For mental-health professionals, Donald Trump is at once easily diagnosed but slightly confounding. “Remarkably narcissistic,” said developmental psychologist Howard Gardner, a professor at Harvard Graduate School of Education. “Textbook narcissistic personality disorder,” echoed clinical psychologist Ben Michaelis. “He’s so classic that I’m archiving video clips of him to use in workshops because there’s no better example of his characteristics,” said clinical psychologist George Simon, who conducts lectures and seminars on manipulative behavior. “Otherwise, I would have had to hire actors and write vignettes. He’s like a dream come true.”

That mental-health professionals are even willing to talk about Trump in the first place may attest to their deep concern about a Trump presidency. As Dr. Robert Klitzman, a professor of psychiatry and the director of the master’s of bioethics program at Columbia University, pointed out, the American Psychiatric Association declares it unethical for psychiatrists to comment on an individual’s mental state without examining him personally and having the patient’s consent to make such comments. This so-called Goldwater rule arose after the publication of a 1964 Fact magazine article in which psychiatrists were polled about Senator Barry Goldwater’s fitness to be president. Senator Goldwater brought a $2 million suit against the magazine and its publisher; the Supreme Court awarded him $1 in compensatory damages and $75,000 in punitive damages.

But you don’t need to have met Donald Trump to feel like you know him; even the smallest exposure can make you feel like you’ve just crossed a large body of water in a small boat with him. Indeed, though narcissistic personality disorder was removed from the most recent issue of the Diagnostic and Statistical Manual of Mental Disorders, for somewhat arcane reasons, the traits that have defined the disorder in the past—grandiosity; an expectation that others will recognize one’s superiority; a lack of empathy—are writ large in Mr. Trump’s behavior.

“He’s very easy to diagnose,” said psychotherapist Charlotte Prozan. “In the first debate, he talked over people and was domineering. He’ll do anything to demean others, like tell Carly Fiorina he doesn’t like her looks. ‘You’re fired!’ would certainly come under lack of empathy. And he wants to deport immigrants, but [two of] his wives have been immigrants.” Michaelis took a slightly different twist on Trump’s desire to deport immigrants: “This man is known for his golf courses, but, with due respect, who does he think works on these golf courses?”

Mr. Trump’s bullying nature—taunting Senator John McCain for being captured in Vietnam, or saying Jeb Bush has “low energy”—is in keeping with the narcissistic profile. “In the field we use clusters of personality disorders,” Michaelis said. “Narcissism is in cluster B, which means it has similarities with histrionic personality disorder, borderline personality disorder, and antisocial personality disorder. There are similarities between them. Regardless of how you feel about John McCain, the man served—and suffered. Narcissism is an extreme defense against one’s own feelings of worthlessness. To degrade people is really part of a cluster-B personality disorder: it’s antisocial and shows a lack of remorse for other people. The way to make it O.K. to attack someone verbally, psychologically, or physically is to lower them. That’s what he’s doing.”

Clockwise from top left: By Andrew Burton (2), Scott Olson (2), Michael Stewart, Tom Pennington, all from Getty Images.

What of Trump’s tendency to position himself as a possible savior to the economy despite the fact that four of his companies have declared bankruptcy? “It’s mind-boggling to me that that’s not the story,” said Michaelis. “This man has been given more than anyone could ever hope for,” he added, referring to the fact that Trump is not wholly self-made, “yet he’s failed miserably time and time again.” Licensed clinical social worker Wendy Terrie Behary, the author of Disarming the Narcissist: Surviving and Thriving with the Self-Absorbed, said, “Narcissists are not necessarily liars, but they are notoriously uncomfortable with the truth. The truth means the potential to feel ashamed. If all they have to show the world as a source of feeling acceptable is their success and performance, be it in business or sports or celebrity, then the risk of people seeing them fail or squander their success is so difficult to their self-esteem that they feel ashamed. We call it the narcissistic injury. They’re uncomfortable with their own limitations. It’s not that they’re cut out to lie, it’s just that they can’t handle what’s real.”