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.
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You are already trying to answer the more important and more difficult question about why do they make more money in that phase. I agree that women might be better at designing the research for that question.
Regards, Dr Shock (male)
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.
That's not true for normally cycling women, only those on the pill. Fig 2 shows that the error bars for normal women are roughly $90 for each of the three phases, but the diff in means between menstrual and fertile is about $180, and between luteal and fertile about $90.
Moreover, for normal cyclers, the error bars for menstrual and luteal overlap quite a bit, but that of the fertile phase does not overlap either of them at all. So, for normally cycling women, the fertile phase really does stand out.
I just really don't see anything here to indicate that men tip more based on increased "soft-tissue body symmetry" during estrus.
They authors don't claim that this is the cause -- in fact, they explicitly say (p.6):
A final limitation is that our study did not identify the precise proximal mechanisms that influence tip earnings. These might include the previously documented shifts in body scent, facial attractiveness, soft-tissue body symmetry, waist-to-hip ratio, and verbal creativity and fluencyor they might include shifts in other phenotypic cues that have not yet been studied.
You're also being uncharitable with this:
Do we really believe that women who are on the pill give off pregnant pheromones that turn men off?
The authors made no such claim about pheromones, remaining agnostic on what the mechanism is.
If you've read some ridiculous interpretations of the study on blogs, etc., that's fine -- quote and criticize those. But the article itself is not what you're making it out to be.
Why do you assume that the study had biased results because it was performed by men? It is ironic that you make the exact same mistakes that you accuse the authors of this paper of in your criticism of it. There are any number of reasons that men could desire to get headline grabbing, career boosting results, few of which have anything to do with sexist wish fulfilment, and no reason to think women won't be just as vulnerable to them. You want to watch out - your agenda is a little obvious.
Pot, kettle, black, and all that.
The researchers made a huge leap to form those conclusions from those data.
The simplest explanation is the most likely. It's not that the men somehow know the women are fertile. It's that the women perform more energetically when they don't feel horrible with PMS, etc.
But naturally these male researchers focused on male motivations.
I also agree with you about the concerns in this research. The fact that three men wanted to gather evidence at a strip club doesn't look so great, even though I'm sure their motivations were purely scientific. I would have also liked to see this question asked from a female researcher's perspective. However, I have never fully trusted the "hidden ovulation" interpretation. Just because humans don't develop large estrus swellings like some other primates shouldn't suggest that we're somehow different from all other mammals. In fact, I think estrus signaling would actually be evidence that female sexuality has been a positive force in human evolution (something that has been adamantly denied by some evolutionary psychologists). The women in this study weren't displaying attraction for their customers, but it's easy to see how signaling ovulation would be a powerful enhancement of reproductive success.
As I wrote in my post on this topic (see Evolutionary Lap Dance):
What I don't get is that it should be common knowledge among lapdancers that they earn less when they are menstruating. Such differences would automatically detected by anyone who works for these tips. So may be it's a statistical mistake.
If you read the paper you'll see that while during the menstrual stage dancers received the lowest amount of tips, the luteal phase also had significantly less than the fertile phase. I think the menstrual phase was just included for comparison. The interesting distinction is between luteal and fertile. The pill-using dancers showed no mid-cycle peak in tip earnings.
What I don't get is that it should be common knowledge among lapdancers that they earn less when they are menstruating. Such differences would automatically detected by anyone who works for these tips. So may be it's a statistical mistake.
Oh for crying out loud people, read the damn paper. It's only 6 pages, and you don't even have to know how to count to understand it. On p.5:
These cycle effects are notable because in previous research on gentlemen's clubs (e.g., Barton, 2006; Deshotels & Forsyth, 2006; Pasko, 2002), summarizing thousands of hours of interviews, dancers are never reported as noticing cycle effects on tip earnings (thus making it less likely that participant expectations or demand characteristics can explain the results).
Why they don't notice is an interesting question, but that they don't notice is not really debateable. You're assuming that, like the narrow group who surrounds you, everyone must have scored above 1400 on the SAT or whatever. In the real world, some people aren't bright at all and don't notice things you'd quickly detect.
Again, the fertile error bar is strictly above both of the other bars -- menstrual as well as luteal; it's not just a menstrual vs. fertile contrast. So cramps have nothing to do with it. Additionally, they cite a reference to the effect that dance moves don't vary much by cycle phase.
