Donald Trump is an iconic System 1 candidate - more clearly so than any party nominee in at least 60 years.
Hillary Clinton is an iconic System 2 candidate - as clearly so as any party nominee in the same period.
That distinction may well end up defining the general election.
Let me explain.
Psychologists, and most prominently Nobel Prize winner Daniel Kahneman, have distinguished between two ways of thinking - fast and slow.
Fast thinking is associated with the brain's System 1: It is intuitive, quick and sometimes emotional. When you think that two plus two equals four, and when you immediately recognise a warm, smiling face, you are using System 1.
System 2 is deliberative and reflective. When you multiple 346 times 213, or struggle to fill out your tax forms, you are relying on System 2. If you are engaging in some kind of complex cost-benefit analysis, System 2 will be working hard.
System 1 is what leads people to fall in love. System 2 helps them decide whom to marry.
In the political arena, you can usually tell System 1 candidates by asking a single question: Do they seem to love running for president? Such candidates can also be polarising. They might appeal to the System 1 of a large part of the electorate - but turn off another large part.
On the Democratic side, the defining System 1 candidate was John F. Kennedy. Charismatic, witty and sharp, he left audiences smiling. In 2008, Barack Obama was also a System 1 candidate, a modern-day Kennedy. George McGovern fell in the same category.
By contrast, Lyndon Johnson, Walter Mondale, Michael Dukakis, Al Gore, and John Kerry were System 2 candidates. Bill Clinton straddled the line, but he was awfully good with System 1.
On the Republican side, the defining System 1 candidate has long been Ronald Reagan. (It helped that he was an actor.) George W. Bush falls in the same category - not exactly charismatic, but sharp, apparently strong, and likeable. John McCain was mostly System 1. Barry Goldwater was a highly reflective person, but he appealed directly to System 1.
Richard Nixon was all System 2. The same holds for George H.W. Bush, Mitt Romney and Bob Dole.
But in its entire history, the US has never had a serious System 1 candidate like Donald Trump.
Much of his success lies in the automatic favourable response that he triggers, at least within a substantial part of the Republican Party. Whether or not you're for him, he's funny and he's quick. When he's hit, he hits back. He's punishing. System 1 likes that.
There's an old adage about speakers: You won't remember what they said, but you'll never forget how they made you feel. Trump knows that in his bones. He gives his supporters - and they are growing - a terrific feeling of safety and security, with a laugh and a smile. Reagan did something similar, but as a former Governor of California, he also seemed, to many, to have genuine policy credentials.
As a candidate, Clinton is mostly System 2. She was first lady for eight years; she served in the Senate; she was Secretary of State. It's hard to doubt her credentials and mastery of policy. In terms of the power to reflect and deliberate, there have been few presidential candidates in her league.
Which kind of candidate has the upper hand? History doesn't say, and no general answer would make sense.
For System 1 candidates, the challenge is to get some kind of permission slip from System 2. That was Reagan's task in 1980, and he accomplished it beautifully. Obama did the same thing in 2008; in 1972, McGovern failed miserably.
Here, then, is one of Trump's major problems. He's a System 1 candidate, but he hardly appeals to everyone's System 1. And in terms of System 2, he has massive work cut out for him. Right now, many millions of Americans appear unwilling to give him that permission slip.
But System 2 candidates have serious challenges of their own. Voters like to fall in love with presidential candidates, at least a little bit. System 2 candidate need to find a way to appeal on an emotional level. George H.W. Bush did that in 1988, with his reference to "a kinder, gentler nation".
System 2 candidates beware: In politics, as in romance, System 1 can be immensely powerful
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If they can't forge an emotional connection, they might try to scare people about their System 1 opponent. Lyndon Johnson achieved that in 1964, with his famous Daisy ad suggesting that Goldwater would start a nuclear war.
If that fails, System 2 candidates can triumph by giving people real confidence that they'd do a good job, so that deliberation and reflection end up governing citizen behaviour in the voting booth. That was a big part of Nixon's winning strategy in 1968.
At the moment, it looks as if Hillary Clinton has formidable advantages over Donald Trump, not least because of the demographics and the electoral map.
But System 2 candidates beware: In politics, as in romance, System 1 can be immensely powerful.
_ Cass Sunstein is director of the Harvard Law School's programme on behavioural economics and public policy.