Discover magazine has an article that looks at the psychology of laughter and humour, noting that the two aren’t necessarily as linked as we’d normally think.
It seems the social context is as powerful as the content of the humour itself in driving our response, because laughter is a communication in itself.
Previous studies of laughter had assumed that laughing and humor were inextricably linked, but Provine’s early research suggested that the connection was only an occasional one. As his research progressed, Provine began to suspect that laughter was in fact about something else‚Äînot humor or gags or incongruity but our social interactions. He found support for this assumption in a study that had already been conducted, one analyzing people‚Äôs laughing patterns in social and solitary contexts.
“You’re 30 times more likely to laugh when you’re with other people than you are when you’re alone‚Äîif you don’t count simulated social environments like laugh tracks on television,” Provine says. Think how rarely you’ll laugh out loud at a funny passage in a book but how quick you’ll be to give a friendly laugh when greeting an old acquaintance. Laughing is not an instinctive physical response to humor, the way a flinch is a response to pain or a shiver to cold. Humor is crafted to exploit a form of instinctive social bonding.
Link to Discover article on laughter.
Mind Hacks 
Neat! The most conclusive proof of this being true, for me, was when I worked at a theatre that hosted a Deaf Film Festival. When you walked into the theatre during any screenings, it was almost always completely quiet (the films are all closed-captioned and silent), even during comedies. The only laughter you’d ever hear was innocent, genuine, child-like, and completely unmoderated and uncontrolled…and even then only from one or two people at a time. It was surreal.