For while O’Donnell crusaded against masturbation in the mid-1990s, denouncing it as “toying” with the organs of procreation and generally undermining baby making, the facts are to the contrary. Evidence from elephants to rodents to humans shows that masturbating is—counterintuitively—an excellent way to make healthy babies, and lots of them. No one who believes in the “family” part of family values can let her claims stand.

You will find that opening paragraph in an entertaining article in Newsweek (lid lob: linkfilter.)  It surveys a variety of stories suggesting that masturbation serves an adaptive role and was selected for by evolution.  The stories given (hygiene, signaling (??)) are mostly of the just-so variety, but this is a case where we don’t need to infer exactly the reason.  We can prove the evolutionary advantage of masturbation by a simple appeal to revealed preference.

There are lots of ways we can touch ourselves and among these, Mother Nature has revealed a very clear preference.  You cannot tickle yourself. Because the brain has a system for distinguishing between stimuli caused by others and stimuli caused by ourselves. Nature puts this system to good use:  such a huge fraction of sensory information comes from incidental contact with yourself that it has to be filtered out so that we can detect contact with others.

Mother Nature could have used this same system to put an end to masturbation once and for all:  simply detect when its us and mute the sensation. No gain, no Spain.  Instead, she made an exception in this case.  She must have had a good reason.