical action," Bryn Mawr University researchers Clark McCauley and Sophia Moskalenko write that there are "at least two profiles for lone-wolf terrorists."

"Statistical studies indicate that may be called a disconnected-disordered profile: Individuals with a grievance and weapons experience who are socially disconnected and stressed with a psychological disorder," they write, while others "have a caring-consistency profile: They felt strongly the suffering of others and a personal responsibility to reduce or revenge this suffering."

McCauley and Moskalenko suspect the first group is more common, "not least because self-sacrifice for others is less common than self-interest." But perhaps because such people fit the stereotype of the dangerous loner, it's arguable that we pay insufficient attention to those in the second category.

McCauley and Moskalenko "emphasize the practical, situational requirements for crossing the gap between radical opinion and radical action." They argue that "the most dangerous indicator of potential for lone-wolf terrorism is the combination of radical opinion with means and opportunity for violent action."

To put it plainly: If terrorists were not able to get their hands on guns, ammunition, bomb-making equipment, and the like, they would not be able to act on their violent impulses.

Aside from the issue of tighter restrictions on firearms, the other question that always arises after such tragedies is the state of the perpetrator's mental health. Indeed, in his look at lone-wolf terrorists, Spaaij finds high rates of "psychological disturbance."

That was certainly the case with "Jihadi Jane," a.k.a Colleen LaRose—the Pennsylvania woman who plead guilty in 2013 to attempting to kill a Swedish cartoonist whose work offended many Muslims. In a paper published earlier this year, philosopher and attorney Caroline Joan S. Picart traces her tragic life.

Picart concludes that, "although contact with particular sites and mentor figures were necessary for her transition from radical thought to radical action, the mere proliferation of radical jihadi sites on the internet alone was not sufficient for her radicalization." In LaRose's case, she writes, "among these factors were: trauma from childhood rape and incest, and prostitution."

Picart notes that "from the age of eight Colleen and her eleven-year-old sister, Pam, were raped repeatedly by their biological father," leading to her running away from home at 13 and becoming a prostitute. "These horrific experiences left deep imprints upon her psyche," she adds, no doubt making her vulnerable to radicalization.

Syed Farook, one of the alleged San Bernardino shooters, also had a traumatic childhood. From the Washington Post:

The home Farook grew up in was troubled and at times apparently violent. In 2002, when both husband and wife were unemployed and had run up credit card and retail debt of about $50,000, the couple filed for bankruptcy protection. And for most of the past decade, right up to early this year, his parents waged war in court against each other. (His mother) repeatedly sought — and at least twice won — restraining orders against a husband who she said abused her verbally and physically. In court papers, she described her husband as a “bipolar” alcoholic and told of harrowing incidents: He threw dishes in the kitchen. He pushed and choked her. He dropped a television on her.

Much research suggests rowing up in such an environment can cause a range of mental-health issues.

In contrast, we know virtually nothing about the childhood of the alleged Colorado gunman, Robert Dear. But interviews with people who knew him paint a picture of severe mental instability. Again, from the Washington Post:

"He complained about everything," said another neighbor who, like others, spoke on the condition of anonymity, saying that he feared for his security. "He said he worked with the government, and everybody was out to get him, and he knew the secrets of the U.S.A. He said, 'Nobody touch me, because I've got enough information to put the whole U.S. of A in danger.' It was very crazy."

Crazy, but not uncommon. McCauley and Moskalenko write there is "a growing consensus" that lone-wolf attackers are likely to suffer from "depression or (some) other mental disorder."

Combine that with an ideology that sanctions violence, and a society that provides easy access to weaponry, and the result is, too often, unspeakable tragedy.

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.