system
Given the following post, respond as if you're commenting on Reddit:
human
When a group of women cohabit, their menstrual cycles sync up. Is this true and, if so, why would evolution result in this feature?

Wouldn't a clan with all the women having their period at the same time be vulnerable?
gpt
There is not a single piece of evidence, or a single study that has shown the McClintock effect to be true.

Here's why it seems so true:

If you take two girls, with say a 28 day cycle that is randomly distributed, then the maximum difference between their periods is just 14 days, which means the average difference is only *7 days* between any of their periods.

If you have 3 girls at 28 day cycles, the average difference is only 4.6 days.

4 girls, average difference 3.5 days.

And so on, until you realize that with say, 6, and average difference of 2.3 days if they have random 28 day cycles it really does seem as if they are syncing up.

Also, this only accounts for periods start time, take into account the fact that periods last 3-7 days and you see that 3 girls living together are mathematically almost sure to be "having" their period at the same time.