"If the misery of the poor be caused not by the laws of nature, but by our institutions, great is our sin."
- Charles Darwin

Aug 10, 2007

Eye of the Beholder

Psychologists determine that eyes are the window to a woman’s desire.

As a species we are consumed by love. Ask yourself, how many cultural productions (films, stories, songs, dances, arts) do not have love, the loss of love or the absence of love as their central theme? Would you be satisfied with what is left over? Love’s power over us is just one reason why evolutionary research is so fascinating.

A well-worn trope of human culture is men’s obsession with female infidelity. Othello. Madame Bovary. Desperate Housewives. These are just three Western examples of this concern that are paralleled in every society and throughout time. Such powerful and universal human emotions suggest a biological commonality that can be revealed through a scientific lens.

Past research has demonstrated that women alter their sexual preferences based on where they are in their menstrual cycle. Women tend to be much more prone to initiate sexual encounters during their ovulatory period when sex could result in reproduction. Further, in studies based on modern Western societies, women are more likely to fantasize about having sex (as well as make those fantasies reality) with a man other than their current partner during this most fertile phase of their cycle.

Now, a study published online today in Hormones and Behavior demonstrates that this increased sexual proceptivity is apparent simply by looking into a woman’s eyes. Using 14 women (7 on birth control and 7 not) Bruno Laeng and Liv Falkenberg from the Department of Psychology at the University of Tromsø, Norway have shown that women’s pupils dilate more widely to photos of men they were sexually attracted to during their period of greatest fertility.

The researchers had the women look at a computer screen during three different periods of their menstrual cycles to view photos of individuals of sexual significance (their boyfriend or a favorite actor) as well as individuals of unknown sexual significance (their favorite actress, a Norwegian weatherman or an elderly politician). Each time the photos appeared on the screen the women’s pupil size was recorded using an infrared eye-tracking device. The authors determined that:

The findings confirmed the presence of cyclic differences in papillary diameters while watching facial portraits of sexually interesting individuals. Remarkably, the participants using contraceptive pills did not show cyclic fluctuations of pupil sizes.

The results suggest that using birth control flattens the responses by releasing higher levels of progesterone and suppressing estrogens during the fertile period. Also, testosterone, which peaks at midcycle and is directly related to sexual motivation, is suppressed in women on the pill and could negatively influence women’s sexual desire (indeed, reduced libido is one of the common side effects of taking birth control).

Of the subjects not on birth control, the study found that women’s pupils grew largest during their fertile period when viewing pictures of their boyfriend. A close second was the women’s favorite actor.

Assuming that a favorite actor represents the most likely candidate for an extra-pair sexual encounter, an increased response to such a target of interest would be entirely consistent with previous reports that women (at least, in Western societies) report to be more prone to fantasize or have sexual intercourse with a man other than their current partner during the fertile phase of the cycle.

From an evolutionary perspective this is sometimes represented as the selective advantage between dads and cads. During most of their cycle women tend to be most attracted to men who demonstrate good paternal characteristics --- tender, patient, attentive, loyal --- qualities that are often present in individuals with lower levels of testosterone. However, during their peak of ovulation, women on average are more attracted to muscular and/or assertive men who are statistically more likely to make poor long-term partners. Not coincidentally, these are also individuals with higher levels of testosterone.

The arms race between male and female sexual strategy is one of maximizing individual reproductive success. It should come as no surprise that these strategies sometimes differ. For women the strategies are to either choose a partner with phenotypes indicative of high status in order to directly benefit their child or to choose a partner who will assist in child-rearing to ensure that child’s survival. However, as primatologist Sarah Hrdy has suggested with what she terms “Female Plan B,” why not have both? Choose a man who will be a devoted partner and assist in childrearing, but then have a child with a different man who has the genetic qualities promoting high status. If the woman can get away with it, the dad will then help to raise the cad’s bastard (there was a time when that term had serious negative connotations for any children labeled as such).

This has been demonstrated in gibbons, a monogamous “lesser ape” native to Asia and the Indian subcontinent. In a study published in the journal Ethology, 12% of the otherwise monogamous females engaged in extra-pair copulations, or matings with individuals who were not their partners. This is somewhat lower than the current statistics for Western women, of whom 26 to 70 percent engage in extra-pair copulations. The differences aren’t so much important as is the fact that this is a common strategy in females of both species. Females, like males, will use flexible strategies to ensure their reproductive success, regardless of the negative stereotypes our culture places on one rather than the other.

So that flicker of delight you see in your girlfriend’s eyes while watching Ocean’s 13, the slight dilation of her pupils that seems a little bit larger than usual during a certain time of the month, that’s our evolutionary legacy playing itself out today in our living rooms. But don’t panic guys, according to Laeng and Falkenberg’s study women are still mostly interested in the man they’re with. Just make sure you continue to earn that interest.


