How come when you live with other girls, you end up on the same cycle?

Answer 1

Studies have shown that groups of women living in close proximity gradually become synchronised with regards to their menstrual cycle.

This was first described in 1971 by researcher Martha McClintock, now with the University of Chicago, although it was more common knowledge long before then - for example in nunneries.

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.

The whole building didn't synchronize, just roommates and close friends although the women were unaware of what had happened. Later research has suggested that synchrony is caused by some sort of scent cue, or pheromone (an airborne hormone).

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. 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.

Why synchrony occurs is pretty much a total mystery. The only published theory treats it as an evolutionary holdover from prehistoric times, when it was common for men to take multiple mates and efficient reproduction was essential to the survival of the species. The author of this theory assumes that women in their brief monthly phase of peak fertility give off some pheromonic signal that drives men wild.

If his wives are on different cycles, hubby has a good chance of picking the wrong one when his nose tells him it's time to go into action, thus wasting precious bodily fluids. But if the women are all menstrually synchronized, he CAN'T pick wrong-they're all fertile. In short, synchronous menstruation compensates for congenital male cluelessness about women's bodies.

There is a little problem with this theory... surveys show sex occurs most frequently around the time of menstruation, when women are LEAST fertile!

Answer 2

I'd have to agree, pheromones are the link. Everyone has a pheromone node in their nose. As to, why does this synchronization of female cycles happen, I'd have to say that it's an innate competition behaviour of the body. One could use their imagination to hypothesize, such as follows:

Humans have multiple sex partners by nature. Although humans may have always had one life long companion, to spread the genetics more widely and insure the survival of the species, humans, historically and biologically speaking, likely had multiple sex partners within a nearby time frame. Thus, to be competitive, the females, let’s say hypothetically, were are all trying to bear the child of the alpha male (the leader, or chief etc.) and the fist one to do so would become his new companion or whatever the case may be. I’m certain, humans have always been very intelligent and fully aware of the female cycles and what they mean. I’m sure certain rituals or fights between the males occurred then the winner copulated with the females vying for him, then the male would only have to wait to see which one, if any, became pregnant. The male would likely have to fend off the other males for the rest of their fertility window.

Additionally, males have competitive behaviours too, the physical ones displayed in public are obvious, but sperm from two separate males will perform offence and defence behaviours as a way to fend off or penetrate through other sperm. Certainly with these kinds of sperm behaviours built in, it is expected that there be sperm of other males present at the time before or after copulation. The sperm of the healthier male will perform better thus the survival of the fittest prevails at this level too, just as the healthier female will be more fertile and have a more regular cycle. It’s obvious that the males will follow the female cycles and be affected by their pheromones and subsequent behaviours too.

Answer 3

Most women who live with or near other women adjust their menstrual cycle timing to each other. A recent study at the University of Chicago by Martha McClintock exposed a group of women to a whiff of perspiration from other women. It caused their menstrual cycles to speed up or slow down depending on the time in the month the sweat was collected -- before, during or after ovulation. This was the first proof that people produce and respond to pheromones.

Although it's now clear that pheromones exist, the way our body processes them has yet to be determined. Animals have a vomeronasal organ (VNO), which perceives the substance and then leads them to mate. Some anatomists don't think humans have a VNO; others think they've found pits inside our nostrils that might be VNOs, but may not work.

The concept of a human pheromone, or sexual scent of attraction, has been debated and researched for years.

In most animals, the relationship between pheromones and mating is straightforward. Sea urchins, for example, release pheromones into the surrounding water, sending a chemical message that triggers other urchins in the colony to eject their sex cells simultaneously.

Human pheromones, on the other hand, are highly individualized, and not always noticeable. In 1986 Dr. Winifred Cutler, a biologist and behavioral endocrinologist, codiscovered pheromones in our underarms. She and her team of researchers found that once any overbearing underarm sweat was removed, what remained were the odorless materials containing the pheromones.
Dr. Cutler's original studies in the '70s showed that women who have regular sex with men have more regular menstrual cycles than women who have sporadic sex. Regular sex delayed the decline of estrogen and made women more fertile. This led the research team to look for what the man was providing in the equation. By 1986 they realized it was pheromones.

Answer 4

"kja63" is right on the money! Early man (and women) depended on eachother for survival. A crucial part of survival is reproduction, the continuance of the species. The alpha female is the one all other women cycle around and their bodies would adjust to her menstrual cycle. This would ensure that all breeding age females would be fertile at the same time. Thus, the maximum number of offspring would be born around the same time. Therefore, offspring could be raised together and if necessary breastfeeding could be a shared task.

The same effect remains today and in a group it is still the alpha females who determine when ovulation occurs. It might be interesting to figure out who is the alpha female in a group. It's not always who you think it is.

Answer 5

That's wierd, isn't it? You would think that it would be contrary to survival of the human species. For example, if a man in ancient times had multiple wives, they would all have similar cycles, so there would be times of the month when he couldn't get any one of them pregnant. Then they suddenly might be wiped out by disease or disaster. Even if one woman survived, she may not be pregnant to carry on the family.

You would think if the women's cycles were different, that would lead to furthering the survival of the species.

Answer 6

It happens when you work together too. It's hormones and/or pheromones. Centuries ago, women of breeding age in the same clan all cycled at the same time. That phenomena continues today. It's amazing! LOL

Answer 7

Well - we all produce pheromones, and for whatever reason, when women live together, their pheromones cause all the women to gradually shift their menstrual cycle to match.

I've always wondered what happens at sorority houses... twenty or forty young women with PMS at the same time... eeek

-dh

Answer 8

Could possibly. When I was in school, me & my friends use to be on the same cycle - or really close to it - because of all the time we spent together or whatever. It's like your body picks up on it.

Answer 9

Yes that's true it's been proven, I've heard the same thing, it can be any two girls that hang out alot can start around the same time.

Answer 10

It is one of the mysteries. Some researchers think that women give off pheromones that affect the others. There is even a name for it: McClintock Effect