Some readers might object that the censor-learning theory of jokes is too narrow to be an explanation of humor in general. What of all the other roles that humor plays in occasions of enjoyment and companionship? Our answer is the same as usual: we can't expect any single, simple theory to explain adult psychology. To ask how humor works in a grown-up person is to ask how everything works in a grown-up person, since humor gets involved with so many other things. I didn't mean to suggest that every aspect of humor is involved in making censors learn. When humor evolved, as when any other mechanism develops in biology, it must have been built upon other mechanisms that already existed, and embodied mixtures of those other functions. Just as the voice is used for many social purposes, the mechanisms involved in humor are also used for other effects that are less involved with memory. In later life the effect of functional autonomy can make it hard to recognize the original function not only of humor, but of many other aspects of adult psychology. To understand how feelings work, we need to understand both their evolutionary and their individual histories.
We've seen how important it is for us to learn about mistakes. To keep from making old mistakes ourselves, we learn about them from our families and friends. But a peculiar problem arises when we tell another person that something is wrong, for if this is interpreted as an expression of disapproval and rejection, it can evoke a sense of pain and loss — and lead to withdrawal and avoidance. Accordingly, to point out mistakes to someone whose loyalty and love we want to keep, we must adopt some pleasant or conciliatory form. Thus humor has evolved its graciously disarming ways to do its basically distasteful job! You don't want the recipient to kill the messenger who brings bad news — especially when you're the messenger.
Many people seem genuinely surprised when shown that humor is so concerned with unpleasant, painful, and disgusting subjects. In a certain sense, there's really nothing humorous about most jokes — except, perhaps, in the skill and subtlety with which their dreadful content is disguised; frequently, the thought itself is little more than See what happened to somebody else; now, aren't you glad it wasn't you? In this sense most jokes are not actually frivolous at all but reflect the most serious of concerns. Why, by the way, are jokes usually less funny when heard again? Because the censors learn some more each time and prepare to act more quickly and effectively.
Why, then, do certain kinds of jokes, particularly those about forbidden sexual subjects, seem to remain persistently funny to so many people? Why do those censors remain unchanged for so long? Here we can reuse our explanation of the prolonged persistence of attachment, infatuation, sexuality, and mourning-grief; because these areas relate to self-ideals, their memories, once formed, are slow to change. Thus the peculiar robustness of sexual humor may mean only that the censors of human sexuality are among the slow learners of the mind, like retarded children. In fact, we could argue that they literally are retarded children — that is, they are among the frozen remnants of our earlier selves.