Geoffrey Miller, Joshua M. Tybur, Brent D. Jordan of the University of New Mexico have done a wonderful study to see whether human beings can tell when a woman is ovulating. Unlike most animals, people have "hidden estrus." One good consequence is that men pay attention to their mates all the time, whereas in other primates females only get attention when they are "in heat." However, human beings may be able to tell when a woman is ovulating in more subtle, even subconscious, ways.
Miller and colleagues asked eighteen lap dancers to keep track of their menstrual periods, work shifts, and tip earnings for 60 days - more than 5300 lap dances. The researchers noted differences between women who were on the pill and those who were not.
Average tips during ovulation = $335 per shift
Halfway through (luteal phase) = $260
During menstruation = $185
Women on the pill, by contrast, showed no ovulation peak for tips.
[Thanks to reader Brittany for the study]