The menstrual Cycle.

A study conducted by UCLA’s Department of Psychiatry has revealed that the kind of face a woman finds attractive on a man can differ depending on where she is in her menstrual cycle. For example: If she is ovulating, she is attracted to men with rugged and masculine features. However, if she is menstruating, or menopausal, she tends to prefer a man with scissors lodged in his temple and a bat jammed up his ass while he is on fire.

Further studies in this area have been cancelled.