This week in Chicago, the defense presented some of its most crucial witnesses—psychiatrists—in the jury trial of John Wayne Gacy, 37, the contractor and charity-benefit clown charged with murdering 33 boys and young men. On trial with Gacy, in effect, is his controversial plea: not guilty by reason of insanity (NGRI). To some that defense is proof of the law's mercy; to others it seems a shield for monstrous criminals. To at least one Chicago attorney, Barbara Weiner, both viewpoints can be supported. Weiner, 31, is counsel to the Department of Psychiatry at Rush-Presbyterian-St. Luke's Hospital, which examined Gacy and found him competent to stand trial. An authority on the NGRI defense, she discussed it—chillingly—last week with PEOPLE correspondent Linda Witt.

What is the basis for an NGRI defense?

There are two standards, used in different states. Under the M'Naghten Rule, the accused is not responsible if mental disease prevented him from knowing right from wrong. Under the more liberal American Law Institute Rule, a person may not be responsible, even if he knows his actions are wrong, if a mental disease or defect left him incapable of conforming to the law.

Why has the NGRI defense come under such attack recently?

In the 1950s, when we had great faith in psychiatry, the NGRI defense was easy to accept, because we assumed that any crazed killer would be locked away in a mental institution for life. But since then, public-interest lawyers have made it very hard to keep patients institutionalized once they have been involuntarily committed. New drugs have made it possible to stabilize almost any patient after a short hospital stay. Nowadays we know the NGRI defendant will probably be out on the street in a few weeks—and the public rightly fears that some crazy, unpredictable person may kill again.

Does that happen?

In one notorious Illinois case a man named Vanda was found NGRI of murder. Shortly after he was released by the Department of Mental Health, he killed again. It was later discovered that he was sane enough to write a letter to a fellow inmate explaining how to get off with an NGRI defense. He wasn't able to pull it off the second time.

Is it possible, then, to fake it?

There's no question about it. Vanda is a perfect example. If I were going to tell someone how to develop a successful NGRI defense for, say, killing his wife, I'd say build up a convincing psychiatric history—weeks of depression, crying, bizarre behavior. Then after you kill her, chop her up and put her through the Cuisinart.

Are there any common characteristics of bona fide NGRI defendants?

One of the odd things about the truly insane is that you can't possibly imagine that they could have committed their crime. We have one patient who murdered two people, chopped off their heads and fingers and mailed them around the world. He's obviously mentally ill, but when you meet him he's a nice quiet guy. One of the frightening things about NGRI is that you cannot predict who will go berserk and shoot all the kids in the neighborhood.

Why isn't every mass murderer automatically considered insane?

The public assumes that anyone who kills a number of people has something seriously wrong with him. But most mass murderers do their killing at one moment—they go to the top of a tower in Texas, like Charles Whitman, and shoot a lot of people. When someone kills a great number of people over a long period and functions normally in between the killings, it's harder to argue that the person is insane.

How can NGRI co-exist with the public safety?

Because of Vanda, there is now a provision in Illinois law that anyone found NGRI stays under the jurisdiction of the court. The judge must approve a hospital's decision to release any NGRI patient. In Maryland and Oregon, the court can continue supervision for five additional years. I'd like to see that apply nationally.

But don't states already supervise the released NGRIs?

Unfortunately not. Most states require that a person who is involuntarily committed must be immediately dangerous to himself or others. Most NGRIs don't fall into that category. If their symptoms weren't in remission, they wouldn't have been declared "fit to stand trial." So the court has no option but to discharge. It's a Catch-22.

Should Gacy be found NGRI?

Our hospital's Isaac Ray Center [a clinic specializing in NGRI cases] did the psychiatric evaluation for the court on Gacy, so I can't talk about him. But I can say that some 50 NGRI defenses a year succeed in Illinois in front of judges, but to my knowledge, not one jury in this state has ever bought an insanity plea. Juries seem to be very good at cutting through jargon and garbage and getting to the facts.