Since the massacre of innocents at Sandy Hook Elementary School, calls have rung out for improving "mental health services." This deflects from actions that would save lives. Such calls blur the distinction between what is crazy and what is evil. Further, they compound our national reluctance to face facts about what can and cannot be changed.In modern scientific parlance, the label crazy centers on delusions, hallucinations and bizarre beliefs. More commonly used technical terms are insane, psychotic or schizophrenic.Evil is at least as ancient a concept as crazy. Its hallmark is a narrow moral circle in which other people are objects of moral indifference or hatred, people deemed not to deserve to live. In this usage, the label evil is not mysterious nor derived from a belief in "the devil." Rather, it is clarifying; it denotes people inclined to be violent and to put many other people at risk.We know evil when we see it. Mean, violent, full of hate, selfish, grandiose, without a conscience and bullying all signal evil. Whatever mental illness he may have had, Adam Lanza died and, most likely, lived at the extreme end of evil.Craziness and evil are separable: One can be crazy, evil, neither or both.Crazy people commit somewhat more violent crimes per capita than non-crazy people, but most crazy people are not evil. John Nash, the Nobel Prize-winning mathematician, was certifiably crazy for much of his life: He had an imaginary roommate and pervasive paranoid delusions. But he remained kind and compassionate. His moral circle was broad and undiminished.Lanza's alleged autism or Asperger's syndrome does not explain what happened at Sandy Hook. Focusing on his crazy part will not help prevent future tragedies; the craziness of evil people almost never comes close to explaining their crimes. Would Lanza have murdered had he not been full of hate, had he not had a very narrow moral circle?You simply do not shoot 6-year-olds repeatedly unless you are exploding with rage and regard the violent suffering of young children and their parents with indifference or worse.What our nation really cares about is not what label to pin on Lanza but how to prevent such incidents.I have spent most of my life working with mental illness. I have been president of the world's largest association of mental-illness workers, and I am all for more funding for mental-health care and research -- but not in the vain hope that it will curb violence.Drugs and therapy offer disappointingly little additional help for the mentally ill than they did 25 years ago -- despite billions of dollars in research funding. And there is zero promise that any developments I am aware of will help curb the violence that mentally ill persons commit.As for progress on restraining or rehabilitating evil people, the past record and future promise are even more dismal. I know of no development that has much reduced recidivism or violence or done more to identify violent offenders in advance than was in place a generation ago.I conclude from all this that progress in reducing violence through either helping the mentally ill or curbing the impulses of violent, non-crazy people will be very slow in coming, perhaps even fruitless. That is not where the leverage is.Crazy people and evil people can commit mass murder, and they always do it with guns. Our only real leverage, at least in the near term, lies in reducing access to guns.Our national experience with another lethal menace, cigarettes, shows that government regulation massively saves lives. High taxation on cigarettes and restricting access to them have markedly cut smoking rates and improved health. High taxes on guns and strong restrictions on their availability are the only realistic hope for avoiding many more Sandy Hooks.Martin Seligman, a former president of the American Psychological Association, is a psychology professor at the University of Pennsylvania and the author of "What You Can Change and What You Can't."