In February, 2009, the British medical journal Brain published an article on the intersection of health and politics titled “Hubris Syndrome: An Acquired Personality Disorder?” The authors were David Owen, the former British Foreign Secretary, who is also a physician and neuroscientist, and Jonathan Davidson, a professor of psychiatry and behavioral sciences at Duke University, who has studied the mental health of politicians. They proposed the creation of a psychiatric disorder for leaders who exhibited, among other qualities, “impetuosity, a refusal to listen to or take advice and a particular form of incompetence when impulsivity, recklessness and frequent inattention to detail predominate.”

Owen and Davidson studied the behavior and medical records of dozens of American and British political leaders, from Prime Minister Herbert Asquith, who took office in 1908, to President George W. Bush, who left office in 2009. Across that century, they identified a tendency among some otherwise high-achieving individuals to close themselves off from critics and to overestimate their odds of success. Neville Chamberlain wrongly believed that he could appease Hitler; Tony Blair supported the invasion of Iraq even after his envoy informed him that the plan had “no leadership, no strategy, no coördination,” among other defects. When a leader succumbs to hubris syndrome, the authors wrote, his experience at the top has distorted his personality and decision-making.

“The Greeks warned us about it,” Owen told me recently, when I called him at home, in Britain. “When you see it, you’ve got to be very conscious that you may be watching somebody who is intoxicated with power.” After training as a doctor, Owen spent thirty-two years in politics, heading the Foreign Office from 1977 to 1979, and he developed a fascination with the ways in which C.E.O.s, dictators, and parliamentarians who are otherwise successful in their professions can be warped by the pressures and self-glorification presented by power. “It takes one to know one,” he said, dryly. “For a lot of us who are in leadership roles, the problem with the word ‘narcissism’ is that it has a very Freudian linkage and, if you use it, people will shy away from it.”

Owen was only partly interested in establishing a formal diagnosis. (Hubris syndrome does not appear in the Diagnostic and Statistical Manual.) More fundamentally, he wanted to call out a kind of public cognitive bias, in which voters and shareholders are often slow to acknowledge signs of irrational behavior in their chosen leaders because that acknowledgment reflects poorly on the decision to put them there. “You get rumors or people are telling you that things aren’t going all that marvellously, and either you’ve made a wrong choice or something has happened to him,” Owen told me. He helped establish a charity, the Daedalus Trust, which raises public awareness of hubris syndrome in public life, and he encourages institutions—banks, schools, political entities—to assess leaders’ mental health on a fixed schedule. “Then it’s easier to spot an incipient intoxication of power,” he said.

President Donald Trump, in the months since he entered the White House, has become a kind of international case study of mental health’s role in politics. To his friends and allies, he elicits an array of anodyne, even appealing, adjectives: unpredictable, fearless, irascible, sly. Many of his counterparts in diplomacy, and in American politics, are rapidly shedding the euphemisms that they once used to express their appraisals, however. When Trump, after a confused viewing of a Fox News segment, urged people at a rally to look at “what’s happening last night in Sweden. Sweden, who would believe this?,” suggesting that an incident—which no one could identify; nothing notable had happened the night before—had something to do with Sweden being overrun by refugees, Swedes reached a judgment. “They thought the man had gone bananas,” Carl Bildt, Sweden’s former Prime Minister and foreign minister, told Susan Glasser, of Politico, in an interview published this week. “It was a somewhat unsettling thing to see the president of the United States without any factual basis whatsoever lunge out against a small country in the way that he did.”

Though politicians often accuse each other of being crazy, Trump has inspired a more clinical and sober discussion. (In the magazine this week, I write about proposals in Congress to assess the President’s mental health.) In recent days, the discussion of Trump’s stability has entered a blunter phase. Over the weekend, Trump made a series of bizarre comments, including questioning the history of the Civil War, saying he was “looking at” breaking up banks (prompting a stock-market slide), and demonstrating unfamiliarity with basics of the health-care bill known as Trumpcare. The Presidential historian Douglas Brinkley told an interviewer that it was “among the most bizarre recent twenty-four hours in American Presidential history,” adding, “It was all just surreal disarray and a confused mental state from the President.” Joe Scarborough, the former Republican congressman, told his television audience, “My mother’s had dementia for ten years. . . . That sounds like the sort of thing my mother would say today.”

In the Washington Post on Thursday, the conservative columnist George Will wrote, “It is urgent for Americans to think and speak clearly about President Trump’s inability to do either. This seems to be not a mere disinclination but a disability.” After months of bemoaning Trump’s policies and incivility, Will is now bluntly warning of his instability. “Americans have placed vast military power at the discretion of this mind, a presidential discretion, that is largely immune to restraint by the Madisonian system of institutional checks and balances,” Will wrote. “So, it is up to the public to quarantine this presidency by insistently communicating to its elected representatives a steady, rational fear of this man whose combination of impulsivity and credulity render him uniquely unfit to take the nation into a military conflict.”

When I asked Owen if Trump meets the threshold of hubris syndrome, he replied that Trump was a hard case, because he reigned over a family business for so long before entering politics. “He has obviously got hubris, but did he acquire it in his business? What was he like when he was twenty? I refuse to put a label on him because I don’t know enough.” Owen added, “Watch him very carefully. It’s a phenomenon that needs to be analyzed, but it will not be very revealing to put labels on it that are inappropriate just because you desperately want to say, ‘He’s crazy.’ ”