Apple #117: Synchronized Menstruation

A while back now, 3d asked:

I don't know if this an unappropriate subject or not, but my wife would like to know why a woman's cycle will change when with different groups of women. Perhaps there's really no explanation for this.

Some brief investigation into this question pulled up some interesting research. Or anyway, I found it pretty fascinating. I always wondered about this. I had heard that maybe the moon had something to do with it, but people's menstrual cycles didn't necessarily seem to match with the full moon, so I didn't think that was it. Turns out, it's scent that does it.

• Scientists have studied women who live together, say in dorms, and also women who just socialize without necessarily sharing living quarters, and they have found that their cycles synchronize after a couple months. This synchronization is now sometimes called "the McClintock effect" after Martha McClintock, Professor of Biopsychology at the University of Chicago, who first studied and wrote about this phenomenon.
  
• Not everybody is equally sensitive to synchronization. Some women's cycles don't change at all, and some women's cycles jump 12 days to match the cycles of others around them.
  
• Scientists also studied menstrual cycles of rats and found that rats also synched up even when they were in separate rooms. The rooms, however, were connected by a shared airflow. This suggested an airborne influence at work.
• To determine what the airborne influence could be, Dr. McClintock studied the effect of pheromones on women's menstrual cycles. She had women bathe, then use an alcohol wipe under their arm and turn and wipe the cloth on the upper lip of the next woman. Sounds kind of gross, but you know, it's all for science. She said there wasn't a strong odor at all, and all the women in the study thought they were in the control group.
• The results of this study found that not only did the women's cycles synch up, but by timing when they wiped each other relative to certain points in the cycles, they could shorten everybody's cycles or lengthen them.
• While further research has corroborated that pheromones from other women influence the synchronization of their menstrual cycles, as far as I've found, no one has yet isolated the particular pheromone, or chemical scent, that triggers the shift in ovulation. In other words, there's no spray that you can go out and buy and squirt on a bunch of women, or female animals, to get them all ovulating on the same date.
  
• As far as why synchronization happens, that's another matter. I've discovered from doing these Apples that while science can verify whether or not something happens the way we think it does, science often has a very hard time explaining why it happens. And the reasons why women's cycles influence each other are still entirely speculative. But the possiblities Dr. McClintock and others have offered essentially boil down to this:
• When women are ovulating, other women's bodies could take this as a signal that it is a good time for them to be fertile. A prepubescent girl, for example, may pick up the signal that now is a good time for her body to start ovulating. An adult woman may get the signal from other women who are currently pregnant that now is a good time to get pregnant. In other words, if other women's bodies are going through and surviving this expenditure of energy, then there must be enough food available, the weather is benign, there will be other women available to assist, and so on.
  
• Another possibility is that if all the females in a group are ovulating at the same time, that would make it more difficult for a dominant male to go around and impregnate every single one of them, thus making it more likely that the male who gets the female pregnant is more available to support its offspring. Therefore, a woman's body will do its best to synch up with the ovulation cycles of other women as a way to protect their potential offspring.
  
• Elsewhere, however, Dr. McClintock says that menstrual synchrony may serve no particular function at all. Thanks a lot.

Hope that helps, Wife of 3d!

I think that, since nobody has added any new requests in a while, we'll call the requests finished for now. Thanks to everybody who posted a question! You've helped make this blog even more interesting and informative for lots of other people.

Sources
Radio National, The Health Report with Norman Swan, "Menstrual Synchrony," June 8, 1998
Martha K. McClintock, "Whither menstrual synchrony?" Annual Review of Sex Research, 1998
Morofushi et al., "Positive Relationship between Menstrual Synchrony and Ability to Smell 5alpha-Androst-16-en-3alpha-ol," Chemical Senses, 2000
"Period living," NewScientist.com, December 14, 2002