Synchronization and suppression of the menstrual cycle

Abstract
Synchronization of menstrual cycles among women living together in a college dormitory suggests that mutual interaction may have a strong influence on the length of the menstrual cycle. Studies on the influence of pheromones on mouse cycles and other species have revealed that social groups affect the functioning of the endocrine system. Although few studies have been conducted among humans, indirect observations indicate that group influence is significant in some aspects of the menstrual cycle. Cycle synchronization is often observed among mothers, daughters, and sisters living together. Unsynchronized cycles become synchronized within 3 months of living together. The study involved 135 women aged 17-22. The study revealed a significant increase in synchronization among roommates (P<=0.0007), among closest friends (P<=0.003), and among closest friends who were roommates (P<=0.0003). The increase in synchronization among roommates did not differ significantly from the increase in synchronization among closest friends. In the second study, female students were divided into 15 groups of closest friends (5<=N<=10). A significant decrease in individual differences within the group was observed (P<=0.001). The greatest decrease was noted within the first 4 months. 47% of women were unaware of their friends’ menstrual days. Among the remaining 48% of women, only partial awareness existed. It is possible that at least one female pheromone affects the frequency of women’s menstrual cycles. Furthermore, women were divided into two groups: those who saw men less than three times a week (N=42) and those who saw men at least three times a week. Women who saw men less frequently reported significantly longer menstrual cycles (P>=0.03) than 28 days. Age did not affect the length of the cycle.

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