It’s difficult to quantify, but the best example I can give is when I watched some Chappelle skits with some southern relatives of mine who would be charitably described as racist. I couldn’t figure out why they were laughing at what was clearly a sketch written to make fun of people with attitudes and beliefs exactly like theirs (The blind, black KKK sketch). I noticed they were laughing not only at the wrong time, but for what appeared to be for the wrong reasons. Later, when they were quoting what they considered the “funny” parts of the skit, it wasn’t what everyone “in on the joke” was quoting.
The e-mail author’s relatives: pre-PC. The e-mail author, who is “in on the joke”: post-PC. The problem with so-called post-PC humor is that it really only works if your audience is exclusively or almost exclusively post-PC, too. A joke among a few friends, for example. But bring it to a mass audience and you’ll hear lots of laughter, but it’s the racists, sexists, and homophobes laughing the loudest… and for reasons that are likely to appall. They’re reinforced in their prejudices while you get your lollies and ignorantly congratulate yourself on your sophisticated, post-ironic sense of humor. Not funny.