Re: Why do women menstruate at the same time?

It is well known that women who live in close quarters with each other tend to 
synchronize their menstrual cycles.  What is not known so well is the exact 
mechanism by which this occurs.  The best idea is that there must be a small 
molecule that acts as a pheromone.  The idea is that it stimulates the lining of 
the nose (like any other molecule which transmits smell information) which in 
turn sends a chemical message to the brain.  The brain, in turn, sends its 
hormone messages to the ovaries and uterus, thereby changing the cycle.

Why this mechanism evolved in the first place is a difficult question.  It 
probably had something to do with keeping all the women of a social group or 
clan on the same cycle, maybe for child-rearing purposes.