What would a Martian visitor think to see a human being laugh? It must look truly horrible: the sight of furious gestures, flailing limbs, and thorax heaving in frenzied contortions. The air is torn with dreadful sounds as though, all at once, that person wheezes, barks, and chokes to death. The face contorts in grimaces that mix smiles and yawns with snarls and frowns. What could cause such a frightful seizure? Our theory suggests a simple answer:
The function of laughing is to disrupt another person's reasoning!
To see and hear a person laugh creates such chaos in the mind that you can't proceed along your present train of thought. Derision makes you feel ridiculous; it prevents you from being serious. What happens then? Our theory has a second part:
Laughter focuses attention on the present state of mind!
Laughter seems to freeze one's present state of mind in its tracks and hold it up to ridicule. All further reasoning is disrupted, and only the joke-thought remains in sharp focus. What is the function of this petrifying effect?
By preventing you from taking seriously your present thought, and thus proceeding to develop it, laughter gives you time to build a censor against that state of mind.
In order to construct or improve a censor, you must retain your records of the recent states of mind that made you think the censored thought. This takes some time, during which your short-term memories are fully occupied — and that will naturally disrupt whichever other processes might try to change those memories.
How could all this have evolved? Like smiling, laughter has a curious ambiguity, combining elements of affection and conciliation with elements of rejection and aggression. Perhaps all these ancestral means of social communication became fused to compose a single, absolutely irresistible way to make another person cease an activity regarded as objectionable or ridiculous. If so, it is no accident that so many jokes mix elements of pleasure, cruelty, sexuality, aggression, and absurdity. Humor must have grown along with our abilities to criticize ourselves, starting with simple internal suppressors that evolved into more sophisticated censors. Perhaps they then split off into B-brain layers that became increasingly able to predict and manipulate what the older A-brains were about to do. At this point, our ancestors must have started to experience what humanists call conscience. For the first time, animals could start to reflect upon their own mental activities and evaluate their purposes, plans, and goals. This endowed us with great new mental powers but, at the same time, exposed us to new and different kinds of conceptual mistakes and inefficiencies.
Our humor-agencies become internalized in adult life as we learn to produce the same effects entirely inside our own minds. We no longer need the ridicule of those other people, once we can make ourselves ashamed by laughing silently at our own mistakes.