“Ultimately, it is the desire, not the desired, that we love.” ― Friedrich Nietzsche

In recent weeks, there has been a flurry of interest surrounding the topic of female sexual desire (largely focused on heterosexual relationships), stemming largely from the publication and surrounding publicity of a book called What Do Women Want? Adventures in the Science of Female Desire by Daniel Bergner. Bergner followed the book’s publication with a widely read New York Times Magazine piece focused on the medical treatment of women with low libido.

All of this discussion about the nature of female desire immediately brought to mind my friend Kim.

Dr. Kimberly Russell holds a Ph.D. in Ecology and Evolutionary Biology from the University of Tennessee. She is currently a Visiting Associate Professor at Princeton University and a Research Scientist at New Jersey Institute of Technology, where she studies native bee communities.

Last semester, Kim taught a course called Evolution and the Behavior of the Sexes, which led us to have a series of lively discussions about how heterosexual men and women differ in libido, interests, and activity. Given all the hype surrounding these topics, I thought it would be a great opportunity to check in with her formally and share some of her ideas and reactions with you.

Below, my questions are in bold and Kim’s responses follow.

Source: courtesy of Kimberly Russell

Usually when one reads evolutionary accounts of how men and women differ in their , you hear a lot about how it is evolutionarily advantageous for men to copulate with many partners, in order to spread their genetic material as widely as possible, whereas it is evolutionarily advantageous for women to focus on a single partner who will be likely to stick around and help her raise her offspring.

However, you view the evolutionary evidence quite differently. What evolutionary pressures do you see impacting males versus females, and how does this differ from the traditional account?

Since at least the mid-90s, evolutionary biologists have known that the dichotomy of "indiscriminate males" and "choosey females" was a gross oversimplification.

In most species where scientists have looked for it, females mate with more males than are required for fertilization – this is true from insects to birds to primates. In the past, probably due to on the part of researchers, females were seen as passive participants – forced into “cheating” by persistent males. Subsequent research has shown that a female actively solicits copulation with males other than her partner and that this strategy is, in fact, adaptive. It yields benefits in how many offspring she has and how well they survive.

In addition, in some non-human animals, sex is not only about reproduction but serves a function either to maintain existing relationships in monogamously breeding species or as a social strategy to reduce .

Overall, the evolutionary benefits of females having multiple sexual partners has been, and perhaps continues to be, underestimated.

But this is all evidence from non-human animals. Even putting aside issues like culture, isn't there a lot of interspecies variation? To what extent do you think the evidence from these other species holds weight for humans?

Absolutely, no two species are exactly alike. Each has a unique evolutionary history. But what we can look for are common pressures that lead to similar outcomes across many species. For instance, for species whose offspring take a great deal of time and energy to raise, male or female abandonment would result in reproductive failure. Thus, males and females in these species forego opportunities with others in order to maximize the survival of offspring they already have.

Humans are a rather extreme example of this because human babies are extraordinarily energetically costly to raise when you consider both their relative helplessness at birth and prolonged .

The implications of this are twofold. In most monogamously breeding species that have been studied, animals adopt a mixed strategy of social without pure sexual monogamy. That is, both males and females engage in discreet copulations outside the socially monogamous relationship to defray the cost of missed mating opportunities. Secondly, because a male raising another male’s child means complete reproductive failure, the evolutionary cost of having a cheating wife is far greater than having a cheating husband.

We expect, then, to have much stronger evolutionary pressures for males to prevent female than for females to prevent male infidelity.

Let’s get back to that in a moment. First, much has been made of this study of heterosexual women experiencing arousal to viewing a variety of sexual stimuli. Can you tell us about that?

In the few studies that I am aware of that tested both physical and mental responses to visually presented sexual stimuli, heterosexual women’s bodies were physically aroused in response to most varieties, whereas heterosexual male physical arousal was more reproduction appropriate – women together and heterosexual human intercourse. This would go along with the hypothesis that women’s bodies are primed for high interest in sex due to the evolutionary benefits of having multiple, diverse partners.

A much-discussed alternative hypothesis is that women’s bodies respond in this way as a protective response to forced copulation. I don’t find this argument compelling, as it implies 1) a long history and frequency of forced copulation (unlikely based on current knowledge) and 2) that being mildly swollen and lubricated would have a significant effect on reproductive fitness in the face of such copulations, which is hard to imagine.

In this study, the women’s self-reported arousal differed a great deal from their physical arousal. They were physically aroused to all of these stimuli, but reported only being turned on by the images of heterosexual sex. Why do you think this was the case?

Ah, this is the interesting part!

As I said before, there is intense evolutionary pressure for males to control paternity, particularly in monogamously breeding species. A variety of strategies are used to combat female infidelity in animals – mate-guarding being the most common.

