The latest person to offer his services as guide in this regard is Kevin Davis, in “The Brain Defense: Murder in Manhattan and the Dawn of Neuroscience in America’s Courtrooms.” Davis, a veteran Chicago reporter, is the author of two previous books, “The Wrong Man” (1996), an engrossing true crime account of a mentally challenged man falsely convicted of a savage murder in Florida, and the brilliant “Defending the Damned” (2007), a revealing portrait of Chicago’s public defenders. In both those works, despite Davis’s efforts to be evenhanded, he could not help but bring his natural proclivities toward the defense’s side to bear; and in those books, that proclivity was entirely appropriate. But is it here?

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To answer that question, we can focus (as Davis does) on the case of Herbert Weinstein, a 65-year-old New Yorker, seemingly reserved in manner, who struck and strangled his second wife in 1991, then threw her (perhaps still alive) out of a 12th-story window to try to make her death appear a suicide. His family and friends were stunned; nothing in his life seemed to indicate that he was capable of such an act. When initially examined, he passed all the standard neurological and psychiatric tests for brain competency easily. But Weinstein had committed his crime in the era of M.R.I.s; and when he was scanned with one such machine, the results appeared shocking. He had a gaping space the size of an orange in the area of his left frontal lobe: the region that, it is generally agreed, governs impulse control. His defense lawyer, when the space was revealed to be an arachnoid cyst (a growth in the weblike lining between the brain and the skull), knew he had his defense.

And it worked — in the short term. When presented with the M.R.I. and other imagery, the prosecutor in the case decided that he could not press the murder charge. If the jury saw this apparent “hole” in Weinstein’s brain, they might bring in a verdict of not guilty by reason of insanity. So he agreed when Weinstein said he would not contest a lesser charge of manslaughter, and the judge decreed the minimum sentence of seven years. It was historic, the first time that brain imagery had been used to mitigate the sentence of a confessed killer. But were the use of the images, the plea bargain and the sentence just?

Davis’s answer is ambiguous and perplexing, just as it is when he considers many similar cases — indeed, far too many, for a book of this length. Perplexing because Davis never adequately explores alternate behavioral theories (the work of forensic psychology) that were and are illuminating. There are general questions — such as the very large numbers of people walking around the United States with arachnoid cysts and other brain irregularities in their heads who never commit crimes or become violent — and there are questions specific to Weinstein. It turns out, for example, that the supposedly genial gentleman may have had financial motives (including gambling debts) for his well-to-do wife’s death, and that he had attended meetings of the Hemlock Society, a national right-to-die group that often discusses methods of suicide: methods that Weinstein may have found useful in covering his crime. And, perhaps most damning of all, the same forensic psychologist who discovered these and more incriminating facts before the plea agreement was reached, Daniel Martell, also found that Weinstein had been an overweight, bullied child, one who had struggled throughout his life to control an inner sense of rage often planted by such experiences. As if to demonstrate as much, it turned out that on the day of the killing, when he and his wife had been arguing about their respective children from their first marriages, his wife had been taunting him — about his grown son’s own weight.