Conservatism: the belief in established and traditional ideas and practices.
In what way is humor a vehicle for conservatism? Well, the fact that in a lot of comedy material the characters don't really develop their personalities - i.e. genuinely learn new ways of looking at the world or gain meaningful insights from their experience - but rather are portrayed as types with certain traits, is a hint towards this idea. Correctively joking about repetitive or 'stupid' behavior is another. Jokes are used as more or less closed units of information that deliver prejudice or message, instead of opening up a conversation about a topic of interest. Also, humor can be quite offensive, enforcing own prejudices while blocking sympathetic relationships.
Progressivism: the belief in developing new ideas and practices.
In what way is humor a vehicle for progressiveness? Well, the fact that humorists are quite often seeking the psychological boundaries of a subject, suggests they are not bound by any particular code of norms and suggests humors openness to other points of view. The way humor is being used as a mechanism for handling difficult or stressful situations (together) is a hint. Pointing out and joking about contradictions in behavior and thinking is another. Also, humor may create a bond between the people that share it, which possibly deepens and develops their understanding of the subject of discussion and their personal relationship.
With these scenario's, a joke can be conservative to some groups, while being progressive to others (it can strengthen mutual understanding and develop a relationship while at the same time insult and make fun of other people their traits). The question that some social psychologists are trying to answer, namely whether 'conservative' or 'progressive' people appreciate humor more, must largely depend on the content and usage of the chosen humor; and it is hard to judge humorous examples in a (politically) neutral way. It's not as if humor is somehow a neutrally given and then people can neutrally decide the amount of conservatism and progressivism in it and then measure whether a conservative or a progressive person likes the humor more. This seems to me to be as much an act of creation as is wanting to prove which group appreciates humor more in the first place. It forms the world as much as it reports it.
Remains an important question to be asked: where's the humor in this? Why, after all, only talk about two groups, while Robert Benchley famously said that there are two kinds of people in the world: those who believe there are two kinds of people in the world and those who don't?