The class of jokes I refer to as "Martian Jokes" are those which poke fun at one particular group, be it defined by religion, class, ethnic origin, or physical appearance. Most of these jokes can be told equally well for one of any number of different groups. I have observed two markedly different responses to this variety of joke. Some people find them amusing, and other people are hurt by them.
When people are amused by these jokes, it puts the idea of the joke into their mind, giving them a frame which describes this type of behavior. However, as soon as they meet someone who contradicts this frame, they will find that the default assumptions they make are, in fact, false. Because the frame is already present in the mind, the person will have to examine their opinions of the situations. This weakens the strengths of the defaults.
Other people find these jokes insulting. This is an example of the joke working in its censorship role. This person will be concerned that the joke is about them, and the joke will allow them to laugh and pretend it does not effect them. (alleviating the need to kill the person who insulted them) This gives the brain a change to build a censor against the behavior either performed in the joke or suggested by the joke.