The Search for Answers
Needless to say, medical researchers and local breast cancer advocates have spent a lot of time trying to figure out what has caused this massive outbreak of the disease. The first potential factor that people focused in on is the collective affluence of Marin County's residents. Breast cancer is different than most medical conditions in that upper middle class and wealthy women are more likely to contract the disease than poorer women. This is not to suggest that socioeconomic status is a risk factor itself. But women from higher economic stratum tend to make certain life choices that can increase their odds of eventually being diagnosed with breast cancer. For example, the age at which a woman has her first child increases risk, as delayed childbirth causes rates of breast cancer to go up. Having fewer children and breastfeeding less frequently also increase the incidence of breast cancer. The child bearing patterns of affluent women tend to conform to these risk factors more closely than women from other economic classes, and Marin County has a higher percentage of affluent women than most other places.
Another factor linking socioeconomic status and breast cancer is hormone replacement therapy (HRT). This practice, which involves the use of the female hormones estrogen and progesterone as a remedy for postmenopausal symptoms, has now clearly been shown to increase the risk of breast cancer. But this is something that was only discovered in recent years, and back in the 1990s when breast cancer cases were exploding in Marin County, postmenopausal women across the country were undergoing HRT in complete ignorance of the possible consequences. Affluent women, generally speaking, have been the ones using hormone replacement therapy over the past couple of decades, and what was going on in the '90s with breast cancer rates in Marin County certainly dovetails nicely with the "glory days" of HRT.
Out of the various studies that have been done looking into the situation in Marin County, two deserve special mention. First, there was the Adolescent Risk Factors study from the late '90s, which found an intriguing correlation between higher alcohol consumption and the diagnosis of breast cancer in individual women. Rates of drinking are higher among women in Marin County than the national average and it has also been shown that alcohol acts synergistically with HRT to push the risk levels associated with the latter up even higher. The second study worth noting was carried out by a researcher with the Buck Institute for Age Research. This analysis revealed that all of the "excess" breast cancer cases in Marin County were estrogen-receptor positive cancers occurring in postmenopausal women - the exact type of cancer that has been strongly linked to hormone replacement therapy.
Analyzing the Numbers
The key to understanding Marin County's astonishing breast cancer rates appears to be related to demographic factors. High socioeconomic status is connected to patterns of behavior and lifestyle choices that lead to higher incidence of breast cancer, and as it happens Marin County is one of the wealthier areas in the nation. However, it is far from the only affluent location in the United States, and in comparison to other wealthy places, Marin County's breast cancer rates still appear to be something of an anomaly. Therefore, the possibility of some kind of hidden factor (something environmental?) playing a supporting role cannot be entirely ruled out.