An evolutionary psychologist at the University of New Mexico and his research conducted a study that showed an interesting, but very obscure fact. Strippers earned more tips by lap dancing when they were ovulating than when they were not, and also more than women on the pill, who don't ovulate.
The research assistant first noticed this because he worked at a strip club assisting the performers. One of his duties was handing out tampons. He noticed women who requested them earned less than the others. This was the basis for their study.
They set up an anonymous website where they gathered data for 296 work shifts, or about 5,300 lap dances. The women would include info about their earnings, productivity and menstrual cycles.
The results? Women ovulating would earn in average $30/ hour than women in their periods and $15/hour more than women at other stage in their cycle. Women on the pill, who don't ovulate, made significantly less than women in their natural cycle.
The theory is that women ovulating are at their peak of fertility and that makes them more attractive to males. What they haven't found yet is how men can subconsciously recognize that a women is in estrus (their fertility peak).