And so it remains elusive...

...the evolutionary story behind female orgasm, that is. A month ago I had a post on female sexuality and the sensory cortex, where I also mentioned that my favorite theory on the evolution of female orgasm is the 'by-product' theory: "Women simply happen to share biology with men, for whom orgasm is important. It’s an accidental byproduct, like men’s nonlactating nipples." (source of the quote here) 

But now a new study was published which offers evidence against this theory -- see here and here for details. As described in the Wired article:

So Zietsch and Santtila [the authors of the new study] devised a test. They surveyed 1,803 pairs of opposite-sex twins and 2,287 pairs of same-sex twins, asking them how often and how easily they reached orgasm. If female orgasm is evolutionarily connected to male, opposite-sex twins should have similar orgasmic function.

In other words, if the by-product theory is correct, being more or less orgasmic should 'run in the family', both among male and female individuals (since it's the male biology that has been 'selected for' and bestowed to the female kin). But... that's not what came out of the study:

Instead, while orgasmic function was genetically shared in same-sex twins — brother tended to share function with brother, or sister with sister — the relationship vanished in opposite-sex twins, though both share the same amount of genetic material. The underlying genetics, and thus the underlying evolutionary pressures, thus appear to differ.

That is, the mechanisms behind the selection of female orgasm may simply not be the same as those behind male orgasm, contrary to the basic idea of the by-product theory. But then what is the evolution of female orgasm? The new study does not have a positive story to tell.

Of course, even the researchers themselves warn that their results are not definitive; but this is the first empirical study that clearly goes against the by-product theory of female orgasm, which so far was considered by many (myself included) to be the strongest contender in the market. So now it looks like we know even less than we did before; the mystery deepens...