DEAR DOCTOR K:

It seems to be commonly accepted that women who live together or who are close friends get their periods at the same time. I've had the same experience. Is this a real biological phenomenon or just a coincidence?

DEAR READER:

The idea that women who spend a lot of time together eventually begin to get their periods at the same time each month is called menstrual synchrony. But how this synchronization occurs -- or even if it happens at all -- is not well understood.

Menstrual synchrony was first described by a graduate student at Wellesley College named Martha McClintock, now a professor at the University of Chicago. McClintock noticed the phenomenon in her dorm and decided to research it. She found that of 135 female fellow students, those who lived closest together tended to cycle together. The research was published in a prestigious scientific journal and caused quite a stir.

What could account for this? McClintock wasn't sure. One interesting possibility was pheromones -- airborne chemical signals. It's well known that pheromones exist in animals. The chemicals given off by one animal travel to the nose of another animal and affect that animal's behavior. Mating, dominance among male animals, weaning patterns of mothers and many other animal behaviors are influenced by pheromones.

In 1998, McClintock published another research report in a prestigious journal that suggested pheromones were the cause of menstrual synchrony. She found that women produce pheromones in their armpits, and showed that these pheromones influenced the length of the menstrual cycle of women around them.

However, not all researchers who have studied this question have found that menstrual synchrony exists. And, to my knowledge, the chemical nature of the pheromones seemingly identified by Dr. McClintock has not yet been determined.

Even though pheromones from one person reach another person by traveling through the air to the other person's nose, pheromones don't actually have a smell. For many years it was believed that the human nose did not have the same ability to receive pheromones as the noses of animals. However, a study in 2006 by Linda Buck, who shared the Nobel Prize for her discoveries of how the sense of smell works, found that the human nose has the structures necessary to receive pheromones.

In short, there is still some controversy about whether menstrual synchrony exists, and about whether there are human pheromones. My bet is that menstrual synchrony does exist: I've heard many women describe their experience with it. And I'll bet that human pheromones exist: Our noses have the "equipment" necessary to sense pheromones, and pheromones are common in other mammals.

The question of whether human pheromones exist is one of the most important and interesting in biology. If we are like animals, then our behavior and even our attitudes about people may be influenced by forces (pheromones) we are entirely unaware of. If science identifies human pheromones, then it is possible that science could influence our behavior -- for better and for worse.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

** ** **

COPYRIGHT 2012 THE PRESIDENT AND FELLOWS OF HARVARD COLLEGE

DISTRIBUTED BY UNIVERSAL UCLICK FOR UFS