How Jokes Work

It is why incongruity should give us pleasure, and why some sorts of incongruity prompt laughter and others (such as Oedipus' parenthood) do not.

Perhaps, but consider Oedipus Rex by Tom Lehrer:

  There once lived a man named Oedipus Rex,
  You may have heard about his odd complex.
  His name appears in Freud's index
  'Cause he loved his mother.

  His rivals used to say quite a bit
  That as a monarch he was most unfit
  But still in all they had to admit
  That he loved his mother.

I don't care who you are, that's funny.