Donald Trump is the Republican frontrunner for president, a fact that has befuddled just about everybody—except perhaps Trump himself—and spawned countless theories: He's leading because Americans are frustrated with politicians and want a straight-talking outsider. Because he shamelessly caters to paranoid conservatives. Because he's famous. He's not politically correct. He never says sorry. He's unfailingly entertaining. And the press can't resist him. But there's another reason that no one has considered yet, a secret weapon that has propelled past charismatic politicians like Bill Clinton and Theodore Roosevelt to the White House: hypomanic temperament

To be clear, I’m not using my authority as a professor of psychiatry to call Trump mentally ill. Hypomanic temperament is not an illness. It is genetically linked to bipolar disorder and manifests the same traits as mania—but crucially, does so to a less severe and more functional degree. Historically, hypomanic temperament has received little attention compared to bipolar disorder, but the founders of modern psychiatry—Eugen Bleuler, Emil Kraepelin, Ernst Kretschmer—first described these personalities around a century ago. "Hypomanics," as I describe them in In Search of Bill Clinton: A Psychological Biography

are whirlwinds of activity who are filled with energy and need little sleep, less than 6 hours. They are restless, impatient and easily bored, needing constant stimulation… and tend to dominate conversations. They are driven, ambitious and veritable forces of nature in pursuit of their goals. While these goals may appear grandiose to others, they are supremely confident of success—and no one can tell them otherwise…. They can be exuberant, charming, witty, gregarious but also arrogant…. They are impulsive in ways that show poor judgment, saying things off the top of their head, and acting on ideas and desires quickly, seemingly oblivious to potentially damaging consequences. They are risk takers who seem oblivious to how risky their behavior truly is. They have large libidos and often act out sexually. Indeed all of their appetites are heightened.

This description doesn't just match Clinton; it also sounds an awful lot like Trump. He reports, for example, “I usually sleep only four hours a night,” which by itself is usually a pretty reliable indicator of hypomania, and something he boasts about: “How can you compete against people like me if I sleep only four hours?” He claims to work seven days a week, and in a typical 18-hour day makes “over a hundred" phone calls and have “at least a dozen meetings.” “Without passion you don't have energy, without energy you have nothing!” Trump has tweeted. Hence his taunt of Jeb Bush as “a low energy person,” by contrast. Like most hypomanics, he is distractible. “Most successful people have very short attention spans. It has a lot to do with imagination,” he once wrote. He is correct. The same rapidity of thought that helps engender creativity makes it difficult to stay on one linear track of ideas without skipping to the next. Like most hypomanics, he follows his “vision, no matter how crazy or idiotic other people think it is.” Trump sees himself as a person of destiny and no one is going to talk him out of it. Trump's inflated self-esteem is illustrated by the fact that his net worth is reported by Forbes to be $4 billion, a fraction of the $10 billion he claims. It’s not just hyperbole: Hypomanics' wild optimism systematically distorts their perceptions.

Dripping with arrogance, Trump is an uber-aggressive alpha male who gleefully dominates, bullies, and colorfully disparages his competitors and critics. His hypomanic energy gives him that elusive charisma: Whether you love him or hate him (and charismatic figures produce such polarized responses) he makes himself the center of attention, the most exciting figure on the stage, who consumes all the oxygen in the room. But his impulsivity is manifested in his impolitic, unfiltered, outrageous statements, like his remark about Mexican immigrant "rapists." Many assume that one of these gaffes will ultimately bring him down, and they may be right. Leaders who live by their hypomania often die by it as well.

Take Bill Clinton.

For my Clinton biography, I interviewed a hundred people who have known him well, from childhood to the present day, and read aloud to each one of them the narrative description of hypomanic temperament. All one hundred of them agreed—enthusiastically in most cases—that it described Bill Clinton. Clinton also requires little sleep and is brimming with energy. When he first ran for Congress, he regularly campaigned for 36 hours at a time without sleeping, and required rotating shifts of drivers; he wore out the soles of three pairs of shoes. George Stephanopoulos recalled that Clinton called him with “fifty ideas a day.” In what seemed like an attempt to “change the world overnight,” the youngest governor in Arkansas history submitted 150 bills to the legislature on its opening day, a packet so thick legislators complained they couldn’t lift it, much less read it.

Clinton also evidences classic hypomanic pressured speech pattern, to a degree so extreme I have never seen the likes of it before. At a three-hour dinner in Africa, seated at a table with a dozen members of the travelling press corps in 2007, I heard him speak off the cuff for three hours, without interruption. “Why didn’t any of you ask him a single question?” I asked a veteran reporter from a national publication. “I was having trouble just keeping up with him,” he confessed. Of course, Clinton is off the charts in his charisma, and his erotic electromagnetism always made him the center of attention in any room. But with these assets came his downfall—the inability to contain his sexual impulses.

The irony is that Donald Trump and all his success, indeed America’s success, has been produced by the very phenomenon Trump claims is destroying America: immigration. In The Hypomanic Edge, I propose that the secret to America’s character and success is that we have more hypomanics than anyone else, precisely because we are a nation of immigrants. Immigrants are unusual people with the restless energy, ambitious drive and risk tolerance to come here. America's gene pool has been seeded these self-selected, highly driven people from all over the world—the type of people most likely to become entrepreneurs.

Andrew Carnegie, a Scottish immigrant himself, called immigration the “golden stream” that contributed more to America’s wealth than “all the silver and gold mines in the world.” Contrary to Trump’s contention that Mexico is sending us their worst, Carnegie argued that the “the best of the workers seek [America's] shores.” “The old and destitute, the idle and contented do not brave the waves of the stormy Atlantic, but sit helplessly at home,” he wrote. Yet each generation of Americans forgets this lesson, insisting that the most recent wave of immigrants is different than previous generations'—immoral, criminal, and a drain on society. Lest we forget, the same was once said of the Irish and Italians. The irony is that the Mexican immigrants Trump disparages are very much like him—thinking big and pursuing their dream of a better life, no matter how big a wall you put in their way. One of them might even be the father of the next Donald Trump.