Menstrual synchrony can be assessed, inherent cycle variability notwithstanding: commentary on Schank (2001)

J. C. Schank (2001) argued that biases in the calculation of synchrony (due to inherent variability in the menstrual cycle) and in respondents' recall of menstrual dates can produce spurious results. In this commentary, the major flaws in J. C. Schank's arguments regarding the calculation of synchrony are presented. Low standard deviations (< 2.0) of interwomen cycle lengths were found in a reanalysis of the Bedouin family data (A. Weller & L. Weller, 1997). This obviates J. C. Schank's major assumption that a high degree of cycle variability exists and his concern that high variability produces a spurious increase of onset differences over time. Furthermore, research that has used random control groups has found significantly greater levels of synchrony in their experimental groups, refuting J. C. Schank's arguments.