Although anthropology is not my area, it seems to me that from an evolutionary perspective, culture gives humans the unique opportunity to take advantage of a different strategy. If you can effectively intervene with cultural rules and expectations and thus control the process involved in female choice, i.e., control whether desire leads to sex, you essentially get control of reproduction. And who controls paternity controls the world!

Put another way, if you have a culture that convinces women that 1) they are less interested in sex than men and 2) they are more interested in monogamy, then you create a situation whereby women learn to ignore or disregard their own physical arousal, particularly in situations that are deemed inappropriate.

Of course, other cultural mechanisms work to reinforce this through slut-shaming and even physical , but surely the psychological strategy would be the most effective because women internalize it so completely.

Let me try to sum this up. You are saying that there may be evolutionary pressure for women to be more interested in frequent and varied sexual encounters than men, and the fact that we see opposite trends in self-report and behavior may have more to do with culture than biology?

Actually, I wouldn’t necessarily say more frequent sex than what men want or even more partners per se, but rather that women evolutionarily would have had nearly as much to gain from multiple partners as men, so desire for sex is probably much more similar than previously thought.

What seems to be different is that while men in long-term monogamous relationships report that they long for variety, their interest in sex (with their partner) remains roughly constant – they still want sex with their wives/partners. Women, however, appear to lose interest in sex in monogamous partnerships. In the past, this was interpreted as confirmation of women’s lower libido and the result of having already “gotten what they wanted” = marriage.

In light of recent research, it seems that women have not lost interest in sex; they have just lost interest in sex with their long-term partners. They carry the evolutionary baggage of a time when seeking new partners would have given them a fitness advantage, especially as they approach the end of their reproductive lives.

But if men and women are under roughly equal pressure to be attracted to extra-monogamous sex, why this dichotomy? Why are women losing sexual interest in their long-term partners but not men?

This is a bit tricky, but I would argue this a consequence of differences in the basic sexual control mechanisms between men and women. Males still show the hallmarks of their indiscriminate past, i.e., the drive was to mate and the choice of whom with was less important. Females have a long history of choice, such that they developed mechanisms to evaluate the quality of their partners and to adjust their level of desire accordingly. If, at some point in their lifespan, quality equals diversity, then they stop responding to their long-term partner and need other stimulation to become sexually interested.

But *how* do these evolutionary pressures impact us now? What do you say to the reader who accepts everything you are saying about other species and even about our past hominid ancestors but just doesn't understand how these historical pressures could impact us in our contemporary lives?

Just as we bring emotional baggage to a new relationship that results in actions or reactions that have little to do with our new partner and everything to do with the previous one, species carry evolutionary baggage that results in behavior that reflects past environmental conditions and selection pressures.

For example, our strong desire for fat and sugar and our willingness to overconsume these when available would have been enormously beneficial to our ancestors who lived in an environment where these resources were rare. Now that they are common, we continue to have the desire to overconsume and find it difficult to stop ourselves from indulging especially when the items are in full view, despite the fact that doing so no longer carries an evolutionary benefit and may even carry a cost.

Why? Because our bodies respond to these foods in a preferential way — they taste good to us, they result in reward processing in the brain, we crave them when we don't have them. Similarly, our motivational systems — at the levels of , brain processing, biases in behavior — may be set up to motivate us to approach new sexual partners. You don't consciously decide to be attracted to your new neighbor, but he draws your , your heart speeds up and skin flushes when your eyes meet, and you experience an intense thrill of pleasure interacting with him that drives you to interact again and again.

Let’s end on an note, shall we?

The point of all this is not to say that both men and women are frustrated with sexual monogamy, and so let’s just forget about marriage and other long-term partnerships because sexual monogamy "isn't natural." It is a ridiculous and flawed argument to say that just because something is "natural" that it is automatically "good." Infanticide is natural! Our desire for fat and sugar and consequent weight gain is natural!

As humans, we have an obvious and strong desire for long-term, steady partners and this is as much a part of our evolutionary history as the desire to engage in extra-partner sex. Understanding our evolutionary baggage and acknowledging that females evolved to be very sexual allows us to approach sexual dissatisfaction in a new way. We need to understand desire to be able to stimulate it.

I hope that this new perspective will lead to more sexually satisfying long-term relationships for both men and women.

Thank you, Kim!

Note: Bergner’s work and some of the studies he describes have been criticized by several well-regarded science bloggers, in particular for drawing too many conclusions from studies limited in their methodology, oversimplification of neurotransmitter function, and medicalization of sexual boredom within marriage. Bergner himself is soliciting people’s ideas for how to increase female satisfaction within monogamous relationships here.