I have to admit that I rather like the findings of this little paper.

A woman’s sex drive begins to plummet once she is in a secure relationship, according to research. Researchers from Germany found that four years into a relationship, less than half of 30-year-old women wanted regular sex. Conversely, the team found a man’s libido remained the same regardless of how long he had been in a relationship.

OK, I’m willing to believe that, it certainly seems to accord with the anecdotal evidence. Indeed, the number of jokes that are made about the way that sex stops when marriage starts would seem to make it that the entire society understands this point.

Women, he said, have evolved to have a high sex drive when they are initially in a relationship in order to form a “pair bond” with their partner. But once this bond is sealed, a woman’s sexual appetite declines, he added.

Again, entirely willing to accept that. For those sufficiently cynical sex is what women give to men in order to gain committment and committment is what men give to women in order to get the sex. (It’s at about this point that the truly cynical would deploy Robert Heinlein’s point that only on Earth could we have a shortage of what every woman has an infininte supply of.)

However, this seems to me to be entirely wrong.

The rationale for why a woman’s sex drive declines may be down to supply and demand. If something is in infinite supply, the perceived value would drop.”

At least, put that baldly it fails. For if it was indeed just this simple supply and demand then it would apply to both men and women, would it not? Male sex drive would fall equally, as sex for the man is quite obviously in just as much infinite supply as it is for the woman.

However, we can construct a scenario where this supply and demand story might actually work. Which is that, in a stable relationship, sex is indeed infinitely avaliable to the man and thus might lose some of its perceived value. Thus the woman (not consciously you understand) will reduce supply of it to increase that perceived value.

And that does indeed provide us with a decent economic model of what is happening. That an artificial restriction of supply increases perceived value. Hey, it works for baseball cards so it’s difficult to see why it wouldn’t with sex.