The McClintock effect (also known as "Menstrual Synchrony") is the observed phenomenon that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, dormitories, etc.) will tend to become synchronized over time.
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. It has also been noted in small animals such as mice and guinea pigs. Females affected will tend to follow the 'lead' of the alpha female.
Since the research on the McClintock effect is inconclusive, many important questions remain, such as why it evolved, and how much pheromones and Jacobson's organ play a role.
Others have doubted whether menstrual synchrony occurs at all. It is apparent from research that some women at least synchronize after living together for a time. Yet others, in virtually identical living conditions, do not.
McClintock, Martha K. “Menstrual Synchrony and Suppression.” Nature. 229: 244-45, 1971.
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