Ask anyone who's spent any time in a strip club, and one of the things he will almost certainly not mention is the ovulatory state of his favorite gal. But, according to a recent paper by Geoffrey Miller et. al., how much money he spent on her may have more to do with where she is in her cycle than he'd comfortably acknowledge.

Miller and his co-authors set out to see if they could find any economic evidence for human estrus, a period of increased sexual attractiveness, receptivity and proceptivity occuring around the time of peak fertility. The prevailing consensus is that human estrus has been mostly (if not entirely) lost through the years of human evolution, possibly as a way to encourage men to stick around and provide even when women are not likely to conceive, or alternatively to obscure paterntiy. Over the last decade or so, it has become trendy to search for evidence that human estrus is alive and kicking, that women do indeed somehow signal to men when they are most likely to conceive, and that men receive these signals and find fertile women more attractive. This paper is (to my knowledge) the first one to look for economic evidence of estrous signalling. It's short, and worth skimming for the background section describing the gentlemen's club scene alone.

For those of you who have never seen an episode of "The Sopranos", a lapdance goes something like this:

In each lap dance, the male patron sits on a chair or couch, fully clothed, with his hands at his sides; he is typically not allowed to touch the dancer. The topless female dancer sits on the man's lap, either facing away from him (to display her buttocks, back, and hair) or facing him (either leaning back to display her breasts, and to make conversation and eye contact, or leaning forward to whisper in his ear). Lap dances typically entail intense rhythmic contact between the female pelvis and the clothed male penis (emphasis mine. Citations removed, and there were actually two of them).

They got 18 women to voluteer for this study, having them provide information about tips earned nightly and report when they started and stopped menstruating over a 60-day period. They also collected information about birth control, and compared women on and off the pill to see if there were any significant differences in tip income between the two groups.

The researchers found that lapdancers did indeed earn more in tips when they were close to ovulation (about $350 per shift for estrous women versus $260 during the luteal phase and $180 during menstruation), and that normally cycling women earned more than women on the pill (averages of $276 per shift versus $193). They take this as evidence that women somehow signal their fertility to men, who in turn find women more attractive when they are most likely to conceive. They explain the low earnings by women on hormonal contraception by describing the hormonal effects of pill use as mimicking early pregnancy, which is decidedly not hot if you're subconsciously looking to impregnate the woman in question.

We found strong ovulatory cycle effects on tip earnings, moderated by whether the participants were normally cycling. All women made less money during their menstrual periods, whether they were on the pill or not. However, the normally cycling women made much more money during estrus (about US$354 per shift)--about US$90 more than during the luteal phase and about US$170 more than during the menstrual phase. Estrous women made about US$70 per hour, luteal women made about US$50 per hour, and menstruating women made about US$35 per hour. By contrast, the pill users had no midcycle peak in tip earnings. As in other previous research, the pill eliminates peak fertility effects on the female body and behavior by putting the body in a state of hormonal pseudopregnancy. This also results in pill users making only US$193 per shift compared to normally cycling women making US$276 per shift--a loss of more than US $80 per shift.

The take-home message that Miller et. al. want us to absorb is that human females do indeed signal when they are most fertile, a signal that men receive and correctly interpret. Invoking the "doctrine of revealed preference," the researchers suggest that men, using subtle visual and pheromonal cues, find women most attractive when they are close to ovulation, and reward them for that attractiveness via tips.

Now, I went back and forth for a while on how to write this study up. A big part of me wants to treat it as mental candy, write something like, "Hey look! It's science about strippers!" and let it go at that. But this is exactly the sort of paper that makes for great sound bites in the news (it's already starting to make the rounds in papers and magazines of varying journalistic integrity). And sure enough, not too long from now it will be treated as common knowledge that women wear their fertility on their sleeve (literally, in some cases, as other research has shown that women dress more provocatively around ovulation), and men respond.

I don't usually let my feminist flag fly, but it seems obvious to me that that if this study had been conducted by three women instead of three men, we would be looking at a very different set of results and a veeeeery different set of conclusions. First of all, 18 women does not a reliable sample make. The 95% confidence intervals for the findings on earnings at different points in the cycle are in every case larger than the earnings differences between the phases. Also, the authors go to the trouble of collecting information on "age, ethnicity, work experience, sexual experience and attitudes, menstrual cycle characteristics, contraception use, physical characteristics, education, intelligence, and personality" and ask the women to report their mood as well as their earnings, but then don't seem to use any of that information when looking at their results. The mood reporting is especially crucial. Anyone who has been intimate enough with a woman to be aware of her cycle shouldn't be surprised that the week when she's riding the cotton pony isn't likely to be her sexiest.

I don't really have a problem with the claim that women feel sexier around the time of ovulation; in fact I think that's pretty well established. And likewise I have no problem with the claim that women don't feel quite so sexy when Aunt Flo is in town. I think the authors really missed out on a prime opportunity for some field work here. They could have made like urban Dian Fosseys of vice, concealing themselves in the shadowy corners of gentlemen's clubs and observing the behavior of the women employed there. Could it be that women who feel sexier give more energetic, arousing performances than women who feel sluggish and irritated? Even professional performers can get more or less "into" the performance depending on how they're feeling. If the authors had said, "women behave differently at different points in their ovulatory cycles, and men respond favorably to confident, flirtatious behavior", I'd have nothing to argue with. I just really don't see anything here to indicate that men tip more based on increased "soft-tissue body symmetry" during estrus.

Similarly, the differences found in earnings between women who are and aren't on hormonal contraception strikes me as suspicious. Do we really believe that women who are on the pill give off pregnant pheromones that turn men off? I think this is a place where the demographic information collected could have been enlightening. There are any number of stories that can be told to explain why lapdancers on the pill earn less. Maybe women on the pill are more highly educated than women who aren't on it, and more educated women are less likely to enjoy their stripping job. Maybe older women are both more likely to be on the pill and less likely to be highly tipped than younger women. Or maybe women on the pill are more likely to have a long-term boyfriend, and therefore less likely to be able to credibly fake sexual interest in whoever they're grinding on at the moment.

All this having been said, the topic is very interesting, and I would definitely take another look if they got a larger sample size and controlled for demographic characteristics and differences in behavior. As it is, though, I think this paper tells us much more about men, and especially men in academia, than it does about women.