The conviction of the rich that the poor are happier is no more foolish than the conviction of the poor that the rich are.
Mark Twain (1835–1910) American writer [pseud. of Samuel Clemens]
(Misattributed)
I cannot find any reference to this phrase prior to 1921, and no association with Twain until the mid-1970s.
The quotation apparently first appears in various newspaper "filler" columns (e.g., 1921-12-07); in no cases is there an attribution to Twain or to anyone else, except some references of it having been originally seen in the Boston Post (e.g., 1921-12-17, 1921-12-16, 1921-12-07).
One place where a name is associated with the quote is where it appears in the "Facts and Fancies" syndicated column of quips by Robert Quillen (1921-12-07). Quillen (1887-1948) was an American journalist and humorist, whose work was syndicated in hundreds of newspapers. He was know for, among other things, his one-liners. It's unclear whether he adopted material from others, or originated everything in "Facts and Fancies." If the latter, and if the column also appeared in the Boston Post, that would indicate Quillen actually is the source of this quotation.
One place for some doubt is that the one Quillen column shows a date of December 7, but so do some other papers which ran the quote. It is possible, as the actual publication dates of syndicated material can vary between papers or be delayed, that Quillen's column in the paper above ran after its original appearance (in the Boston Globe?), which other papers then stole from as filler material without crediting Quillen.
Twain, who died in 1910, does not seem associated with the quote until the mid-1970s, and it does not show up in more authoritative collections of Twain material. The association to Twain seems to come from Laurence J Peter, Peter's Quotations (1977). Peter included the phrase as a parenthetical comment to a Mark Twain quotation. The proximity may have led to Twain being associated with it (as here, which duplicates the entry from Peter, but with the attribution following the combined two quotes).
In sum, the quotation first appeared in December 1921, a decade after Twain's death, and was possibly created by Robert Quillen. It's association to Mark Twain came from its use by Lawrence Peter as an editorial comment to a different Twain quotation.
