Do the menstrual cycles of women living together tend to synchronize?

This amazing phenomenon was first described in 1971 by researcher Martha McClintock, now with the University of Chicago. Having asked around a bit, I'd say it's common knowledge among women, but I'll bet not one male in 50 has ever heard of it. Women do have their little secrets.

Synchronous menstruation has been observed among mothers, sisters, and daughters who live together, and sometimes among women who simply work together. McClintock tells of seven female lifeguards who started out one summer with widely scattered periods. Three months later they were all menstruating within four days of one another.

A study of 135 residents of a women's college dorm confirmed the effect. Most of the cycle shifting occurred within the first four months and was usually complete after seven months. Fortunately for the dorm's plumbing, the whole building didn't synchronize, just roommates and close friends. As often as not, the women were unaware of what had happened. Later research has suggested that synchrony is caused by some sort of scent cue, or pheromone.

Scientists at the Sonoma State Hospital Brain Behavior Research Center in California identified several women who were believed to be menstrual pacesetters--they made other women conform to their cycles. The scientists placed cotton pads under the dominant women's arms for a day, and then wiped the pads on the upper lips of five female subjects three times a week. (One wonders how much the subjects got paid for this.) Within five months, four of the recipients were menstruating at the same time as their donors.

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.

Reprinted