In 2007, based on statistics from eighteen dancers over a period of 60 days, it was noted that female lap dancers earned the highest tips around the time of ovulation, during the most fertile period of their menstrual cycle and the lowest tips during menstruation; the average difference in earning between those two times amounted to about $30 per hour. Women on the pill earned overall less than those not on the pill. The results were interpreted as evidence of estrus in humans: females apparently advertise their fertility status to males in some manner. This finding earned its authors the 2008 Ig Nobel Prize (a parody of the Nobel Prize given for unusual or trivial achievements in scientific research) in Economics.