Two studies of menstrual synchrony: negative results

In 1971 McClintock reported menstrual synchrony in pairs and groups of women who spent time together. The two studies reported herein, based on the methods and research design introduced by McClintock, fail to replicate her results. The first study is of a sample of 132 women who were sorority members or roommates of sorority members living on the campus of a large coeducational state university. The second study is of a sample of 24 women who were members of a cooperative house near the same university. In the subjects from both studies, the final mean onset absolute difference is greater than the initial mean onset absolute difference, and there are more nonsynchronous pairs of subjects than synchronous pairs. The sample from the sorority study was progressively reduced to exclude those subjects with irregular menstrual cycle lengths and those pairs in which one subject was taking oral contraceptives. Menstrual synchrony did not emerge in the reduced sample. Thus, whether or not menstrual synchrony occurs among women who spend time together should still be considered a hypothesis requiring further investigation.