The
McClintock effect, also known as
menstrual synchrony or the
dormitory effect, is a theory that proposes that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, or dormitories) tend to become synchronized over time.
It is thought to be analogous to the
Whitten effectThe Whitten effect is a phenomenon observed by W. K. Whitten , whereby male mouse pheromone-laden urine synchronizes the estrus cycle "among unisexually grouped females."...
, which is the synchronization of the
estrous cycleThe estrous cycle comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. Humans undergo a menstrual cycle instead...
and has been noted in small animals such as
miceA mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. The American white-footed mouse and the deer mouse also sometimes live in houses. In some places, certain kinds of field mice are also common...
and
guinea pigThe guinea pig , also commonly called the Cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not pigs, nor do they come from Guinea...
s.
The
McClintock effect, also known as
menstrual synchrony or the
dormitory effect, is a theory that proposes that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, or dormitories) tend to become synchronized over time.
It is thought to be analogous to the
Whitten effectThe Whitten effect is a phenomenon observed by W. K. Whitten , whereby male mouse pheromone-laden urine synchronizes the estrus cycle "among unisexually grouped females."...
, which is the synchronization of the
estrous cycleThe estrous cycle comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. Humans undergo a menstrual cycle instead...
and has been noted in small animals such as
miceA mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. The American white-footed mouse and the deer mouse also sometimes live in houses. In some places, certain kinds of field mice are also common...
and
guinea pigThe guinea pig , also commonly called the Cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not pigs, nor do they come from Guinea...
s. In contrast to the Whitten effect, which is driven by male
pheromoneA pheromone is a chemical signal that triggers a natural response in another member of the same species. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Their use among insects has been particularly well documented...
s, the McClintock effect is postulated to have only female pheromonal involvement.
Research support
The phenomenon, sometimes referred to as the "social regulation of ovulation," was first formally studied by psychologist
Martha McClintockMartha McClintock is an American psychologist best known for her discoveries of the existence of human pheromones and menstrual synchrony. She is the David Lee Shillinglaw Distinguished Service Professor in Psychology at the University of Chicago and is the Founder and Director of the Institute for...
, who reported her findings in
NatureNature is a prominent British scientific journal, first published on 4 November 1869. Most scientific journals are now highly specialized, and Nature is among the few journals that still publish original research articles across a wide range of scientific...
in 1971.
A prospective study by McClintock in 1998 was claimed to support her earlier findings by showing that underarm (or axillary) materials (collected from female donors), when wiped under the noses of (other) women, influenced cycle length. Odors taken on the day that donors ovulated (and the next two days) delayed ovulation and hence lengthened the total cycle of the recipients. Therefore, these phase-advancing and phase-delaying effects show how human axillary compounds can regulate biological rhythms.
Criticism
If all women had an average-length menstrual cycle (of 28 days duration), the maximum time between two women's onsets would be 14 days and the minimum time between onsets would be zero days (synchronization). On average, the difference would be seven days, and (in small groups) half the time would be less (if one assumes there is no McClintock effect). McClintock observed a five day difference in her 1971 study and some have suggested this could have been a random occurrence.
Most studies of menstrual synchrony have been retrospective, introducing
recall biasIn psychology, recall bias is a type of systematic bias which occurs when the way a survey respondent answers a question is affected not just by the correct answer, but also by the respondent's memory....
into the data.
The interaction of theorized menstrual synchrony with differing cycle lengths has not been explained. Two women with cycle lengths that differed by two days might initially begin menstruating on the same day, but the next month would be two days apart, the month after that four days, and so on. No studies have claimed to show that the McClintock effect causes women with historical cycles of different lengths to synchronize.
Methodological errors have also been proposed. A critical review of the evidence for menstrual synchrony gave this example:
This type of error is more likely in smaller sample sizes, like those used in studies of menstrual synchrony. One researcher has claimed that when the studies are corrected for such errors, the evidence for menstrual synchrony disappears.
A small minority of scientists believe the influence of the
moonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
synchronizes menstruation. If this were the case, the McClintock effect would be a statistical artifact or fallacy of causation. While nightlighting has been shown to affect reproductive cycles, the idea that menstrual cycles synchronize with the moon (
lunaceptionLunaception is a practice that purports to enable a woman to know ahead of time when she will be fertile each month, based on practices found in many non-urban cultures...
) is at present not accepted as plausible by the scientific community.
External links