Female Orgasm and the Logic of Evolutionary Hypotheses

Is the female orgasm an adaptation, or a byproduct?

As a matter of scientific fact, about 20% of women reliably has orgasm with intercourse, though 90% overall report having an orgasm at some point in their life. These conclusions have emerged from some 37 studies conducted over the years on 148,346 women, and are part of an intriguing argument on the logic of evolutionary explanations laid out in a paper by Elisabeth A. Lloyd, of Indiana University [1].

Lloyd's paper concerns a long standing discussion in evolutionary biology, in which philosophers of science have often been involved. The debate has been between those who think that adaptive explanations by way of natural selection should be the default mode of thinking in evolutionary biology (i.e., assume an organismal trait or behavior is an adaptation, unless you can demonstrate it isn't), and those who propose that biologists should be pluralistic about explanations, considering multiple hypotheses (adaptive and non-adaptive) simultaneusly, weighing the evidence accordingly.

The famous, and highly influential, evolutionary biologist Ernst Mayr explain the adaptationist approach this way: "When one attempts to explain the features of something that is the product of evolution ... [one] must first attempt to explain biological phenomena and processes as the product of natural selection. Only after all attempts to do so have failed, is he justified in designating the unexplained residue tentatively as a product of chance." This, according to Mayr, because it is almost impossible to prove that a trait has evolved by non-adaptive processes, while it is always possible to test adaptive scenarios.

Not so fast, wrote Stephen J. Gould (a paleontologist) and Richard Lewontin (a geneticist) in a famous paper published in 1979: "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" [2]. They pointed out that adaptive hypotheses are actually very difficult to test, and far too easy to make up. They warned their colleagues about the danger of making up "just-so stories" about the adaptiveness of a trait and then, should one such story be in conflict with the evidence, simply move to the next story, without ever seriously exploring the possibility that some characters may not be adaptive [3].

To give you a fairly uncontroversial example -- one that Lloyd herself brings up in her paper -- consider nipples: no evolutionary biologist would argue against the proposition that female nipples have adaptive value. In particular, they serve both to lactate the infant child (natural selection) and as secondary sexual characters to attract males (under what Darwin called sexual selection).

But what about male nipples? Clearly, they don't serve any lactation function, and their role as secondary sexual characters is highly debatable to say the least. Indeed, the best explanation for the existence of male nipples is that they are a developmental byproduct of their female counterpart: the genes to make nipples are present in both males and female, and are expressed in both. But because male nipples don't serve any reproductive or survival function, they are pared down, so to speak, during development, with the result we are all familiar with.

The story of the male and female nipples illustrates what Lloyd calls the difference between "methodological adaptation" and the "evolutionary framework" ways of approaching questions in biology. The first is often deployed by behavioral ecologists and evolutionary psychologists and, echoing Mayr above, begins by posing the question: ‘‘What adaptive explanation can account for this trait?’’ The second one, by contrast, asks ‘‘What evolutionary factors account for the form and distribution of this trait?’’ In the first approach, adaptive explanations are treated as default, with anything else to be considered only as a matter of last resort (if at all). The second approach, instead, includes adaptation by natural selection as one possible (sometimes even most likely) explanation for a trait, but also explicitly takes into account other possibilities.

Lloyd claims that the evolutionary framework is a better way of doing science than methodological adaptation, and I agree. To see why, we need to go back to female orgasms.

The most sensible explanation of female orgasms is, in a sense, the reverse of the one we have just seen for male nipples. As Lloyd and others have painstakingly shown over the years, not only there is no evidence whatsoever of any effect of the ability to have orgams on reprocutive fitness (yes, people have looked), there is also plenty of positive evidence to think that women have orgams -- biologically speaking -- courtesy of the developmental and neurobiological apparatus that allows males to have that experience.

(For males the most likely story is adaptive: given the cost of seeking a sexual partner and engaging in sex, rather than, say, looking for food and shielding oneself from predators, there had to be a strong incentive in place so that males would divert time and forgo resources in order to have sex. That incentive is the acute pleasure of orgasms.)

This isn't a made up story. The evidence comes from the well known fact that the tissues that allow both females and males to have orgasms are what biologists call homologoues: they have a common evolutionary origin. Males and females have the same nerve tissues, erectile tissues and muscle fibers, which is accounted for by a common inheritance over the generations. Moreover, developmentally it has been shown that female orgasm relies on the same biological apparatus as its male counterpart. As Lloyd puts it: "females get the functioning orgasmic tissues, and are often capable of having orgasms under the right conditions of rhythmic stimulation. Female orgasm is seen, technically, as a byproduct or bonus of selection on male orgasm."

What's the problem, then? That a number of evolutionary psychologists just can't seem to be able to accept the byproduct explanation, because there has to be an adaptive one. As Andrews and colleagues (quoted by Lloyd) put it: "demonstrating that the female clitoris and orgasm are byproducts requires the failure to find evidence for its special design and, hence, an adaptationist testing strategy."

But in fact people have looked for evidence supporting adaptive explanations for the female orgasm and have just not found it. For instance, an analysis of 8,000 women investigating the possible existence of a genetic correlation between occurrence of orgasms and number of offspring has utterly failed, which means that there is no reason at all to believe that female orgasms are, in fact, adaptive. (Another way to put this is that women who have orgasms have no fitness advantage over those who don't, though presumably they have more fun.)

This is were the contrast between methodological adaptationism and the evolutionary framework becomes stark and relevant: the first one devolves (pun intended) into a never ending quest for an answer that is assumed a priori, not exactly the best scientific practice ever devided. The second one, by contrast, takes seriously -- gives initial precedence to, even -- the idea that lots of organismal characteristics appear to have a function and are therefore likely the result of natural selection, but at the same time considers alternative hypotheses and weighs the overall evidence. In this case, the balance of that evidence seems clear: female orgasms are a byproduct of males ones, just like male nipples are a byproduct of female ones.


[1] Adaptationism and the Logic of Research Questions: How to Think Clearly About Evolutionary Causes, by E.A. Lloyd, Biological Theory, published online on 9 July 2015.

[2] Spandrels are architectonic features of certain buildings, like churches, with archs that meet at the square base on which a dome is erected. Spandrels are painted with scenes that fit their unusual shape perfectly, giving the impression that the spandrels themselves were put in place on purpose to showcase the painting. But in fact spandrels are inevitable byproducts of the architeture of the building, and they constraint how the artist works to fill them. Analogously, argued Gould and Lewontin, sometimes organismal characteristics appear to be well crafted by natural selection, but in reality are the byproducts of developmental and genetic processes that have to take place anyway.

[3] My colleague Jonathan Kaplan and I wrote a commentary on the Spandrels paper a few years back, it can be downloaded here.