Ever since Hillary Clinton lost to Donald Trump in the 2016 presidential election, political spectators have been wondering how it happened.  She was supposed to win by a landslide.

Now Democrats are debating which of their candidates is most "electable."  They're afraid to nominate another Hillary Clinton.  But no one can agree on why she lost.  Was it because of globalization, sexism, populism, or Russian interference?

Eighty-four years of electoral history show that Trump's victory was not at all surprising.  In fact, it was entirely predictable.

In our forthcoming book, Personality Wins: Who Will Take the White House and How We Know, we show that personality has determined the winner of the last twenty-two U.S. presidential elections, and it will decide who wins in 2020.

Policy, platform, and ideology just don't seem to matter.  Ever since the rise of radio, followed by TV and social media, undecided voters have been drawn to the bigger personality.

There are many systems for categorizing personalities.  Most involve a smorgasbord of letters that no one can remember.  Let's use a system that is easy to apply to presidential politics. All you need to remember is four birds: Eagle, Parrot, Dove, and Owl.  In brief:

• Eagles are confident, assertive, blunt, and results-oriented. 
• Parrots are positive, charismatic, playful, and inspiring.
• Doves are empathetic, collaborative, caring, and diplomatic.
• Owls are analytical, detail-oriented, inquisitive, and tactical.

Since 1932, the energetic, bigger personalities of Eagles and Parrots have beaten the more reserved, soft-spoken Doves and Owls in all but one presidential election (Richard Nixon is the exception you can read about in our book).  Doves and Owls have won only when they've gone up against other Doves and Owls.  No Eagle has lost to a Parrot, but we think it could happen.

Trump is an extreme Eagle.  Franklin Delano Roosevelt, Harry Truman, Lyndon Johnson, George W. Bush, and Barack Obama were Eagles, too.  People often reflect two of the personality styles.  Obama, for example, is equal parts Eagle and Dove.  Candidates with Trump's personality have won many elections, and they will win again.

Hillary Clinton is an Owl. So were a lot of candidates: Herbert Hoover, Alfred Landon, Thomas Dewey, Michael Dukakis, George H.W. Bush, Bob Dole, Al Gore, and Mitt Romney.  What do they all have in common?  They've each lost at least one presidential election, and their personality will lose many more.

Is that a good thing?  No.  Some of our best presidents, including George Washington, John Adams, and Thomas Jefferson, were Owls.  Abraham Lincoln was a Dove.  Arguably, our country has suffered from the lack of diverse personalities in the White House.

So what about this year's candidates?  At this point, there are seven prominent Democrats left. Have you guessed their birds yet?

• Bernie Sanders: Eagle-Dove
• Joe Biden: Parrot
• Elizabeth Warren: Owl-Eagle
• Michael Bloomberg: Eagle/Owl
• Pete Buttigieg: Dove-Owl
• Amy Klobuchar: Eagle-Owl
• Tom Steyer: Eagle/Owl

History suggests that Sanders, Bloomberg, Klobuchar, and Steyer would have the best odds against Trump.  They each have demonstrated Eagle toughness, competitiveness, and candor.  Although we think Biden would have a chance, he's not inspiring voters the way Parrots normally do — least of all in Iowa and New Hampshire. 

Warren or Buttigieg would have a particularly hard time.  We haven't seen their personalities beat an Eagle since before 1928.

Remember that Hillary Clinton, like Warren, had a plan for everything.  Her campaign announcement speech laid out over 20 policies in more detail than Trump would ever give during his campaign or presidency.  She could run circles around Trump in debates about policy.  But who looked like the winner in the sound bites and highlight reels from the 2016 debates?  Trump.

Like the Owl Bob Dole, who lost to Bill Clinton in 1996, Warren can be wickedly funny and personable.  But that's not what she usually demonstrates to voters.

Although Mayor Pete has channeled some more Eagle energy in the debates, he's too much of a Ferdinand the Bull.  You have to sting him before he's willing to charge.

"I'm not going to take lectures on family values from the likes of Rush Limbaugh," Buttigieg recently said, after the conservative radio host attacked his sexual identity.  That's a classic Dove move to take the higher ground.  And it's commendable.  But what if Pete had to fight with Donald Trump over undecided voters who, for 84 years, have fallen for the more dominant personality every single time?  That response wouldn't cut it.

Personality isn't destiny.  The most successful candidates learn to flex into each style.  Bill Clinton and Ronald Reagan (both Parrots, coincidentally) mastered that skill.  They could be resolute.  They could be visionary.  They could tug at our heartstrings.  And they could talk policy when they had to.  They could hit all four styles in one speech.  Buttigieg or Warren could learn to do the same, but not without effort.

Our point is that any Democratic could beat Trump in 2020, but some have better odds than others.  There is no mirror image of Hillary Clinton in 2020.  In any year, under any conditions, Trump would be a difficult opponent.

Democrats need a candidate who can go toe-to-toe with Trump in the war of attention and sound bites.  Policies won't make a difference to undecided voters.  Personality will.

Merrick Rosenberg and Richard Ellis are the authors of Personality Wins: Who Will Take the White House and How We Know.