Everybody knows that most women go a little crazy right before they get their period, that their reproductive hormones cause their emotions to fluctuate wildly. Except: There's very little scientific consensus about premenstrual syndrome. Says psychologist Robyn Stein DeLuca, science doesn't agree on the definition, cause, treatment or even existence of PMS. She explores what we know and don't know about it — and why the popular myth has persisted.