He attempted to define evil by plumbing the biographies and motivations of hundreds of violent felons who had committed heinous crimes.Dr. Michael H. Stone, a psychiatrist and scholar who sought to define evil and to differentiate its manifestations from the typical behavior of people who are mentally ill, died on Dec. 6 at his home in Manhattan. He was 90.The cause was complications of a stroke he had in January, his son David said.Dr. Stone was best known to the public as the author of the book “The Anatomy of Evil” (2009) and as the host from 2006 to 2008 of the television program “Most Evil,” for which he interviewed people imprisoned for murder to determine what motivated them to engage in an evil criminal act.He ranked the acts on a 22-category scale of his creation. Modeled on Dante’s nine circles of hell, his taxonomic scale ranged from justifiable homicide to murders committed by people whose primary motivation was to torture their victims.Only human beings are capable of evil, Dr. Stone wrote in “The Anatomy of Evil,” although evil is not a characteristic that people are born with. He acknowledged that while acts of evil were difficult to define, the word “evil” was derived from “over” or “beyond,” and could apply to “certain acts done by people who clearly intended to hurt or to kill others in an excruciatingly painful way.”For an act to be evil, he wrote, it must be “breathtakingly horrible” and premeditated, inflict “wildly excessive” suffering and “appear incomprehensible, bewildering, beyond the imagination of ordinary people in the community.”“Mike’s major contribution to psychiatry was sharpening the distinction between mental illness and evil,” Dr. Allen Frances. a former student of Dr. Stone’s who is now chairman emeritus of the department of psychiatry and behavioral sciences at Duke University School of Medicine in Durham, N.C., said in a phone interview.“The problem,” Dr. Frances said, “is that with every mass murderer, every crazy politician, every serial killer, the first tendency in the public mind and the media is that he’s mentally ill.” Dr. Stone, he said, helped to change that default position.Dr. Stone became known for his book “The Anatomy of Evil” and for hosting the TV program “Most Evil.”Prometheus BooksAnalyzing the biographies of more than 600 violent criminals, Dr. Stone identified two predominant personality traits: narcissism, to the point of having little or no ability to care about their victims; and aggression, in terms of exerting power over another person to inflict humiliation, suffering and death.In “The New Evil: Understanding the Emergence of Modern Violent Crime” (2019), a sequel to Dr. Stone’s 2009 book, he and Dr. Gary Brucato warned that since the 1960s there had been an “undeniable intensification and diversification” of evil acts committed mostly by criminals who “are not ‘sick’ in the psychiatric and legal sense, as much as psychopathic and morally depraved.”The reasons, they wrote, included greater civilian access to military weaponry; the diminution of both individual and personal responsibility, as preached by fascist and communist governments earlier in the 20th century; sexual liberation, which unleashed other inhibitions; the ease of communication on cellphones and the internet; the rise of moral relativism; and a backlash against feminism.In 2000, Dr. Stone figured in a sensational murder trial that tested the limits of doctor-patient confidentiality. He wanted to testify in the murder trial of Robert Bierenbaum, a plastic surgeon and former patient of his who was accused of killing his wife, Gail Katz-Bierenbaum, in 1985.Dr. Stone had written a letter to his patient’s wife two years before her death, advising her to live apart from her husband for her own safety. He had asked that she sign and return it, but she never did. He had also contacted Dr. Bierenbaum’s parents, with his permission.The judge ultimately excluded Dr. Stone’s testimony from the trial on the basis of professional confidentiality. But the testimony of several other witnesses about the letter contributed to Dr. Bierenbaum’s conviction.Dr. Stone identified two predominant personality traits in those who commit evil acts: narcissism and aggression.Librado Romero/The New York TimesMichael Howard Stone was born on Oct. 27, 1933, in Syracuse, N.Y., the grandson of Eastern European immigrants. His father, Moses Howard Stone, owned a wholesale paper business. His mother, Corinne (Gittleman) Stone, was a homemaker.A prodigy who learned Latin and Greek as a child, he was only 10 years old when he began seventh grade. As the youngest and smallest student in the school, as well as the only Jewish one, he formed an alliance with a 17-year-old classmate who was a boxer, his son David said: Mike would do the classmate’s homework, and the classmate would protect him from local antisemitic bullies.He entered Cornell University in Ithaca, N.Y., when he was 16, enrolling in a premedical curriculum but double-majoring in classics in case he was rejected by medical schools that had already met their quota of Jewish students. He enrolled in Cornell Medical School in Manhattan after graduating from Cornell in 1954 and received his medical degree in 1958.He originally studied hematology and cancer chemotherapy at Sloan Kettering Institute in Manhattan, but his mother’s chronic pain disorder prompted him to switch to neurology and then, eventually, to psychiatry. He did his residency at the New York State Psychiatric Institute at Columbia Presbyterian Hospital, where he met Dr. Clarice Kestenbaum, whom he married in 1965.He is survived by two sons, David and John Stone, from that marriage, which ended in divorce in 1978; his wife, Beth Eichstaedt; his stepchildren, Wendy Turner and Thomas Penders; three grandchildren; and one great-grandson.Dr. Stone spoke 16 languages and, like a vestige from another era, customarily wore three-piece suits. He was known for his impish sense of humor: His latest book, “The Funny Bone,” published this year, is a collection of his cartoons, jokes and poems.An amateur carpenter, he built the shelves that housed his library of 11,000 books. His collection included about 60 books on Hitler — further evidence, like his memories of childhood bullying, of his yearning to define evil.As a psychiatrist, a psychoanalyst and for many years a professor of clinical psychiatry at Columbia University College of Physicians and Surgeons, Dr. Stone also conducted a long-term study of patients with borderline personality disorders, including those who had contemplated suicide. He concluded that, often as a result of therapy and other treatment, the condition of about two-thirds of them had improved appreciably some 25 years later.In “The New Evil,” Dr. Stone and Dr. Brucato offered a possible explanation for why “particularly heinous and spectacular crimes,” especially those committed in America and by men, had been on the rise since the 1960s. They warned against “the rise of a sort of ‘false compassion,’ in which the most relentless, psychopathic persons are sometimes viewed as ‘victims.’”The two concluded by invoking a familiar metaphor: A frog dropped in a pot of boiling water will immediately try to escape; but, if placed in cold water that is gradually heated, the frog will remain complacent until it’s too late.“It is our ardent hope that, after a period of terrible growing pains, our culture will eventually learn that true power and control come only after a lifelong process of mastering and inhibiting the self,” they wrote. “Perhaps, as a first step, we should admit that the water in our collective pot is growing disquietingly warmer, day by day.” More