Steven Johnson in Discover Magazine:
We take it for granted that tickling causes laughter and that one person’s laughter will easily “infect” other people within earshot. Even a child knows these things. (Tickling and contagious laughter are two of the distinguishing characteristics of childhood.) But when you think about them from a distance, they are strange conventions. We can understand readily enough why natural selection would have implanted the fight-or-flight response in us or endowed us with a sex drive. But the tendency to laugh when others laugh in our presence or to laugh when someone strokes our belly with a feather—what’s the evolutionary advantage of that?
There is a long, semi-illustrious history of scholarly investigation into the nature of humor, from Freud’s Jokes and Their Relation to the Unconscious, which may well be the least funny book about humor ever written, to a British research group who claimed they had determined the world’s funniest joke. Despite the fact that the researchers sampled a massive international audience in making this judgment, the winning joke revolved around New Jersey residents: A couple of New Jersey hunters are out in the woods when one of them falls to the ground. He doesn’t seem to be breathing; his eyes are rolled back in his head. The other guy whips out his cell phone and calls the emergency service. He gasps to the operator: “My friend is dead! What can I do?” The operator says: “Take it easy. I can help. First, let’s make sure he’s dead.” There is silence, then a shot is heard. The guy’s voice comes back on the line. He says, “OK, now what?”
This joke illustrates the notion of controlled incongruity: You’re expecting x, and you get y.
More here.