After Anders Breivik detonated a car bomb, killing eight people, in central Olso in the summer of 2011, he took a ferry ride to an island in a peaceful Norwegian lake. Armed with a semiautomatic rifle and pistol, he killed 69 people, most of them teenagers at a summer camp.

A team of psychiatrists who examined Breivik declared him psychotic and criminally insane — a diagnosis that, given the magnitude of the slaughter, made intuitive sense. Who could kill that many people without being insane?

But Breivik, a white supremacist who told police that he carried out the attacks to prevent a Muslim takeover, thought the diagnosis was offensive. He argued that he was of sound mind. A second team of doctors examined him and agreed.

Tahir Rahman, a forensic psychiatrist at the University of Missouri, uses the Breivik story in a new paper to explain how actions such as Breivik’s can be mistaken for psychosis instead of calculated evil.

This is a difficult conclusion for people to understand, as I wrote this month in a story examining whether mass shooters are seriously mentally ill. Most, it turns out, are not. What are they? Psychopathic. Angry. Revenge-seeking. Jilted. Antisocial, not delusional.

“This is a behavior,” Rahman said in an interview. “It’s separate from psychosis.”

And it often results, Rahman argues in the paper, from an “extreme overvalued belief.”

The concept of an “overvalued idea” was first identified a century ago by Carl Wernicke, a German neuropsychiatrist who used the phrase to describe ideas in society that dominate a person’s mind, causing nondelusional but extreme behaviors. Anorexia nervosa is seen as a classic example.

“These individuals,” Rahman writes, “have extreme beliefs that fulminate and dominate their minds to the point of starvation.”

The same can be said of racists, anti-Semites and people gripped with hatred toward their employers, co-workers, classmates, etc. Most don’t act out in violence. But at some point, the idea becomes extremely overvalued and “violence becomes a way to make a statement for their belief systems,” Rahman said.

The concept of the “overvalued idea” has largely been ignored by his profession, Rahman said, but he thinks it’s a crucial concept that can help forensic psychiatrists sort out the motivation for behavior that appears delusional but is not.

Rahman and his colleagues take the concept further, coming up with the phrase “extreme overvalued belief” and defining it this way:

An extreme overvalued belief is one that is shared by others in a person’s cultural, religious, or subcultural group. The belief is often relished, amplified, and defended by the possessor of the belief and should be differentiated from a delusion or obsession. The idea fulminates in the mind of the individual, growing more dominant over time, more refined, and more resistant to challenge. The individual has an intense emotional commitment to the belief and may carry out violent behavior in its service. It is usually associated with an abnormal personality.

Such a description would certainly apply to Dylann Roof, the 21-year-old charged with murdering nine African Americans at their South Carolina church last year. Roof, who faces the death penalty, voiced deep hatred of blacks and other minorities in an online manifesto and allegedly told his African American victims: “You rape our women, and you’re taking over our country. And you have to go.”

Eric Harris, the 18-year-old ringleader of the Columbine High School shooting, idolized Hitler, felt that the human race was totally worthless, and decided, like Breivik, that there was nothing left to do but annihilate as many people as he could. He and his 17-year-old classmate Dylan Klebold killed 13 people and wounded dozens more before killing themselves on April 20, 1999: Hitler’s birthday.