I don’t think it really happens either, but I still have friends who believe it to be true and say they cycled in sync with college roomates. My girlfriend said that she thinks she and her daughter are possibly getting into sync, her daughter started menstruating 6 months ago, and has been every 23 days! God that sucks for a young girl. I’m thinking if one is 23 and the other 28, eventually they will appear to sync for a few months, and then get out of sync again for obvious mathematical reasons.
The thought was pheromones I think. That pheromones from a women you spend a lot of time with would influence your own hormones.
I’m just thinking It doesn’t make sense in nature. A man can make a lot more babies if his harem is ovulating at different times during the month.
I once heard men help keep women regular. That being around testosterone keeps women cycling well. That sounds like a loud of crap to me too.