Human menstrual synchrony in families and among close friends: examining the importance of mutual exposure.

Human menstrual synchrony was examined in a sample of 73 urban households with a relatively high degree of mutual contact and interaction but less than that of a previously studied, possibly unique sample of Bedouin families with an extremely high degree of contact, interaction, and sexual segregation (A. Weller & Weller, 1997). In addition, for the first time, menstrual synchrony was studied among best friends who did not live together and among sisters who did versus did not share a bedroom. A very high degree of synchrony was found among families (51%) and among sisters (51%), and a relatively high degree of synchrony was found among close friends (30%). The results suggest that synchrony can be attained without sleeping in the same bedroom or even in the same house. The findings also suggest that an effective range of mutual exposure is an attribute of human menstrual synchrony; below or above this range, the phenomenon may not occur.