Some joke researchers—all very serious frog-dissectors—separate jokes into three categories: linguistic jokes, cultural jokes, and universal jokes. Cultural jokes are mostly about stereotypes for nationalities, religions, and things like that (I guess that Chuck Norris-type jokes fit in this category as well). At this point I should make it clear that my friend's partner, and all the other Japanese that I describe not getting jokes, are highly intelligent, reasonable, and funny people.