There are two positions regarding the role of humor in the persuasion process. The most commonly acknowledged position is that humor advances the persuasion process because it adds to a sense of shared sensibility and common interest; it creates a "positive affect" (positive emotional state) by adding a dimension of likability to the speaker; it enhances the speaker's trustability; and, perhaps most importantly, humor deflects interest in and attempts at finding logical contradictions and building counter-arguments (emotionally loaded rhetoric accomplishes the same deflection as in presidential campaign speeches).
There are two modes of persuasion processing. The most active is the systematic (also called central) processing during which analysis of the message is critical and ongoing until a conclusion is reached. The other is a more passive mode and, some say, the "default" mode during normal circumstances, indicating a general human predisposition to be persuaded. This mode is heuristic (also called peripheral) processing during which evaluation is made of the speaker's credibility, level of self-interest, likability, attractiveness, similarity to message receiver, and situational comfort.
The other position contends that research that disproves the above ideas of how humor advances persuasion have been generally withheld from publication and left out of the consideration when analysing the effect of humor on persuasion. According to this position, humor at best has a neutral effect on persuasiveness. This position specifies that in the processes of attending to, analyzing, understanding, being persuaded, and "retaining acceptance of persuasion," humor most likely plays no role at all.
It is difficult to give examples of this, but one way to look at this is that if humor used matches your own, then you are persuaded to the individual then to his message. Yet if the humor used is offensive to your own, you are persuaded against the individual and unwilling to even listen to their position. A surprising auxiliary example to this is that if the humor offends on the cultural level, yet is repeated often enough and given a level of prestige by one or more groups, then that humor persuades the culture of the values attached to it, thereby the humor and values become embedded in the culture. This last example contradicts the position that humor does not affect the persuasion process.