I think there was a thread about this a while ago, but there was also an article about it in a recent Discover or Scientific American magazine. If I remember correctly, it has something to do with needing to forge social bonds. I was unable to find the article on Discover's site, but you can check this link out - it's from last year but still relevant.
Desmond Morris proposed that a laugh is an aborted cry. He cites the fact that an individual's first laugh is usually for that individuals mother or father, and is a response to a situation that would normally be considered alarming, or even frightening. His take is that the infant then begins to cry, and stops themselves abruptly with a realization something akin to, "this is frightening, but Mom does not appear alarmed, so it must be OK".
At least on the surface, this reasoning seems sound. If you examine the nature of humor, throughout all cultures one common denominator is that humor must, as a surprise. So the connection between laughter and being startled has some support.
I agree to a certain extent, but not 100% fully. For example, I can watch a movie that I think is funny more than once and laugh at the jokes that I know are coming. It may just be a function of memory (IE, I remember being startled and laugh at that) but I do not believe this to be the case. I think being surprised is important for laughter, but not necessary.
Another interesting thing to note is that almost everyone in the world laughs the same. It's one of the few things that is almost universal as a species.
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