Humour often involves saying something that isn't true (why don't ants catch colds? Because of they have anty bodies - great joke but not true on a number of levels). A surprising number of humans don't have the mental cycles available to consider counterfactuals, hypotheticals or entertain ideas that aren't directly rooted in reality. I suspect that means they can't process humour and they just laugh if the crowd is.

It goes beyond humour, you can see it in a lot of scenarios and it ruins politics. There are people who appear literally unable to consider hypothetical scenarios. Not in a nasty way, they're sometimes wonderful people to have on hand. They simply only deal in reality as they see it. You can walk them slowly through a "and what if ... happens?" and they can't do it.