The female orgasm: a story about sex and questions about natural selection

It's an Epipactus gigantea, from a spring in the Mojave Desert.

In general, science writers love to tell stories about the quirks of evolution, the strange ways in which Darwinian natural selection has created the living world around us. And there are tons of great stories to tell.  The danger, however, lies in assuming that natural selection has created every interesting trait we find in the living world.  It can be too easy to theorize adaptive advantages and imagine evolutionary stories about how traits could have increased the reproductive success of those who possess it.

My favorite un-adaptive story comes from Elisabeth Lloyd’s brilliant book, The Case for the Female Orgasm: Bias in the Science of Evolution, published in 2006.  She writes about two of my favorite things: sex and science. But this is real science, a serious meta-analysis, not pop-sci sex fluff (which can be fun too sometimes, don’t get me wrong) But Lloyd’s book is fascinating. In it, she chronicles 21 different accounts scholars have proposed for the reproductive advantages for the female orgasm.  The important story Lloyd tells isn’t the 21 accounts that she breaks down, instead the bias in evolutionary explanations she exposes steal the show.

At least two main biases have negatively affected the evolutionary explanations of the female orgasm. One of them is the bias of assuming that the female orgasm evolved to its present form in human beings because it contributed to the reproductive success of it’s possessor in some way. It may seem to be obvious that the female orgasm has an evolutionary function, but the obviousness of this conclusion must be reevaluated after looking at the relevant evidence. It turns out that no one has ever adequately shown a function for the female orgasm in increasing either fertility or reproductive success. The second bias is that of being male-centered (androcentric) or of assuming that female sexuality is like male sexuality. There have been various trends over the years away from or towards the view that men and women have similar sexual responses, but, I shall argue, the overwhelming evidence is that women respond differently to the act of intercourse than men do.

Lloyd starts with the scientific understanding of the female orgasm, and the research it took to reach our current understanding, which is fascinating all on its own.  She then outlines the major themes in female orgasm evolutionary theory. Orgasm might encourages pair bonding, which might enhance reproductive success. Or that the physical contractions of orgasm might help move sperm up toward the egg waiting in the wings, which is literally called the “upsuck hypothesis.”  Or that sexual pleasure might have encouraged promiscuous behavior, which could have lead to increased reproductive success.

This is Krameria grayii, also from the Mojave.

The book makes for great reading because of Lloyd’s detailed eye on sex research. You have to feel bad for the graduate students involved in studies of primates, equipped with sensors to monitor vaginal contractions, that were stimulated to orgasm by hand. Or admire the  bravery of people who agree to orgasm for science? Researchers (admittedly, mostly men) made the same mistakes, over and over, assuming that women typically orgasm during intercourse.  Instead, data on female sexual pleasure, (and casual conversations with friends) will show that intercourse does not always lead to orgasm. In an analysis of 25 studies (1921-1995) on reported sexual behavior, between 23 to 75 percent  of women reported that they usually or regularly experience orgasm during intercourse, with a mean and median of 55 percent.

In every adaptive argument, Lloyd identifies the assumptions that stand without evidence. No one has found direct links between female orgasm and fertility, birth rates, or reproductive success. It definitely isn’t bound to intercourse, and women don’t need to orgasm to reproduce, the way men typically do. Humans aren’t the only species where the females have orgasms, and, there is no solid evidence for the “upsuck” motion.

Instead of searching for another plausible adaptive theory, Lloyd suggests that the female orgasm may exist as a byproduct of the selection pressure for male orgasm.  Consider how men have continue to have nipples that they never use for reproductive success.  Men have nipples because they share embryological development with females, whose nipples are adaptive. The penis and the clitoris start as the same organ in an undifferentiated embryo, until at about 8 weeks when, in males, the hormones that lead to developing into a male embryo are released. “The male and female orgasms are remarkably similar,” Lloyd writes, explaining the studies that have shown the physiological commonalities, in muscle contraction and nerve response. Because there is a strong selection effect for male orgasm, to deliver sperm for reproduction, females continue to develop comprable sexual tissues.

This one's Opuntia phaeacantha

However, this story of non-adaption is not meant to down-grade the status of the female orgasm. Llyod writes that “female orgasm may be both advantageous to the female and very rewarding: this still does not mean it is evolutionarily advantageous.” She argues  that we need to acknowledge a separation between our understanding of evolutionarily advantageous and currently important. The female orgasm (obviously!) is important socially, culturally, and personally, but the importance does not have to arise from evolution theory. Having an orgasm releases dopamine in the brain, which is associated with all kinds of healthy, happy feelings. Neuroscientists using orgasm to study how the brain process pleasure released this remarkable video of the female brain experiencing an orgasm last week. Very cool, and very important.

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