Do the menstrual cycles of women living together tend to synchronize?
Source: http://www.straightdope.com/classics/a2_306.html
"Interestingly, men also have an effect on women's menstrual cycles--and not just because they make women pregnant. Women who associate with males frequently find that their periods become shorter and more regular. One woman told McClintock that she had a six-month cycle length until she began hanging out with guys, at which point her periods began occurring every 4.5 weeks. When she resumed her solitary ways, her cycle lengthened again. Another round of cotton pad experiments, this time using males as donors, confirmed this. Having sex with a man at least once a week will also do the trick."
