Communications and Media

The empirical article Ovulatory cycle effects on tip earnings by lap dancers economic evidence for human estrus, Myler, Tybur and Jordan, Evolution and Human Behavior 28 (2007) 375  381 examines whether human estrus was really lost during evolution. The estrus refers to estrous (or oestrous) cycle that comprises the recurring physiologic changes induced by reproductive hormones in most of mammalian females with a placental reproductive system. Humans undergo a menstrual cycle instead and it is believed by many theorists that human estrus has been lost during evolution.

The authors have used an economic model that analyzes the effects of estrus on tip earnings by lap dancers. Typically males of a mammalian species are more solicitous towards a female of the same species. The hypothesis of existence of human estrus is sought to be proved by higher earnings reported by lap dancers during the productive period corresponding to estrus in other placental mammalian species. Here the logical assumption is that an estrous lap dancer would receive more solicitations for private show increasing her tip earnings during estrus.

Design of the experiment involved multiple observations (i.e., tips per shift) for dancers who were nested within contraception use and crossed with cycle phase. The data gathered was used to analyze effects of cycle phase and contraception use on tip earnings using multilevel modeling. Result of the empirical work confirmed the authors prediction that pill using and normally cycling participants would demonstrate a similar difference in tip earnings between the menstrual and luteal phases. Second prediction that cycling participants would demonstrate a larger increase in the fertile phase relative to the other phases than pill-using participants was also confirmed by the findings. Experiment found strong ovulatory cycle effects on tip earnings moderated by whether the participants were normally cycling.

This path breaking paper provides the first direct economic evidence for the existence of estrus in contemporary human females. Real consumer spending patterns reveal human preferences more reliably than verbally stated judgments do. This is particularly true for socially stigmatized products such as pornography or sex work. The experiment involved only 18 participants which can be considered a relatively small size for an experiment with such vast conclusions. But the small size was adequately offset by an observation period of three months and a sufficiently large number of shifts for which observations were recorded. Moreover, the tip earnings of lap dancers are a result of changes in behavior pattern of a much larger number of their clients who were influenced by the estrous state of the lap dancer. When women and men interact intimately over the course of several minutes through conversation and body contact, women apparently either signal or leak cues of their fertility status, and these cues influence spending patterns by male consumers. These results argue against the view that human estrus evolved to be lost or hidden from males. Logical next step, despite its difficulties, would be further research to clarify whether women have evolved special adaptations to signal estrus through such cues  or whether the cues are leaking to sexually discriminating men as unselected side effects of cycle physiology.