Agnosic: the mechanism does matter. The authors certainly do present their results as indicative of male detection of estrus. But unfortunately their empirical findings cannot support this. They wouldn't have been published without that disclaimer, but they're not exactly selling an unbiased story. (The disclaimer itself is the stuff of phdcomics.com...) In my view, the only real result of the paper is that the earnings of normally ovulating lapdancers are cyclic. That's what might be called `no shit' science. No other parts of their story are convincing.
Kara's point is that there is a very obvious mechanism for the cycling tips, ie, mood. About half way though reading, I in fact checked the authors' names (I'm a guy BTW); I agree that female authors probably wouldn't have been so blase about ignoring it (in contrast to Vavanth's comment).
Basically, the only thing I can take away from this paper is a result about women -- not men. Maybe I want a cheerful flirty girl to dance for me, and those who are ovulating provide this. Maybe I don't detect ovulation at all. But Miller et al. certainly write up their paper implying I can.
Their one sensible conclusion is that lapdancers might schedule their shifts for their fertile period. But as Markss says, why isn't this observed already, in light of the large magnitude of fluctuations?
This is a silly objection. Mammals don't detect ovulation, they detect signals of ovulation. If women are significantly more likely to be cheerful and flirty when they're ovulating, and men prefer cheerful and flirty women, being cheerful and flirty is a signal of ovulation - not necessarily the most reliable signal, but a signal nevertheless.
I seem to have gotten myself in a bit of trouble for first saying I wouldn't make light of the paper and then keeping the tone largely glib for the remainder of my post.
Agnostic, you are absolutely right about the confidence intervals. I checked again, and I was looking at the wrong stats in the text while writing up. Figure 2 does show the relationship between the phases.
Eric, your point about the interesting difference being the one between the luteal and fertile phases is taken, but I still don't think the authors have demonstrated what they want to. I assert that mood could account for all the results, for example both the increase at estrus over a luteal baseline and the decrease at menstruation below a luteal baseline.
I do not deny that human estrus exists. There is enough controlled behavioral evidence in humans (photo studies, wardrobe studies, etc) to suggest that something is going on at peak fertility. In fact I think it would be quite strange if human females did not retain some way of demonstrating when they are most fertile. But it is one thing to say that women somehow convey their ovulatory state, say through provocative dressing and flirtation, in ways that men then pick up on behaviorally, and quite another thing to say that, regardless of what women do, men pick up on involuntary cues.
This is exactly what the authors suggest, and this is where I will respond to the criticism above. Regarding their disclaimer: rather than confirming their scientific objectivity, it is a necessary nod to the fact that they cannot, given their statistical findings, claim to have found anything more significant than a regularity in income data. When reading the paper as a whole, it is obvious from the title, the introduction, and the discussion, that the story put forward for this regularity is the detection (by men) of female ovulation. It is intellectually dishonest of the authors to so strongly imply a particular mechanism, and then weasel out of the fact that their data in no way indicates that mechanism by inserting a token disclaimer. I grant that research in general involves drawing conclusions from incomplete data. But this is too much.
i think the larger point of this study IS a good one. the sexuality and reproductive capacity of primates (non-human or not) is amazingly divorced from strict hormonal regulation, unlike in most other species. many small mammals have imperforate sex organs that can only be penetrated given a particular hormonal milieu. if human sexuality has become increasingly detached from hormonal regulation (i.e. it can happen anytime and irregardless of cycle stage), the study presents another way in which hormonally regulated changes may substantially affect the 1. proceptivity of females (e.g. dress, solicitations) and 2. locomotor ability/coordination of females across the menstrual cycle. which, despite their face-value "silly science" value, are actually interesting little contributions to behavioral endocrinology lit.
in the writeup in Science, co-author Grammer states that he readily admits that the difference is attributable to changes in the female, not the male. As humans lack vomeronasal organs and, especially given the study's locations (smoky, dark, smell strip clubs), i doubt that anyone could justifiedly claim that males were responsive to bitty little pheromonal cues emitted by the females. i didn't find the paper misleading, but it could have been my (unwitting) selective reading...
Kara,
Would you mind changing the date of this post to OCTOBER 13, 2007?
Kara,
Please change the date of this post to the day on October you meant it posted! It's killing my RSS reader.
Thanks,
Jeb, FCD
Very valid comments, all kinds of alarm bells were ringing as I read the study- the conclusions seems pretty shaky based on the evidence they provide. Interesting all the same though.