A good topic to explore for believability. Regarding laughter, one theory I heard was that it's misapplied joy-of-learning. e.g. someone sets up a situation where you realise your mental picture is horribly wrong, but they resolve it and you "learn" the solution before the discomfort gets to you, and the consequence of the wrong idea is minimal to you. This is different to being given a Sodoku puzzle then instantly being given the solution because you can't grasp the "problem" quickly (or the solution), and there's little emotional hook to it.
I've been thinking about this for a while. What about when something if funny to you and you only? A prime example would be when a child is developing their sense of humor. Not every joke is worth hacking your lungs to unlike when I go see a comedian on stage.
Also, what about inappropriately placed humor? i.e. a bunch of "hooligans" trashing some poor bloke's lawn laughing all the while.
I'm that imaginary number in the parabola of life.
Maybe that is not actually funny, but its something that generates another source of emotion. These people do this kind of thing not because it is FUNNY, I believe the feeling would be more of achievement of a social status within the group, which generates happiness, not exactly humorous, but satisfactory.
Now, humor based on other peoples misery or pain may be connected to certain psychological issues, because not all brains work exactly the same way, so this is something that should be taken into consideration.
With humour you need to consider the background of the person. Going by the theory I suggested, it depends whether you understand the way that your mental model is wrong, and understand the solution too. Lacking the former means you don't know it's a joke, lacking the latter means you don't get the punchline. Already knowing both well means you've heard the joke too many times. Different people have different knowledge and experience. That's why the same joke can be funny to one person but not another.
As for inappropriate humour, it may be that their outlook is so different that it is funny to them but not you. Or in some awful way it may be truly funny but we can't appreciate it because we're shocked, fearful or empathising with the victim. For someone with a violent history, someone being badly hurt may be the "funny banana peel" times 10.
