A: First, I'll ask you a question: In what percentage of felony criminal cases do you think accused offenders beat the rap using the insanity defense?

If you're like most ordinary citizens surveyed following John Hinckley's acquittal by reason of insanity for shooting President Reagan in 1981, you probably guessed a figure of little more than 50 percent. The correct answer appears at the end of this column. Meanwhile, let's consider what the insanity defense is and isn't.

A basic tenet of the American legal system is that to be justly punished for a crime, a defendant must be found to have actually committed the offense in question (actus reus) and to have had the mental capacity to have committed the act consciously, knowingly, and purposefully (mens rea).

The two major principles that guide insanity evaluations in the United States are the M'Naughten test and the American Law Institute (ALI) test; different states employ different standards. According to M'Naughten, in order to establish a defense on the grounds of insanity: "It must be clearly proved that, at the time of committing an act, the party accused was laboring under such a defect of reason, from disease of the mind, as to not know the nature and quality of the act he was doing, or if he did know it, that he did not know he was doing wrong."

According to the ALI test: "A person is not responsible for criminal conduct if at the time of such conduct, as a result of mental disease or defect, he lacks substantial capacity either to appreciate the criminality (wrongfulness) of his conduct or to conform his conduct to the requirements of the law."

What both standards share in common is a fairly strict set of exclusion criteria to invoke a defense of not guilty by reason of insanity (NGRI), or not guilty by reason of mental disease or defect. Both standards require the presence of a diagnosable mental disorder, but this is not in itself sufficient. The symptoms of that disorder must so impair the defendant's mental functioning that he literally does not know what he is doing or doesn't know right from wrong at the time of the offense (M'Naughten) or make it impossible for him to control his actions (ALI).

Clinically, there are very few mental disorders whose effects are profound enough to produce that level of impairment. Most would fall into the category of severe psychosis like schizophrenia or a manic episode, or some type of organic brain syndrome such as dementia, temporal lobe epilepsy, or toxic-metabolic psychosis. Note that voluntary intoxication is not a defense.

As distinct from NGRI per se, the law allows for a defense of diminished capacity, usually raised at the time of sentencing. Since the purpose here is not to determine legal guilt or innocence, as in an insanity defense, issues of mitigation due to diminshed capacity typically require a less severe degree of impairment than NGRI.

If there has been an abuse of the insanity defense by mental health experts, we partly have ourselves to blame. Too often, I've listened to or read the transcripts of clinicians arguing for NGRI on the basis of a diagnosed mental disorder alone. The part of the fault that lies with the lawyers is not rigorously holding those clinicians to the strict M'Naughten or ALI standards.

So how bad is the "insanity defense epidemic"? In fact, NGRI is raised as an affirmative defense in about one out of every hundred felony cases. In about half of those, the defendant is so obviously impaired that both prosecution and defense agree to adjudicate the case as NGRI without trial. In another quarter of those cases, the two sides come to an agreement during the trial, and so in only one-quarter of one percent of felony cases, do you get a Hollywood-like "battle of the experts" - hardly an epidemic.