Laeng, B. and Falkenberg, L. (2007). Women’s pupillary responses to sexually significant others during the hormonal cycle. Hormones and Behavior. doi: 10.1016/j.yhbeh.2007.07.013

Reichard, U. (1995) Extra-pair copulations in a monogamous gibbon (Hylobates lar). Ethology 100(2):99-112

Eye of the BeholderSocialTwist Tell-a-Friend


Paul said...

No surprise. As you point out, these findings are consistent with the well supported notion women (along with men) evolved more than one reproductive strategy. What puzzles me is why societies are more forgiving of the male who "cheats" than of the female who "cheats" -- a double standard that seems almost ubiquitous.

Chris Chatham said...

This is incredibly cool - I stumbled upon this via Encephalon. Yet another example of the utility of eye-based measurements (also as a measure of cognitive workload and possibly also central dopamine levels, according to various sources.)

Anonymous said...

In this 'world view' you find significance, according to what the lens of your belief allows. Finding significances of this nature - which are couched within the evolutionary paradigm - is but one way to view it all. It's an incredibly complex interwoven view, but merely a contrived one. If we cannot view reality, without using a 'lens of belief, are we not then, prisoners of our mental constructions?

Christopher & Cacilda said...

"A well-worn trope of human culture is men's obsession with female infidelity. Othello. Madame Bovary. Desperate Housewives. These are just three Western examples of this concern that are paralleled in every society and throughout time."

It never ceases to amaze me how comfortable some people are in declaring human universals: "every society and throughout time."

Verbal bravado doesn't fill the huge gaps in knowledge behind such statements. Not "every" society even acknowledges the existence of romantic love. “For the Mehinaku, the idea of romantic love is absurd. Nothing,” writes Thomas Gregor (Anxious Pleasures), “is more ridiculous to the young men who understand some Portuguese than the love songs they hear on their transistor radios.”

In this, the Mehinaku are not alone. Researchers Jankowiak and Fischer conducted a cross-cultural survey on romantic love and found that over 10% of cultures displayed no evidence for romantic love (19 of 166).

But why should reality stand in the way of promulgating "scientific" fairy tales? If Shakespeare, Flaubert, and NBC say it's universal, it must be true!

Christopher & Cacilda said...

P.S. In rereading my comment, I want to clarify that the "huge gaps in knowledge" I referred to weren't meant as a personal slight. You are obviously very well informed. My point was that we all have huge gaps in our knowledge (due to limitations of time, gaps in the fossil record, and so on), and that we should therefore be more circumspect in making such broad generalizations about "all cultures" and "throughout time."

Eric Michael Johnson said...

Christopher, your point is well taken. However, in reading through Gregor I was struck by how many social taboos are set up to prevent what he claims doesn't exist. A new couple (autsapalui) is "permitted" expressions of affection, to sleep together in the same hammock and spend most of their time together. "Those who persist risk the laughter of their comrades and supernatural dangers. Excessive thoughts about one's spouse, the villagers say, attract snakes, jaguars and deadly spirits."

This sounds more like a cultural taboo forcing out a behavior that is otherwise experienced by everyone.

But my argument wasn't about romantic love, it was about female sexual desire and male jealousy. Gregor gives multiple examples of this in his book. Girls are married soon after their first menses. "Virtually all affairs, therefore, are extramarital affairs. These are fraught with danger because husbands and wives are sexually jealous." Myths in this culture talk about a "fury of the jealous spouse" where "adulterous couples are beaten, dismembered, put to death".

And yet, young women and men still persist. This would seem to be an excellent example of just what I'm talking about. Sexual desire is an evolved trait and sexual jealousy has evolved as a counter strategy. However, I think you're right to be cautious about using such universal statements. I've been very critical of others who do the same without having first done their homework to make sure the behavior is, in fact, universal.

Christopher & Cacilda said...

Hi Eric,

I think you're correct to argue that some of what Gregor recounts can be seen as indoctrination against romantic love. In fact, such cultural constraints are common in cultures around the world.

But to your original point, re: sexual jealousy being an "evolved strategy," I think you're on less solid ground. Like most other behaviors, the potential certainly exists in the repertoire of human behavior, but I see it more as a response to specific contexts. It's an involved argument, which I'll detail in a forthcoming book, co-authored with my wife, who's an African psychiatrist. (Sex at Dawn: The Prehistoric Origins of Modern Sexuality -- HarperCollins this fall).

If you're not familiar with the Mosuo people of China, I'd be happy to send you some material. They provide a fascinating counter-example to the whole male parental investment based understanding of human sexual behavior.

Eric Michael Johnson said...

Yes, please send it on to I'm also intrigued by your book. I've been exploring these questions myself over the years. As I wrote in my post The Origins of Forbidden Love the role of male jealousy seems to vary between cultures. This could suggest that the evolved hormonal responses involved in sexual jealousy are context dependent and that environment (including cultural environment) can modify these systems to a significant degree.

My blog is currently hosted by Nature. I hope you'll stop in from time to time.