This is an interesting study, in a “Whoa! Somebody actually thought to do a study of that” kind of way. And that’s saying something, coming from someone who studies discourses of menstruation. Two economists designed a study to try to demonstrate that women bid differently in auctions depending on the phase of their menstrual cycle. They found that women bid significantly higher than men in their menstrual and premenstrual phases, but do not bid significantly differently in other phases of the menstrual cycle. They extrapolate from this that women are greater risk-takers during the fertile phase of their cycle.

The detailed statistical modeling and analysis is beyond my expertise as a humanities scholar, but I find the underlying premises of the study and its conclusions problematic. First, they assess which phase of her cycle their research participants are in by self-report and the assumption of a 28-day cycle: “We distinguish the menstrual phase (days 1 to 5), the follicular phase (days 6 to 13), the peri-ovulatory phase (days 14 to 15), the luteal phase (days 16 to 23), and the premenstrual phase (days 24 to 28).” As my colleagues at CeMCOR will tell you, one cannot assess ovulation merely from self-report of date of last menstrual period and projected date of next period. Regular menstruation ≠ ovulation. And pretty much any menstruator can tell you that the average 28-day cycle is just that, an average. The researchers also noted that 15% of their participants used hormonal birth control, but

Specifications (5), (10), (13), (16), (19), and (22) reveal that our results remain robust when controlling for hormonal methods of birth control. Hormonal methods of birth control do not have a significant effect on bids.

Since hormonal contraceptives (such as the birth control pill, ring, or patch) suppress the hypothalamic-pituitary axis, these women do not ovulate or experience the five phases of the cycle the researchers enumerate.

Additionally, the participants in this study appear to be students at UC Davis; college-aged women often have irregular, anovulatory cycles due to their youth and other factors, such as poor diet, stress, and irregular sleep patterns. Even age at menarche (the first menstrual period) affects whether a young woman ovulates regularly.One must always be suspect of sweeping generalizations from a such small, elite sample.

And I find the whole evo-psych approach and conclusions troubling. While I cannot deny that hormones exert an influence on behavior, I think it takes an awful lot of inferential leaps to get from the data in this study that “that women bid significantly higher than men in their menstrual and premenstrual phase but do not bid significantly different in other phases of the menstrual cycle” to “an evolutionary hypothesis according to which women are genetically predisposed by hormones to generally behave more riskily during their fertile phase of their menstrual cycle in order to increase the probability of conception, quality of offspring, and genetic variety.” So many factors could influence the higher bids that it’s an extraordinary reach to attribute it to phases of the menstrual cycle, especially given the unscientific way the cycles were charted.