What was John du Pont thinking at the instant he pulled the trigger and shot wrestler Dave Schultz? Did du Pont think he was in danger? Why did he kill Schultz?

These are among the questions the jury must wrestle with to decide if du Pont was legally insane. More broadly, what exactly is the insanity defense and is it being used legitimately in our criminal justice system?

Stephen Morse is well-qualified to address both the legal and psychological issues. A lawyer and law professor at Penn, Morse also has a Ph.D. in psychology. He is professor of psychology and of law in psychiatry in Penn's Medical School and is a board certified forensic psychologist.

Although he says he is a strong defender of the legitimacy of the insanity defense, Morse has also testified for the prosecution in insanity cases. He is currently working on a book about excuses in the criminal law.

The insanity defense "goes way back," according to Morse. There are Biblical references to people who lost their reason, he says.

The modern history of the insanity defense begins in England in 1843, however, when Daniel MacNaughton tried to assassinate Prime Minister Peel, but mistakenly killed Peel's secretary instead. MacNaughton suffered from paranoid delusions and believed that Peel's party, the Tories, were conspiring to kill him. At his trail he was found not guilty by reason of insanity.

In response to pubic and official outcry about the verdict, the House of Lords was asked to define the test for when a person suffering from a mental disorder should be excused from responsibility for criminal conduct.

The Lords, says Morse, devised the test that bears MacNaughton's name, which says that the defendant is not responsible if, at the time of the crime as a result of mental disease, the defendant either did not know what he was doing or did not know that what he was doing was wrong.

Familiarly called the right/wrong test, the MacNaughton rule is still the standard for legal insanity in Pennsylvania, and is one of the two predominant tests used by states, he says.

People who are legally insane are excused, says Morse, because they are nonculpably irrational as a result of mental disorder. "Such people do not deserve to be blamed and punished," he says.

"It is not sufficient that the defendant has a mental disorder, even a severe disorder," says Morse. "The defendant's reason for committing the crime must be so irrational, so out of touch with reality, that it is not just to hold the defendant morally responsible."

All insanity defense cases, including the du Pont case, Morse contends, are morality plays. "John du Pont shot Dave Schultz. No question. The real question is why he did it.

"If du Pont had a very crazy reason because he was mentally ill, he doesn't deserve to be found guilty," says Morse. "Suppose, like MacNaughton, du Pont believed as a result of mental disorder that Schultz was threatening him, was planning to kill him at that moment. But even if he had a severe mental disorder, he may not have had a crazy reason or a sufficiently crazy reason. Suppose du Pont and Schultz were simply having an argument and du Pont became angry and shot him. In that case, he deserves to be convicted and punished.

"Unfortunately, it's hard to know why a defendant committed the crime. The diagnosis won't answer this question. You have to know what's going on at the moment. And a defendant may have both crazy and normal reasons."

The jury must ultimately decide if du Pont's reason for killing Schultz was so crazy that du Pont morally should not be blamed. If the jury finds that du Pont was legally insane when he killed, he will be committed to a state hospital for observation and treatment and to protect the public, says Morse.

The law in Pennsylvania and other states interprets the right/wrong test narrowly, says Morse, but he says that the test should be understood more broadly.

"A deluded killer knows that he is killing and intends to ill. In the abstract, he knows that killing wrong" Morse says. "But if he has a crazy reason for killing, he may not know facts that are morally relevant. For example, he may not know or be capable of knowing that his life is not in danger. In this case, blame is not deserved."

Morse also notes that irrationality can be produced by many factors, such as stress, fatigue and trauma. "If a defendant acted for irrational reasons that weren't his fault, the defendant ought to have an excuse, even if his condition doesn't meet the psychological or psychiatric criteria for a mental disorder."

Legal insanity is a moral issue, not a scientific or clinical question, Morse says. "Why should psychological categories drive our criminal justice standards?" he asks.