Then, every day for four months, the researchers rubbed thawed gauze pads above the upper lips of 20 volunteer women who had agreed to have any of 30 different "natural essences" rubbed under their noses. "Sweat" was among the 30. "We buried it in the list," McClintock said.
For two months, 10 women sniffed sweat from women in the early phase of their menstrual cycle, while the other 10 sniffed sweat from women in a later phase of their cycle. Then the groups switched and spent two months getting the opposite scent.
The women smelled nothing, but the results were striking: Those exposed to "early phase" sweat saw their own cycles shortened by an average of 1.7 days per month, and as much as 14 days a month. Those who sniffed "later phase" sweat saw their cycles lengthened by an average of 1.4 days a month, and up to 12 days a month.
Computer models indicated there must be two substances in the sweat -- one that lengthens cycles and one that shortens them -- and that together they can quickly lead to groups of women having synchronous periods.
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Notice the `early phase', `late phase'... That's something like the optimal and superoptimal distinction I was trying to make. That is definitely a signally system marker. It's a cool article. Really interesting for discussion on the whole range of topics, instinct, learned, etc. etc...
CG