Washington: People who commit violent acts such as mass shootings may do so because of their extreme beliefs and not due to the fact that they are mentally ill, scientists say.

Researchers from University of Missouri (MU) studied the 2011 case of Norwegian mass murderer Anders Breivik, and suggested a new forensic term to classify non-psychotic behaviour that leads to criminal acts of violence.

“When these types of tragedies occur, we question the reason behind them,” said Tahrir Rahman from MU. “Sometimes people think that violent actions must be the byproduct of psychotic mental illness, but this is not always the case,” said Rahman.

“Our study of the Breivik case was meant to explain how extreme beliefs can be mistaken for psychosis, and to suggest a new legal term that clearly defines this behaviour,” he said.

Breivik, a Norwegian terrorist, killed 77 people on July 22 in 2011, in a car bombing in Oslo and a mass shooting at a youth camp on the island of Utoya in Norway. Claiming to be a “Knights Templar” and a “saviour of Christianity,” Breivik stated that the purpose of the attacks was to save Europe from multiculturalism.

Two teams of court-appointed forensic psychiatrists later examined Breivik. The first psychiatric team diagnosed him with paranoid schizophrenia.

However, after widespread criticism, a second team concluded that Breivik was not psychotic and diagnosed him with narcissistic personality disorder. Breivik was sentenced to 21 years in prison, researchers said.

“Breivik believed that killing innocent people was justifiable, which seems irrational and psychotic. However, some people without psychotic mental illness feel so strongly about their beliefs that they take extreme actions,” said Rahman.

“Our suggested term for criminally violent behaviour when psychosis can be ruled out is ‘extreme overvalued belief,'” he said.

Rahman defines “extreme overvalued belief” as a belief that is shared by others and often relished, amplified and defended by the accused.

The individual has an intense emotional commitment to the belief and may act violently as a result of that belief, researchers said.

Although the individual may suffer from other forms of mental illness, the belief and the actions associated with it are not the result of insanity, they said.

“Certain psychological factors may make people more vulnerable to developing dominating and amplified beliefs,” said Rahman.

“However, amplification of beliefs about issues such as immigration, religion, abortion or politics also may occur through the internet, group dynamics or obedience to charismatic authority figures,” he said.

The findings were published in the Journal of the American Academy of Psychiatry and the Law.