So while it would be unethical for a psychiatrist to say that President Trump has narcissistic personality disorder, he or she could discuss common narcissistic character traits, like grandiosity and intolerance of criticism, and how they might explain Mr. Trump’s behavior. In other words, psychiatrists can talk about the psychology and symptoms of narcissism in general, and the public is free to decide whether the information could apply to the individual.

This may seem like splitting hairs, but it isn’t. Diagnosis requires a thorough examination of a patient, a detailed history and all relevant clinical data — none of which can be gathered from afar. Narcissism, for instance, isn’t the only explanation for impulsive, inattentive and grandiose behavior. Someone could be suffering instead from another clinical problem like attention deficit hyperactivity disorder; the abuse of drugs, alcohol or stimulants; or a variant of bipolar disorder, to name just a few.

This is all to say that when mental health professionals label public figures with mental illnesses, it is not just unethical — it’s intellectually suspect. We don’t have the requisite clinical data to know what we are talking about.

Besides, even if you posit that a president has a mental disorder, that in itself may say little about his fitness to serve. After all, Lincoln had severe depression. Theodore Roosevelt was probably bipolar. Ulysses S. Grant was an alcoholic. According to a study based on biographical data, 18 of America’s first 37 presidents met criteria suggesting they suffered from a psychiatric disorder during their lifetime: 24 percent from depression, 8 percent from anxiety, 8 percent from bipolar disorder and 8 percent from alcohol abuse or dependence. And 10 of those presidents showed signs of mental illness while they were in office.

You can be psychiatrically ill and be perfectly competent, just as you can be mentally healthy but totally unfit. (Of course, certain mental states, like florid psychosis or dementia, would render a president unfit to serve.)

There is one last reason we should avoid psychiatrically labeling our leaders: It lets them off the moral hook. Not all misbehavior reflects psychopathology; the fact is that ordinary human meanness and incompetence are far more common than mental illness. We should not be in the business of medicalizing bad actors.

So the nation doesn’t need a shrink to help it to decide whether President Trump is fit to serve, mentally or otherwise. Presidents should be judged on the merits of their actions, statements and, I suppose, their tweets. No experts are needed for that — just common sense.