ON A SUMMER EVENING in 1974, Charles Decker (not his real name) picked up a couple of high-school girls in his convertible and took them for a joy ride. Decker "didn't seem mean or anything," one girl said later: he was "a nice guy." A policeman stopped the car because one taillight was out, and Decker explained he was teaching his girl friend to drive. He "seemed a little mad" when he got back in the car and didn't say much as he and the girls cruised again. A few minutes later, he stopped the car and told the girls he had to relieve himself; when he returned, the girls said they wanted to go home. Decker sat silently in the dark, rubbing his hands on his legs. Suddenly he picked up a four-pound stonemason's hammer, which had slid from under his seat during the ride, and he attacked the girls, fracturing each of their skulls. One girl recalled, "He seemed real mad. I don't know, really strange." After a second attack, Decker calmed down "like a little kid" and looked "frightened or scared." He took the girls to a local market where he knew they would get help, and then called his father to confess. "What happened?" Decker was asked. "Daddy, I don't know," he replied. "I went ape."

Charles Decker is "The Crocodile Man" -- after the reptile Evelyn Waugh called "a type and sign for us of our own unredeemed nature." Decker's brief trial in Massachusetts in 1976 avoided the public cries for vengeance that distort more famous cases, and all of the key players (the judge, lawyers, a victim, the defendant, and his family) cooperated to allow a model inquiry into the young man's criminal responsibility. At trial, Decker presented an unusual insanity defense. Based on the theory that some human violence and irrational assaults relate to a dysfunction of the limbic system in the lower brain, the defense argued that Decker's body produced a mysterious substance (the prosecutor dubbed it "Brand X") that caused limbic damage and led to violent seizures. Governing emotion and the urge to survive, the limbic system in man knits the brain into a working organ, provides the connection between thinking and feeling, and makes self-control possible. When a slumbering crocodile lunges at prey, swiping it under water or tearing it limb from limb, the attack is triggered by the reptile's limbic system. As a result of Decker's physical problem, his defense contended, he did not know right from wrong when he hammered in the girls' skulls, and he could not abide by the law.

Andre Mayer, who is a historian of science, and Michael Wheeler, who teaches law at New England School of Law and MIT, draw on their respective disciplines in The Crocodile Man to offer a concise, balanced, and literate report on the insanity defense, through Charles Decker's story. They turn it into a mystery, luring the reader to a thought-provoking end, which shouldn't be spoiled. For anyone interested in how medicine and the law have developed in the past few centuries to yield the current American version of the insanity defense, their study gives the best available popular summary of two complex bodies of knowledge. Along with Abraham Goldstein's excellent book on the legal aspects of criminal responsibility, called The Insanity Defense, and Charles Rosenberg's The Trial of the Assassin Guiteau: Psychiatry and the Law in the Gilded Age, (a first-rate history of the ideas and facts that led to the hanging of President James Garfield's crazy killer a hundred years ago), this new work offsets with clarity, learning, and poise dozens of dull, arcane, and bombastic writings that crowd the insanity-defense shelf.

More important, in an area of social policy that now storms with controversy because of John Hinckley's insanity acquittal, Mayer and Wheeler provide a clear account of the dispute on the horizon. "No one today questions the fact that some mental effects have underlying physical causes," they write. If the law could rely on medicine to pinpoint the causes of violence in specific insanity cases, and, with psychosurgery or drugs, assure restraint, what values would be served by finding a defendant guilty and sending him to prison? There would be no reason to quarantine him: medicine would protect society from him. There would be no reason to deter him by imprisonment, and he would need no rehabilitation. Only revenge would remain a motive for putting him away, and marking a once-violent offender as different from the rest of us.

To date, medicine cannot play this unequivocal role in insanity cases, nor do physicians and others agree it should. "Yet while medical knowledge of the mind has become more sophisticated, the problem of applying the insanity defense has, if anything, grown more difficult," the authors declare, using the case of Charles Decker to demonstrate. In the Hinckley trial, CAT-scans of his shrunken brain and testimony about the tie between this anomaly and schizophrenia teased the gallery and suggested how inconclusive medical evidence can be. The Decker case forced a judge in Massachusetts to think about the law's purpose in holding a defendant criminally accountable, in light of stronger physical evidence freeing him from moral blame. The Crocodile Man provides intelligent digressions on crime, violence, medicine, and the law, before grappling with this hard problem and letting us know how one judge, at least, solved it.