A psychological condition that occurs a day or two before and/or during menstruation that anyone who's ever lived with a woman for longer than 2 months can confirm is not a myth.
This is a serious phenomenon in which a woman will say things that can irreparably damage a relationship, treat their partner with a disrepect he (or she) hasn't earned, and estrange them from anyone who does not suffer from the condition.
It took me a year to figure out that the times my girlfriend was acting like a nut were the few days before she was on her rag.
Now, she puts her period on the calendar, and I mark the two days prior as PMS times in which I make sure to hang out with my friends in another town. I don't try to argue with her during this time because I can't deal with the enormous logical fallacies that a woman suffering this condition produces.
dude: ite i'm out. cya tomorrow.
chick: you're going to fuck another woman aren't you. AREN'T YOU?!?
dude: yes, that makes sense. see you tomorrow.