Washington, July 27 (ANI): Mass murderers don't necessarily share similar histories of mental illness, but one of the best predictors of later violent behaviour is aggressiveness during childhood, it has been revealed.

More aware parents can get odd childhood behaviour treated before kids leave home for good.

According to psychologists, it's nearly impossible to accurately predict human behaviour, although they do agree that parents, teacher and friends need to heed warning signs that can signal the possibility of extreme violence; signals like depression, changes in mood or personality, drug or alcohol abuse, or a change in school performance.

Paul J. Frick, professor of psychology at the University of New Orleans, and a specialist in treating psychotic youths, says that the best predictor of adult violence is violence or aggression as a young person.

"Most people who have killed have a history of hurting other people," Discovery News quoted him as saying.

"The key is what sets them off to where they take it to such an extreme," he said.

Lawrence Steinberg, professor of psychology at Temple University and an expert in the psychology of adolescent development, also said it was "hard to generalise" the symptoms.

This issue has been brought up in the light of accused movie theatre shooter James Holmes, who went out on a shopping spree in Colorado last week killing 12 people.

"It's awfully hard to generalize about these things," Steinberg said.

"Some (mass murderers) have long histories of depression, others don't. Right now, we don't know yet about this guy's (Holmes) history. Some of these are loners, and he has been described as a loner, but many don't. Some are criminally aggressive before committing these crimes, others are not," he added.

While a person's violent tendencies may not be immediately obvious, Frick explained snapping "usually requires them to be callous and unemotional-maybe things happen that their anger builds up to where they don't care anymore."

Frick, like other psychologists interviewed, said that there isn't enough data on mass killers to make generalizations about the exact factors that cause their crimes.

However, Frick, who studies children who exhibit psychotic and violent behaviour, also notes that it's rare that violence comes from out of nowhere and there are usually some signs in childhood.

"People who are highly intelligent can get away with it more," he said.

"They may be better at covering it up from parents or teachers," he added.

He concluded by saying that with early intervention, these violent children can be treated before they grow up to kill as adults. (ANI)