And they call economics the dismal science...

MILLER, G., TYBUR, J., & JORDAN, B. (2007). Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?☆☆ Evolution and Human Behavior, 28 (6), 375-381 DOI: 10.1016/j.evolhumbehav.2007.06.002

Sometimes, science reporters find a really interesting paper that you wouldn't have known about and you think, "Okay, I have to read that." This was one. The issue of human estrus is interesting to start with, and when you throw lap dancers into the title, well, that's paper that warrants a trip to the library. I took a risk and suggested it for our journal club.

So did the paper get attention for the obvious prurient reasons, or is it good science? Some of the other participants in journal club were very down on it, and didn't think there was much to be concluded from it.

Humans are definitely unusual in that it is not obvious when females are ovulating, In many mammals, including many primates, it's blazingly obvious when a female is fertile, and this is the only time females are receptive to mating. I've heard sort of "science legends" that humans and dolphins are about the only mammals that don't have obvious estrus cycles. To my surprise, it seems to be true that dolphins don't show pronounced behavioural changes with fertility. I don't know whether there are other mammals that show very reduced estrus or not.

Since the whole idea of estrus is to indicate readiness for mating, it follows that if human females show some subtle signs of when they are fertile, males should consider fertile female more attractive. Problem is, it's really hard to get people to talk about what turns them on to a complete stranger -- like a researcher.

The idea here is simple: people vote with their dollars.

The authors of this paper contacted exotic dancers indirectly and set up a website where the study participants could enter in their data. And here's one of the worries about this paper: they had a small sample size, with only 18 participants.

The subjects were asked basic demographic information, but the key pieces of information were how much money did they make dancing each night, where were they in their menstrual cycle, and were they on the pill?

And here's the second worry. The subjects didn't log in as often as they were asked to (i.e., daily). Over 70% of the data were missing. That's a lot of missing data.

Nevertheless, the researchers found big swings in earnings over the course of the month, but not for dancers who were on the pill (which suppresses ovulation). Dancers who were not on the pill earned over 80% more during their estimated fertile period.

The effect is large, so I'm inclined to believe that the effect is real. In fact, it's almost puzzling that the dancers hadn't learned that they can make more money being off the pill (say).

The interpretation of what's causing that effect is more more difficult, however. In particular, is there any cue above and beyond some behavioural differences? Some manner of pheremone? Can those behavioural differences be identified? (I can just imagine being the ethics reviewer on that proposal...)

The third worry is that self-report data is always tricky, particularly when the key question is, "When were subjects ovulating?" This is not easy for anyone to determine, as the low success rate of using the rhythm method of contraception testifies. Testing this rigourously would require regular blood samples to test for hormone levels.

Assuming the effect is real, one way to test whether is is specifically related to estrus would be to look at tipping in a non-sexual context. For example, waitress's tips. In some ways, probably easier in that you'd probably be more likely to be able to find more waitresses who were willing to participate. In some ways, it might be harder, because the tips would probably be lower and it would be harder to see any effect. But if waitress's tips varied over the course of the month, it would suggest that whatever is going on may be more related to general feelings well-being than estrus as such.

Regardless of its limitations, this paper should be a leading candidate for an Ig Nobel prize for Economics this year.