Was this petty, snarling man, utterly lacking in presidential temperament and heedless of the established rules and norms of politics, going to get us into a nuclear war? That’s what a lot of Americans wondered about Richard Nixon back in 1956. Chosen as Dwight Eisenhower’s running-mate four years before, Nixon was seen as unscrupulous, mean-spirited and reckless in smearing his opponents. So in the 1956 campaign, with Ike still wildly popular, Democrats made Nixon’s character an issue. The Soviet Union had just detonated its first hydrogen bomb, and Eisenhower had recently suffered a heart attack and a bout of ileitis (a gastrointestinal illness)—putting Nixon, they warned, perilously close to the Oval Office. In the fall Adlai Stevenson, the Democrats’ nominee, invoked what the liberal New York Post columnist Max Lerner called “the triple issue”: should Eisenhower suddenly die, the unreliable Nixon would have his finger on the button. 

Today, of course, we’re wondering the same things about Donald Trump, whose recent tweets about big nuclear buttons and his own “stability” have pundits buzzing about his mental health. His behavior—impulsive, erratic, petty and often cruel (to name just a few of the most salient qualities)—especially amid the prospect of a nuclear showdown with North Korea, makes it impossible not to think about his psychology, just as it was during the Cold War.

It’s an understandable impulse: The public needs to understand what moves our leaders to act as they do, including not just “rational” motives—pursuing policy objectives, responding to political pressures, working within economic and international constraints—but also the drives and responses that operate deep in an individual’s unconscious, formed by experiences and relationships long ago. 

But more than a century after Sigmund Freud revolutionized the understanding of the human mind, the use of psychology to understand our political leaders has failed to realize its promise. The diagnoses we’re now throwing around for Trump, from incipient senility or venereal disease, are simplistic, facile and unhelpful—and possibly fueled by politics rather than objective analysis. As crucial as it is to recognize the psychological dimension of Trump’s behavior, history suggests that analyzing public figures is a dangerously fraught project—one that tends to descend into subjectivity, politicization and self-caricature.

Applying psychological insights to political leaders’ behavior started way back in Freud’s own day. Stanley Allen Renshon, a political scientist and psychoanalyst, cites as the first such effort a March 1912 New York Times Magazine cover story titled “Roosevelt as Analyzed by the New Psychology,” about Theodore Roosevelt’s relationship with his successor as president, William Howard Taft. In it, Dr. Morton Prince, a professor of “nervous diseases” at Tufts, argued that Roosevelt was acting strangely because of an internal struggle: On the one hand, he consciously wanted to support his hand-picked successor and stand by his avowals to forswear a third term. On the other hand, he had a repressed, subconscious wish to win back the office he had loved so much. Over time, Prince explained, “the subconscious wish to be an active candidate became acceptable to his consciousness.” ...