In the last twenty or thirty years, researchers have moved from looking at PMS as a cultural issue to examining it increasingly from biological perspective.

Although researchers have not yet established a definite genetic link for PMS, there does appear to be a genetic link to sensitivity hormonal changes. Studies show, for example, that identical twins are twice as likely to have PMS than fraternal twins. So while no PMS gene has been found, experts agree that, biologically speaking, some women are more likely to get PMS than others.

Some women appear to be more sensitive to hormonal fluctuations and are more likely to experience PMS. A recent National Institute of Mental Health study showed that women with a preexisting vulnerability to PMS experienced relief from symptoms when their sex hormones were suppressed, but once the hormones were reintroduced, they again developed symptoms. In contrast, women without a history of PMS reported no effects from hormonal manipulation.

The prevailing theory among experts is that women with low levels of the brain chemical serotonin are particularly sensitive to changes in the level of progesterone, which might lead to symptoms of PMS. Experts also believe that some women may have a genetic predisposition to get PMDD. The neurotransmitter serotonin affects emotions, behavior, and thought, and is known to play a role in depression. It also appears to play a role in PMDD. A 2001 Canadian study, published in the Archives of Women's Health, is one of several studies that suggest women with PMDD may have a dysfunction of the serotonin transporter gene. However, the evidence for this theory is not conclusive. A more recent study, published in November 2006 in the American Journal of Obstetrics & Gynecology, compared fiftythree women diagnosed with PMDD with fifty-two healthy women and found no evidence that the genes that control serotonin are associated with PMDD. The conclusion is that some women may have a biological tendency to get PMDD, but experts haven't conclusively determined why.

Researchers are looking at the following possible culprits of PMS and PMDD:

• Serotonin: A chemical in the nervous system believed to play an important role in the regulation of mood, sleep sexuality, and appetite, as well as in disorders such as migraines, anxiety, and depression. Low levels of serotonin or a dysfunction in the serotonin transporter gene are thought to be responsible for PMS and PMDD.

• Gamma-aminobutyric acid (GABA): The body’s main chemical messenger is found in the nervous system. There increasing evidence it plays a role in mood disorders.

Experts once believed that PMS and PMDD were caused by hormonal imbalance in women. Now, the prevailing belief is that normal cyclical changes in the sex hormones trigger the conditions. The cyclical changes dramatically shift the levels of progesterone and estrogen, essentially putting women’s bodies into a tailspin. Those with great sensitivity to the fluctuations experience severe PMS PMDD. In other words, normal hormonal changes trigger PMS and PMDD.