What is the common element when we laugh at a self-important man slipping on a banana, when a baby laughs when tickled by its mother, and when we laugh at a joke? Arthur Koestler referred to a bisociation of matrices. There is a contrast between the self-important man and the slipping fool, between genuine aggression and the "friendly" aggression of tickling, and such a contrast is present in every joke I ever heard.
What, then, is the biological purpose of laughter and of becoming weak at the knees? I believe the answer is sexual in origin. Every animal species can indulge in mock aggression during sexual behaviour. Laughter is a signal that the victim recognises the apparent aggression as mock aggression. This contrast is at the root of the laughter mechanism.
Tickling is mock aggression, and I believe it originated as a part of courting behaviour. The borderline can be thin. A baby can howl with rage if tickled by someone it does not know or trust. Similarly, an unwanted sexual assault, or even unwanted tickling, can reduce the victim to tears.
This readiness of the brain to recognise the contrast between true aggression and mock aggression has spread so that many contrasts, even in non-sexual situations, have become linked to the laughter response.
D P EVANS