A critical review of menstrual synchrony research.

Two experiments and three studies reported a significant level of menstrual synchrony after subjects had been treated with applications of axillary extract from a donor subject or after subjects have spent time together. Four studies failed to replicate these results. A comparison of the studies shows the only consistent difference is that those studies not finding menstrual synchrony reported problems with subjects who had irregular cycle lengths, while those finding menstrual synchrony reported no such problems. All experiments and studies were based on the methods and research design introduced by McClintock (1971). Three errors are inherent in research based on her model: (1) an implicit assumption that differences between menses onsets of randomly paired subjects vary randomly over consecutive onsets, (2) an incorrect procedure for determining the initial onset absolute difference between subjects, and (3) exclusion of subjects or some onsets of subjects who do not have the number of onsets specified by the research design. All of these errors increase the probability of finding menstrual synchrony in a sample. One or more of these errors occurred in the experiments and studies reporting synchrony; no significant levels of menstrual synchrony occur when these errors are corrected. Menstrual synchrony is not demonstrated in any of the experiments or studies.