What’s So Funny? Well, Maybe Nothing – New York Times
He and Professor Provine figure that the first primate joke — that is, the first action to produce a laugh without physical contact — was the feigned tickle, the same kind of coo-chi-coo move parents make when they thrust their wiggling fingers at a baby. Professor Panksepp thinks the brain has ancient wiring to produce laughter so that young animals learn to play with one another. The laughter stimulates euphoria circuits in the brain and also reassures the other animals that they’re playing, not fighting.
Humans are laughing by the age of four months and then progress from tickling to the Three Stooges to more sophisticated triggers for laughter (or, in some inexplicable cases, to Jim Carrey movies). Laughter can be used cruelly to reinforce a group’s solidarity and pride by mocking deviants and insulting outsiders, but mainly it’s a subtle social lubricant. It’s a way to make friends and also make clear who belongs where in the status hierarchy.