Carlo M. Cipolla

A stupid person is a person who causes losses to another person or to a group of persons while himself deriving no gain and even possibly incurring losses.

No matter how high are one's estimates of human stupidity, one is repeatedly and recurrently startled by the fact that:a) people whom one had once judged rational and intelligent turn out to be unashamedly stupid.b) day after day, with unceasing monotony, one is harassed in one's activities by stupid individuals who appear suddenly and unexpectedly in the most inconvenient places and at the most improbable moments.