The verdict of not guilty by reason of insanity at the trial of John W. Hinckley Jr. in 1982 set in motion the most radical overhaul in the history of our federal criminal laws. New legislation signed into law Oct. 12 by President Reagan was a direct outgrowth of the Hinckley decision. It reverses more than a century of legal history in which court decisions gradually, but increasingly, expanded the concept of legal insanity.

In the 19th century the insanity defense was based on the M'Naghten rule: A person is not responsible for his criminal acts when, because of a "disease of the mind," he does not know the "nature and quality" of his acts and does not know that they are "wrong." But in 1954, Judge David Bazelon extended the concept of insanity by writing that an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect.

Bazelon's profound modification of the insanity defense, the Durham rule, invited psychiatrists to testify about all elements of a defendant's mental condition, instead of confining psychiatric testimony to the determination of the defendant's ability to tell right from wrong.

In 1972, the Court of Appeals for the District of Columbia replaced the Durham rule with the Brawner rule: A person is not responsible if, as a result of mental disease or defect, he "lacks substantial capacity to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law."

Both Durham and Brawner enlarged the scope of the insanity defense to include not only a cognitive standard ("did the defendant know what he was doing?") but a volitional aspect as well ("could he control his actions?").

Under M'Naghten, John Hinckley would have been found guilty. Under Brawner, he was declared not guilty by reason of insanity. Under the new law signed by President Reagan, Hinckley would have undoubtedly been found guilty since, according to the new standard, the insanity defense in federal cases is limited to those who are unable to appreciate the wrongfulness of their act. In essence, sanity or insanity is very much dependent on the arbitrary standard held in a particular jurisdiction at a particular time.

Here lawyer-journalist Lincoln Caplan describes and interprets the Hinckley trial in the context of the insanity defense. The book succeeds on the level of a colorful narrative of the trial and the participants who testified for and against Hinckley's alleged insanity. But it is too brief and superficial to convey the com- plexities involved in limiting or expanding the insanity defense. Indeed, Caplan breezes over in a sentence or two some of the most profound issues in forensic psychiatry:

"Of thirty thousand American psychiatrists," he writes, "less than a thousand are forensic psychiatrists, and only one hundred and twenty-five of these regularly testify on criminal responsibility." Caplan fails to provide an explanation for this courtroom "shyness" on the part of the majority of American psychiatrists. To do so would have required him to expand his material beyond his once-over-lightly approach and explore the differences in the training and philosophy of lawyers compared with doctors.

Medical training was once characterized (or caricatured) as learning the art of asking questions and writing prescriptions. To this one might add the everyday necessity of drawing conclusions from inadequate data. Lawyers, in contrast, operate on the basis of "law logic," defined by John Quincy Adams as "an artificial system of reasoning, exclusively used in courts of justice, but good for nothing anywhere else." Psychiatrists must function in the courtroom without an education in "law logic," and most of them forgo the courtroom experience whenever possible.

More important, however, are the ethical differences of the two professions. Lawyers routinely engage in practices that most psychiatrists, and others, consider reprehensible: relying on technicalities to obtain acquittals for guilty defendants, confusing and intimidating adverse witnesses, and so on. In the face of such discrepancies between medicine and the law, a self-selection process takes place whereby those who practice forensic psychiatry often have more in common with lawyers than they do with their fellow doctors.

"Many distinguished forensic psychiatrists have felt more comfortable acting on behalf of criminal defendants," according to Alan A. Stone, professor of law and psychiatry at Harvard. "Indeed, it seems there is a very comfortable ideological fit between being a forensic psychiatrist and being against capital punishment."

Stone's concern about the influence of ideology on psychiatric testimony is supported by the Hinckley experience. Without exception, the psychiatrists involved in the case rendered conclusions consistent with the side that sought their opinion and paid their bills.

The foremost problem of forensic psychiatry -- barely touched on in Caplan's breezy and abbreviated survey of the subject -- remains. Should psychiatrists be involved in the legal process? Do they have the answers to the legal questions posed by the courts? Unless these questions can be answered affirmatively, the dictum enumerated by Immanuel Kant two centuries ago still holds: "Concerning the question whether the mental condition of the agent was one of derangement or of a fixed purpose held with a sound understanding, forensic medicine is meddling with alien business."