McClintock effect

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 effect, which is the synchronization of the estrous cycle and has been noted in small animals such as mice and guinea pigs. In contrast to the Whitten effect, which is driven by male pheromones, 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 McClintock, who reported her findings in Nature 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. These phase-advancing and phase-delaying effects may show human axillary compounds regulate biological rhythms.

Another study experimented with 20 lesbian couples had results which showed that more than half of the couples tested had the same synchronization within a two day period of each other.

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 bias 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:

"Suppose a study starts on October 1. Subject A, with a 28-day cycle, has an onset on September 27, another on October 25, and a third on November 22. Subject B, with a 30-day cycle, has an onset on October 5 and another on November 4. A naive investigator could report that these subjects were 20 days apart at the outset (October 25 vs October 5) and 18 days apart at their second onset (November 4 vs November 22). Therefore, the two are synchronizing. In fact, the two subjects were eight days apart to start with (September 27 vs October 5) and are diverging."

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 moon 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 (lunaception) is at present not accepted as plausible by the scientific community.