Democracy is the theory that two thieves will steal less than one, and three less than two, and four less than three, and so on ad infinitum.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 5, § 25 (1916)
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Variant:

DEMOCRACY. The theory that two thieves will steal less than one, and three less than two, and four less than three, and so on ad infinitum.
A Book of Burlesques, "The Jazz Webster" (1924)