Why do jokes have such peculiar psychological effects? In 1905, Sigmund Freud published a book explaining that we form censors in our minds as barriers against forbidden thoughts. Most jokes, he said, are stories designed to fool the censors. A joke's power comes from a description that fits two different frames at once. The first meaning must be transparent and innocent, while the second meaning is disguised and reprehensible. The censors recognize only the innocent meaning because they are too simple-minded to penetrate the forbidden meaning's disguise. Then, once that first interpretation is firmly planted in the mind, a final turn of word or phrase suddenly replaces it with the other one. The censored thought has been slipped through; a prohibited wish has been enjoyed.
Freud suggested that children construct censors in response to prohibitions by their parents or peers. This explains why so many jokes involve taboos concerning cruelty, sexuality, and other subjects that human communities typically link to guilt, disgust, or shame. But it troubled Freud that this theory did not account for the nonsense jokes people seem to enjoy so much. The trouble was that these seemed unrelated to social prohibitions. He could not explain why people find humor in the idea of a knife that has lost both its blade and its handle.
Freud considered several explanations to account for pointless nonsense jokes but concluded that none of those theories was good enough. One theory was that people tell nonsense jokes for the pleasure of arousing the expectation of a real joke and then frustrating the listener. Another theory was that senselessness reflects a wish to return to carefree childhood, when one was permitted to think without any compulsion to be logical, and to put words together without sense, for the simpler pleasures of rhythm or rhyme. Freud put it this way:
Little by little the child is forbidden this enjoyment, till there remain only significant combinations of words. But attempts still emerge to disregard restrictions which were learned.
In yet a third theory, Freud conjectured that humor is a way to ward off suffering — as when, in desperate situations, we make jokes as though the world were nothing but a game. Freud suggested that this is when the superego tries to comfort the childlike ego by rejecting all reality; but he was uneasy about this idea because such kindliness conflicted with his image of the superego's usual stern, strict character.
Despite Freud's complicated doubts, I'll argue that he was right all along. Once we recognize that ordinary thinking, too, requires censors to suppress ineffectual mental processes, then all the different-seeming forms of jokes will seem more similar. Absurd results of reasoning must be tabooed as thoroughly as social mistakes and inanities! And that's why stupid thoughts can seem as humorous as antisocial ones.