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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.
Family composition (e.g., the absence of a father) is associated with pubertal timing in women, although the socioendocrinology of the human primate is poorly understood. To better understand social influences on sexual maturation, retrospective data were collected on menarcheal age and family composition from a sample of approximately 1,938 participants from a college population. Absence of a biological father, the presence of half- and step-brothers, and living in an urban environment were associated with earlier menarche. The presence of sisters in the household while growing up, especially older sisters, was associated with delayed menarche. Menarcheal age was not affected by number of brothers in the household, nor was there an effect of birth order. Body weight and race were also associated with menarche. The present findings advance the literature as they are suggestive of putative human pheromones that modulate sexual maturation to promote gene survival and prevent inbreeding, as occurs in rodents and nonhuman primates. Am. J. Hum. Biol. 18:481-491, 2006.