Jim Crow laws: A contributing factor to more lethal breast cancer among U.S. black women now?

Nancy Krieger Big 3 Poster

November 1, 2017 – Jim Crow laws—which legalized racial discrimination in Southern U.S. states from the late 1870s through the mid-1960s—have been linked with negative health impacts. A new study led by Harvard Chan School’s Nancy Krieger, professor of social epidemiology, suggests that, among U.S. women currently diagnosed with breast cancer, being born in a Jim Crow state heightened black women’s risk of being diagnosed with tumors that have a worse prognosis. 

What prompted you and your colleagues to undertake this study?

We undertook this study to re-insert history and societal context into the analysis of black versus white disparities in breast cancer mortality in the U.S. We focused on the breast cancer estrogen receptor (ER) because breast cancer ER status is a critical prognostic indicator for breast cancer treatment and survival. The vast majority—80% or higher—of women in the U.S. who are diagnosed with breast cancer among all racial/ethnic groups have ER-positive (ER+) tumors, which can be treated with hormonal medications, and these women have better survival rates than patients with ER-negative (ER-) tumors. However, the current prevalence of ER+ tumors is highest among women who are white and/or affluent, and some researchers have suggested that the higher risk for ER- tumors observed among black versus white women is primarily due to “racial” differences that are presumed to be genetic.

Yet this is an odd hypothesis when considered from the vantage point of evolutionary biology, which suggests that environmental influences could have played a role in the differences in breast cancer risk among black and white women. The ER has an exquisitely evolved sensitivity to different kinds of cues and is subject to epigenetic regulation. Whether the ER is turned on or off in a cell can be affected by changes in hormone levels, such as changes in levels due to pregnancy, to exposure to hormone therapy, or to exposure to environmental agents that act like the hormone estrogen.

Has any prior research looked at links between Jim Crow laws and health?

To date, only a handful of epidemiologic studies, including some I have led, have focused explicitly on the health impact of Jim Crow or its abolition, whether in relation to breast cancer or any other health outcome. Of note, although the geography of cancer mortality and incidence has been well-documented, no previous studies have investigated the association between exposure to Jim Crow and risk of cancer. Having previously done research showing that white versus black risk of having an ER+ tumor rose and fell with the rise and fall of hormone therapy, and also having conducted studies on the impact of the abolition of Jim Crow on U.S. black versus white disparities in infant mortality and premature mortality, I decided to look at associations between Jim Crow birthplace and breast cancer ER status.

The dearth of research on possible links between Jim Crow laws and health is surprising, for two reasons. First, Jim Crow can plausibly be hypothesized to be a major societal determinant of health, given how its legally codified system of white supremacy, bolstered by extrajudicial terror, exposed black Americans to noxious social, economic, and physical conditions, above and beyond the types of discrimination experienced in the non-Jim Crow states. Second, the abolition of Jim Crow was 50 years ago, during the time when most women currently diagnosed with breast cancer in the U.S. were children—meaning that Jim Crow laws could have had an impact on breast cancer cases we are seeing now, given the importance of early life exposures to risk of breast cancer and type of breast cancer.

What were the study’s major findings?

Our study provides novel data on the pace of change of breast cancer ER status at the time of diagnosis among contemporary U.S.-born black and white women. We analyzed changes in ER status in relation to Jim Crow birthplace among the 46,417 black and 339,830 white U.S.-born non-Hispanic women in the National Cancer Institute’s 13 SEER (Surveillance, Epidemiology, and End Results) Registry Group who were born between 1915-1979 and diagnosed, from age 25-84, between 1992-2012.

Key findings were that, among the women diagnosed before age 55, the percentage of ER+ cases rose among each four-year birth cohort—from the 1915-1919 cohort to the 1975-1979 cohort—and that the percentage increase in ER+ tumors was greater for the black versus white women. Further, among black women, the increase in the percentage of ER+ tumors was greatest for those born in the Jim Crow states versus those born in the non-Jim Crow states. These results imply that Jim Crow contributed to the higher percentage of ER- tumors among black women, and its abolition led to a rise in the percentage of tumors that were ER+ and to markedly reducing the black versus white gap in ER+ tumors. The rapid change in the percent of ER+ tumors we observed among the black women, especially among those born in Jim Crow states, and the shrinkage of the black excess risk of ER- tumors, can plausibly be driven only by non-genetic factors despite emphasis in the literature on genetic contributions to black versus white risk of ER- breast tumors.

Our study’s analytic approach and findings underscore the need to consider history and societal context when analyzing breast cancer ER status and racial/ethnic inequities in its distribution.

– Karen Feldscher