Period Syncing (McClintock effect) – Is It a Myth or Truth?

It is commonly believed that women who live together start getting their period at the same time every month. Many women will tell you that this happened to them when they were in college or living with a friend. Is period syncing an old wives’ tale, or is there any scientific evidence behind the same? Let us find out.

The origin

Menstrual syncing is a long-held belief that when women spend a long time together, their menstrual cycles sync up. The idea started when the findings of a study were published in Nature, a scientific journal, in 1971.


During this study, Martha McClintock studied the menstrual cycles of 135 female residents of a college dormitory.  McClintock found that there was a significant increase in the synchronization of menstruation’s onset dates among friends and roommates than a random pairing of women (1).

During the study, McClintock hypothesized that women experienced menstrual syncing due to the time spent together, eating meals, and experiencing stress. She also put forward the theory of alpha uterus. According to this theory, the alpha uterus has a ‘strong hormonal pull that causes the cycles around it to menstruate in unison’ (2). The study suggested that physical closeness caused the women’s pheromones to communicate with each other, which triggers cycle syncing.

This effect thus came to be known as the McClintock effect.

It’s a myth

When this study came out in the 1970s, feminism was emerging as an important movement. The idea of period syncing became extremely popular due to this reason. Few other studies in humans and primates also seemed to show similar results.

However, later research debunked the findings of this study. When H Clyde Wilson from the Department of Anthropology, University of Missouri, reviewed this research in 1992, he discovered several errors in the process. When these errors were corrected, he found no significant levels of menstrual synchrony (3).

A 2006 study collected data on menstrual cycles from 186 Chinese women living in dorms for over a year. This study found that women living in dorms did not synchronize their cycles and that any synchrony found was only a matter of chance (4).

In 2016, Clue, a period tracking app, partnered with Dr. Alexandra Alvergne from the University of Oxford to study period syncing.  This study collected data from over 1500 women. They found that out of the 360 pairs that they studied, 273 pairs had a larger difference in cycle start dates at the end of the study than at the beginning. Only 79 of the pairs that were studied behaved the opposite manner, with the gap between their menstrual cycle start dates narrowing over the course of the study. The results showed that cycles are more likely to diverse over time (5).

Do pheromones play a role in period syncing?

Pheromones are chemicals that an animal produces, which changes the behavior of other animals of the same species. According to a study published in 1998, pheromones can regulate the menstrual cycles of women (6). The researchers studied the pheromones of the participants at various points in their menstrual cycles and concluded that the release of pheromones could manipulate ovulation in other females. Later studies, however, failed to confirm this theory.

There is a lot of evidence to pheromones impact and change behavior in animals of the same species; the same cannot be said about humans. Pheromones are emitted mainly through the armpits and groin. However, when we bathe and shower, we wash off the pheromones as well. This shows that pheromones cannot have much of an effect on modern life.

Do phases of the moon play a role in period syncing?

A women’s menstrual cycle has been linked to the moon cycle because both are about the same length. While the average length of a woman’s menstrual cycle is 28 days, the length of the moon cycle is 29.5 days. The word ‘menstruation’ comes from a combination of Greek and Latin words that mean ‘moon’ and ‘month.’  This may have lead people to believe that a women’s menstrual cycle is linked to the lunar cycle.

A 1986 research, studied 826 female volunteers with the regular menstrual cycle and found that significant proportions (28.3%) of menstruations occurred around the new moon (7). However, later research that studied 74 women over a year did not find any synchrony of the lunar phase with the menstrual cycle (8).

Why is the idea of period syncing so popular?

Even though there is no scientific proof that period syncing is real, this myth persists. The idea of period syncing appeals to women as it creates a sense of solidarity among them. Traditionally the experience of having your period is considered by society as shameful or embarrassing. Menstrual synchrony helps women to feel positive and empowered.

The menstrual cycles occur between 26 to 31 days, and there is bound to be some overlap. The synchrony appears more due to laws of probability than any other reason. It is quite likely that women are looking for patterns where none exist. The idea that your period syncs with that of your friend create closeness or a bond.

What can affect your period?

While living with someone, pheromones, and phases of the moon cannot impact your menstrual cycle, some other factors can influence your period. These include –

1. Birth control pills

Birth control pills alter the estrogen and progesterone hormone levels in the body and control when your period arrives. Many women use this method to induce their periods early.

2. Diet

Your diet can also have an impact on your periods. If your nutrition is poor and you do not consume enough calories, it can cause you to miss your periods. Extreme weight gain or weight loss due to an unhealthy diet can also make your periods irregular. Health conditions like anorexia and bulimia can also impact your periods.

3. Excessive exercise

Your regular exercise routine does not have much of an impact on your menstrual cycle. However, an intense new exercise regime (for a marathon or triathlon training) can cause you to menstruate irregularly or not at all. The intensity of the exercise can lead to hormonal changes, which can cause menstrual changes.

4. Stress

Stress can have a massive impact on your menstrual cycle. Stress hormones can impact your menstrual cycle and make it longer or shorter than average.

5. Prescription drugs

Prescription drugs like thyroid medication, steroids, and antipsychotics can affect your period. These medicines can make your period prolonged, heavier, and irregular.


6. Sleep

Sleeping poorly can also affect your menstrual cycle. Women who work odd hours like nurses and flight attendants are more likely to experience irregular periods.

Final thoughts

Period syncing or menstrual synchrony, also known as McClintock effect, is a process whereby women who live together start getting their period at the same time every month. The idea began when the findings of a study were published in Nature, a scientific journal, in 1971. Over the years, however, many other studies have proved period syncing to be a myth. Given that menstruation usually lasts around five days, it is not uncommon for friends to experience overlapping menses. The idea of period syncing appeals to women as it creates a sense of solidarity among them.


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