The image of Jacqueline F. McLean -- battered, broken and apparently suicidal -- collapsing in a Baltimore courtroom as her lawyers argued that she was unfit for trial raises an issue that comes up in hundreds of less publicized criminal cases each year:

How does a judge determine if a someone is fit to be tried?

In theory, the question of competence for trial is simple: A defendant has to be capable of understanding the proceedings and able to assist in his or her defense.

But as the adage goes, the truth is seldom pure and never simple.

It may be easy to judge as incompetent the raving lunatic -- one who hears voices, says the judge is out to get him or exhibits other bizarre conduct in the courtroom.

But the seemingly simple standard established in 1960 by the Supreme Court in Dusky vs. United States is more difficult to apply in cases where defendants have more subtle symptoms that raise the question of competence, but where the answers are not so clear-cut.

What if a defendant is clinically depressed, manic-depressive or unusually lethargic? Should someone facing prison who threatens suicide be judged incompetent because of seeing death as an option?

Not necessarily.

Dr. Jonas Rappaport, the former chief of medical services for the Baltimore Circuit Court, who teaches forensic psychiatry at the Johns Hopkins and University of Maryland medical schools, said that just because someone is suicidal or depressed does not mean he is incompetent to stand trial.

A defendant may be suicidal but understand everything that is going on in the courtroom. He may be manic-depressive, but be lucid on the witness stand.

"If I had someone arrested for shoplifting and that person admits they did it and thinks they should get the death penalty, that would raise the flag, but that alone is not determinative," Dr. Rappaport said.

Legal experts say competence is an issue of justice.

The rule is designed so that defendants should not be able to duck out of the criminal justice system, but also to guarantee that they have the ability to defend themselves when confronted with it.

The competence issue is based in the Sixth Amendment guarantee of a defendant's right to confront his accusers and and the Fifth Amendment right to due process of law.

"What we're saying in the Constitution is, 'Don't try me if I'm not all there, and I'm not all there if I'm not [mentally] ALL THERE,' " said Byron L. Warnken, a professor of criminal law at the University of Baltimore Law School.

Legal and psychiatric experts say a defendant's competence to stand trial boils down to his ability to communicate -- with his lawyer, with psychiatrists and with the court.

But that can be tricky.

"It's really a question of how do you function, primarily with your lawyer, and that can be a very difficult issue," said George Lipman, who heads the Maryland Public Defender's Office mental health division. In that position he trains the office's new lawyers on defendant competence.

Procedurally, the issue of competence is different from insanity. Insanity is a defense at trial, with the key being the defendant's mental state at the time the offense was committed. Competence is kind of a gate-keeper to the trial process, with the key being the mental state at trial time.

The issue of competence may be raised anytime before or during a trial, right up until the verdict, and may be raised by the defendant, his lawyer, the prosecutor or the judge.

If someone is found incompetent, his trial is delayed until he is found competent, and if that means being involuntarily held for an extended period, a judge may eventually dismiss the charges out of fairness to the defendant.

As was the case with Mrs. McLean, the judge can appoint a psychiatrist to examine a defendant to determine fitness for trial.

Defense lawyers hate it when that happens -- for one very good reason. Anything a defendant tells that court-appointed psychiatrist may be used as evidence.

"You're throwing your client into a hospital full of psychiatric interrogators," Mr. Lipman said.

But Dr. Rappaport said that when a defendant is sent to a psychiatrist, the therapist is less interested in providing evidence for either side than in determining competence. Generally, therapists do that by using a 13-point test, known as the Competency Assessment Instrument, that probes the defendant's ability to understand a variety of concepts ranging from his appreciation of the charges to his understanding the role of the judge.

"If there is an illness, the question is: How does the illness affect comprehension?" said Dr. Rappaport.

"The threshold for competency is really very low, because of the feeling we all share that everyone's entitled to a trial, to have their day in court," he added.

That much is sometimes apparent.

In 1992, a 31-year-old homeless man in Anne Arundel County charged with breaking into a house told everyone who would listen that he had committed no crime, that the house in fact belonged to him -- despite deeds and other land records showing that it just wasn't his.

In his odyssey through the criminal courts, Stephen Lamm cursed out two judges and two prosecutors, fired two public defenders, came to court barefoot and complained that his jailers were putting drugs into his food.

Yet when Judge Eugene M. Lerner agreed with a prosecutor's request to have him assessed for competence, doctors at Crownsville State Hospital found him competent to stand trial.

Mr. Lamm had his day in court, represented himself in a brief trial and ended up being convicted of breaking and entering last September. He is serving a three-year sentence.

Defense lawyers say that Mr. Lamm's case underscores what may be the most serious flaw in the system: Just about everybody who walks into a courthouse is found competent for trial.

But finding someone competent doesn't make him any easier to defend.

"You've got to flat out lack the ability to comprehend what's going on, and a person's mental state is such a complex thing that in a lot of cases that can be extremely frustrating," said assistant public defender Gary Christopher, one of Mr. Lamm's former lawyers. And that seems to apply to almost everyone -- whether they collapse in court or arrive barefoot.

Dennis O'Brien is a reporter for The Baltimore Sun.