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“It ain’t what you don’t know that gets you into trouble,” which must be why The Big Short opens with a fake Mark Twain quote.

If you’ve seen The Big Short—and you should, it’s great—you’ll know that the movie opens with a quote from Twain: “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” 

It’s a perfect quote for the film, only it does not appear in any of Twain’s books, essays, letters, or speeches. Because Mark Twain never wrote it or said it or anything like it. The closest he came is “Faith is believing what you know ain’t so,” which appears in Pudd’nhead Wilson’s New Calendar and is a long way away from the quote that opens The Big Short

In fact, as far as I can tell no one said that exact quote. According to Quote Investigator, the quote should be attributed to Josh Billings, who in 1874 wrote this in what is perhaps best described as “Krazy Kat English”: 

A) I honestly beleave it iz better tew know nothing than two know what ain’t so.
B) Wisdum don’t konsist in knowing more that iz new, but in knowing less that iz false.

I haven’t been able to find any source for the exact quote used in The Big Short—Google Books, which is unreliable, only traces it to 2004. 

Interestingly, Michael Lewis’s book The Big Short opens with this similar quote, from Leo Tolstoy: 

“The most difficult subjects can be explained to the most slow-witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of a doubt, what is laid before him.”

That quote is credited to “Leo Tolstoy, 1897.” It’s less folksy than the fake quote that the film uses, but it’s real: it appears in The Kingdom of God is Within You, a nonfiction work dedicated to Tolstoy’s theories about Christianity and Christian anarchism. (Weirdly enough, I have no idea where Lewis’s publisher, W.W. Norton, got 1897 from—the book was published in 1894—but I’m just splitting hairs at this point.) 

So The Big Short traded a perfectly legitimate Tolstoy quote for a phony Twain one, even though both quotes make the same point. Why make the switch? Was Tolstoy’s not folksy enough? Too long? Too Russian? I’ve reached out to Paramount Pictures for comment.