It is apparent that the premenstrual disorders are biologically driven and are not only psychological or cultural phenomena. PMDD has been reported by menstruating women worldwide, indicating a biological basis that is not geographically selective. Most psychologists infer that this disorder is caused by both a reaction to hormone flux and also genetic components. There is evidence of heritability of (retrospectively-reported) premenstrual symptoms from several twin and family studies done in the 1990s, with the heritability of PMDD proving to be about 56%.