I study liars. I've never seen one like Donald Trump.

Then along came President Donald Trump. In research beginning in the mid-1990s, when I was a professor at the University of Virginia, my colleagues and I asked 77 college students and 70 people from the nearby community to keep diaries of all the lies they told every day for a week. At The Washington Post, the Fact Checker feature has been tracking every false and misleading claim and flip-flop made by Trump this year. The inclusion of misleading statements and flip-flops is consistent with the definition of lying my colleagues and I gave to our participants: "A lie occurs any time you intentionally try to mislead someone." In the case of Trump's claims, though, it is possible to ascertain only whether they were false or misleading, and not what the president's intentions were.

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