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The McClintock effect (also known as menstrual synchrony; not to be confused with the Whitten effect) 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.

Research on the McClintock effect is inconclusive. Important questions remain, such as why it evolved, and to what extent pheromones and Jacobson's organ play a role. One possible explanation on why it evolved is that it allows more males to mate and pass their genes. The argument is that if only one female of the group was ready to mate at a time, the most probable outcome would be that only the more dominant male would have access to her.

Others have doubted whether menstrual synchrony occurs at all. It is apparent from research that at least some women synchronize after living together for a time. Yet others, in virtually identical living conditions, do not. One hypothesis is that whether or not the women get on well with one another may be the factor in this, women who like each other in groups will synchronize and women who are antagonists to one another will not.

Some consider the McClintock effect to be a statistical artifact or fallacy of causation, and the actual cause of the synchrony to be the influence of the moon. Systems such as lunaception and cosmo-fertility, which provide complex explanations of the moon's effect on reproductive cycles, are at present not regarded as having a scientific basis.

McClintock, Martha K. “Menstrual Synchrony and Suppression.” Nature. 229: 244-45, 1971.