“Laughter is an honest social signal because it’s hard to fake,” Professor Provine says. “We’re dealing with something powerful, ancient and crude. It’s a kind of behavioral fossil showing the roots that all human beings, maybe all mammals, have in common.”

The human ha-ha evolved from the rhythmic sound — pant-pant — made by primates like chimpanzees when they tickle and chase one other while playing. Jaak Panksepp, a neuroscientist and psychologist at Washington State University, discovered that rats emit an ultrasonic chirp (inaudible to humans without special equipment) when they’re tickled, and they like the sensation so much they keep coming back for more tickling.

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.

“Primal laughter evolved as a signaling device to highlight readiness for friendly interaction,” Professor Panksepp says. “Sophisticated social animals such as mammals need an emotionally positive mechanism to help create social brains and to weave organisms effectively into the social fabric.”

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.

Which brings us back to the muffin joke. It was inflicted by social psychologists at Florida State University on undergraduate women last year, during interviews for what was ostensibly a study of their spending habits. Some of the women were told the interviewer would be awarding a substantial cash prize to a few of the participants, like a boss deciding which underling deserved a bonus.

The women put in the underling position were a lot more likely to laugh at the muffin joke (and others almost as lame) than were women in the control group. But it wasn’t just because these underlings were trying to manipulate the boss, as was demonstrated in a follow-up experiment.