A new study from the University of New Mexico found that, on average, strippers make the most money in tips during the most “fertile” days of their monthly cycles, Psychology Today reports.

Researchers also found that women who take the birth control pill make less in tips overall than women who do not take the pill, $37 an hour versus $53 an hour, respectively.

For their research, psychologist Geoffrey Miller and colleagues visited local gentlemen's clubs and counted tips made on lap dances.

Click here to read the Psychology Today study

Dancers made about $70 an hour during their peak period of fertility, versus about $35 while menstruating and $50 in between. Researchers attributed the fluctuation in tips to the changes in body odor, waist-to-hip ratio and facial features that occur throughout a woman's cycle.