KANSAS CITY, Mo. – One only need look to the tale of Cain and Abel to see how difficult it is to explain why men are so violent.

Here they are, brothers, born to the same parents and raised in the same place. One turns good. The other, Cain, becomes his brother’s killer.

Was the problem nature, nurture or an interplay? Could Cain have been mentally ill? And exactly how much did the fact that he was male rather than female have to do with his aggressive nature?

People look for answers after tragedies such as last month’s killing spree at a Kansas City mall or the April 16 massacre at Virginia Tech.

Scientists who study violent criminals say that, although they are gathering more clues, cause-and-effect answers remain a mystery.

Consider some of what is known:

Testosterone: Men cause 80 percent of violent crimes in the United States, according to the Justice Department.

In the 1980s, scientists wondered whether testosterone played a role. James M. Dabbs of Georgia State University tested the testosterone levels of 89 male inmates. Nine out of the 11 inmates with the lowest testosterone levels had committed nonviolent crimes. Ten of the 11 inmates with the highest testosterone levels had committed crimes ranging from rape to murder. Subsequent studies have turned up conflicting results. Some people with high testosterone are passive; some with low levels of the hormone are not.

“You’d think that testosterone would be the obvious explanation. But it’s an unclear, fuzzy relationship,” said Grant Harris, a researcher at the Mental Health Centre in Penetanguishene, Ontario, a maximum-security facility for mentally ill criminals.

Evolution: Aggression and the fight-or-flight response is part of human instinct, a result of natural selection. In males, aggression has helped them survive and compete, Harris said. But toss in other such factors as stress, guns, mental illness, drugs and alcohol, and what was designed to be a competitive advantage in nature becomes a danger to society.

Mental illness: The results are mixed and confusing. Many criminals suffer some form of mental illness, but then again, the American Psychiatric Association estimates that 20 percent of adults and children in the United States do, too. The definition of mental illness is vast, ranging from the mildest depressions to major psychoses.

In 2006, a U.S. Bureau of Justice Statistics report, “Mental Health Problems of Prison and Jail Inmates,” showed that 64 percent of inmates in local jails, 56 percent in state prisons and 45 percent in federal prisons have some form of mental illness.

Women inmates have more mental illness than men – about 75 percent, compared with between 55 percent and 65 percent in men.

But of those inmates who were mentally ill, most did not commit violent crimes.

In other words, most violent crimes are committed by people who are not mentally ill.

Drugs and alcohol: Between 31 percent and 53 percent of inmates used drugs or alcohol at the time of their offenses. Substance abuse raises the risk of committing a violent crime.

Sexual or physical abuse: It is widely assumed that the vast majority of inmates, male or female, have been physically or sexually abused. Another assumption: Abused and neglected people are more likely to commit violent crimes. But the numbers vary.

In federal prisons (where many inmates are incarcerated for drug trafficking, as opposed to violent crimes), most inmates report no past physical or sexual abuse.

Among women, the rate of past physical or sexual abuse is high, ranging from 40 percent among federal inmates to 47 percent among prisoners in local jails and 57 percent among those in state prisons.

The tie to violent crime, however, is murky.

Of those men in state prison who said they were abused, 70 percent reported committing a past or current violent crime. Then again, 60 percent of men who were not abused also reported committing a past or current violent crime.

Among sexually and/or physically abused women, 45 percent reported committing a violent crime, compared with 30 percent of women who were not abused.

The brain: Scientists have been searching both the structure and function. They have found many links to high levels of aggression: lesions in the front of the brain; slower metabolism of glucose also in the front part of the brain; high levels of the hormone norepinephrine, which is vital in the fight-and-flight mechanism; low levels of the neurotransmitter serotonin, which is also implicated in depression.

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