Donald Trump Is a LiarOk, you probably already knew that. But Bella DePaulo is a social scientist who has studied lying extensively, producing several books on the subject, like The Hows and Whys of Lies and Behind the Door of Deceit: Understanding the Biggest Liars in Our Lives. She has written a very interesting op-ed for the Washington Post in which she declares that she's never seen a liar anything like Donald Trump in her 20 years of studying the subject.
DePaulo's observation applies to both the quantity and the quality of Trump's lies. The quantity is well known; in his first year in office Trump has averaged about six lies a day, though recently that number has been closer to nine. And those are just his public lies; only his staff knows what happens when the cameras are off and the phone with Twitter is in the pocket. DePaulo says this is more lying than she's ever seen from anyone, in all the folks she's studied.
What is really, unusual, however, is the quality of the lies. Generally speaking, people tend to tell self-serving lies ("I wasn't speeding, officer!") about half the time, and they tend to tell kind lies, often called "little white lies," about a quarter of the time ("That shirt looks very nice on you!") That is a ratio of about 2 to 1. In Trump's case, however, he tends to tell self-serving lies about two-thirds of the time, and he tells kind lies about 10% of the time. That's a ratio of 6.5 to 1, which is the worst that DePaulo has ever seen. Even more unusual, however, is that most people are loath to tell cruel or mean lies ("Your wife is cheating on you!"); those tend to constitute only 1-2% of all lies for the average person. Not for Donald Trump, however, where fully 50% of his lies are meant to inflict harm on a person or group. Either he really does have a brain problem (see above), or else we now have statistical evidence that he's an unusually nasty person, or both.
http://www.electoral-vote.com/evp2017/Senate/Maps/Dec10.html#item-4