From http://en.wikipedia.org/wiki/McClintock_effect:

Menstrual synchrony refers to the alleged process whereby women who 
begin living together in close proximity experience their menstrual 
cycle onsets (i.e., the onset of menstruation or menses) becoming closer 
together in time than previously.

I think the important words are: "close proximity". If the trigger was 
the moon all women would be synchronized, which I think is not true.

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