Dubious Quality

Nothing else would be needed. 

We could chart the rise and fall of social mores by which types of jokes evaporated over time. Slavery jokes. Blackface jokes. Drunk jokes. Fat jokes. Women jokes. Disability jokes. Gay jokes. Transgender jokes. All of these, at some point, became less and less acceptable. Some of them (thankfully) disappeared altogether. 

Punching down used to constitute the vast majority of jokes. That's changed, though sporadically. More comedians punch up now, and those jokes have always been funnier to me. 

One of my many problems with Trump and his ilk is that they mock people with disabilities. They think it's funny to imitate them. That's in public, too. I can only imagine what they joke about in private. 

People tell you so much about who they are by what they find funny. 

Comedy doesn't just change by changing its targets, though. Some gags just lose their appeal over time. "The old grey mare just ain't what she used to be" didn't age well, and nobody riffs on it anymore. Jokes heavily dependent on popular culture faded away, too, but they also help you understand what was popular at the time. 

You could compare comedy across cultures, too, to see which kind of jokes are emphasized and which are ignored. There were lots of jokes in the U.S. about Germany and Japan after WWII, but it wasn't mentioned much in German and Japanese comedy, I bet. That would tell you all you needed to know about who was on the winning side.

Maybe someone's already done this. If not, I wish someone would.