These leaders share four qualities, Dr. Post said: extreme self-absorption, paranoia, no constraints of conscience and a willingness to use whatever means necessary to accomplish goals. They have little empathy for the pain and suffering of their own people, Dr. Post said, but they also can't empathize with their enemies, a critical vulnerability in that ''it's very important as an effective leader to get into the mind of your adversaries.''

Mr. bin Laden in particular has little empathy for others, Dr. Post said, ''and is really consumed with being God's prophet on earth.'' Mr. Kim, who Dr. Post says is consumed by self-doubt because he lives in the enormous shadow of his father, the founding leader of North Korea, once punished a subordinate who displeased him by sending him home naked. As for Mr. Hussein, Dr. Post says that he is not irrational and is in fact entirely predictable and over three decades in power ''worked the international system to a fare-thee-well.''

While Dr. Post carefully shied away from any criticism of Mr. Bush, he did say that the president's failure to amass a large international coalition in the Iraq war -- in contrast to the large one his father assembled against Mr. Hussein in the first Persian Gulf war -- played to Mr. Hussein's strengths.

''Saddam desperately wants to be seen as a major world leader,'' Dr. Post said. ''And having both a large majority of the Arab world as well as the very broad coalition arrayed against him in 1990 had a very powerful negative impact on him.'' But in the Iraq war of 2003, Dr. Post said, he was ''able to say defiantly to his radical Arab and moderate Arab supporters, 'I have the courage to stand up to the mightiest nation on earth and its president.' ''

Dr. Post said he was not privy to the interrogation techniques used on Mr. Hussein, who was captured by American forces in December and is being held by the F.B.I. in Iraq. But he said standard interrogation techniques -- at least those consistent with the Geneva Conventions -- would probably not work on as strong a personality as Mr. Hussein. Although counterterrorism officials said this week that the C.I.A. had used coercive interrogation methods against a select group of high-level Al Qaeda operatives, American and British officials said this year that they were taking a gentle approach in their questioning of Mr. Hussein.

Dr. Post said the best technique would be to play to Mr. Hussein's ego and tell him with admiration that he had managed to convince the government of the United States and much of the Western world that he had weapons of mass destruction. But if he still had some, where might they be? ''Interrogation is part psychological karate,'' Dr. Post said, ''which is going with the strengths of the individual rather than attempting to pressure him.''

Dr. Post got into the business of analyzing the world's leading bad guys more or less by accident. In 1965 as he was planning a career in academic medicine at Harvard, he was approached by an acquaintance who took him on a drive, pulled into an overlook and asked if he would sign a secrecy agreement to help start a pilot program at the C.I.A. to study the world's leaders from a distance. The information would be used to prepare for international summit meetings and crises. Dr. Post thought it would be an interesting thing to do for a few years, but he remained for more than two decades.