Deaths from breast cancer have been dropping in the U.S. since the 1990s. However, African-American women still have a 40 percent higher risk of dying from the disease compared to white women, according to an American Cancer Society report.

Part of the reason for the unsettling statistic has been that these women are at a higher risk of getting the especially tough-to-treat estrogen receptor- and progesterone receptor-negative (ER-/PR-) version of the disease, which make up about a quarter of the 207,000 new cases of breast cancer diagnosed in U.S. women each year.

But why African-American women would be more likely to have ER-/PR- breast cancer has been unclear to researchers, who have been looking for clues in hopes of closing the race-delineated mortality gap.

A new study finds that the risks of developing these types of tough tumors might be related to how many children a woman has—and whether or not she breastfeeds them. The research was published online August 16 in Cancer Epidemiology, Biomarkers & Prevention.

"The higher incidence of ER-/PR- breast cancer in African-American women may be explained in part by their higher parity and lower prevalence of breastfeeding relative to white women," Julie Palmer, a professor of epidemiology at Boston University and co-author of the new study, said in a prepared statement.

The researchers analyzed data from the Black Women's Health Study, which has collected health information from some 59,000 women for the past 16 years, focusing on 318 cases of ER-/PR- breast cancer and 457 cases of estrogen receptor- and progesterone receptor-positive (ER+/PR+) cancer. Palmer and her team found that black women with breast cancer who had two or more children and didn't breastfeed them were 50 percent more likely to have the ER-/PR- form of breast cancer than those who had two children and breastfed them. Breastfeeding in general has been recognized to lower a mother's chances of getting breast cancer—as well as ovarian cancer, cardiovascular disease and possibly type 2 diabetes.

Although the biological basis for this correlation between breastfeeding and ER-/PR- breast cancer is still unclear, some researchers note that lactation might help minimize inflammation in the body during pregnancy and childbirth.

The good news is that the findings suggest that modifiable aspects of life history—rather than underlying and relatively constant genetics—might be partially responsible for these higher rates of tough breast cancer in black women.

But genes might not be in the clear entirely. As the researchers note in their paper, women of African heritage tend to have higher levels of markers of inflammation in general, a tendency that might be traced back to tougher immune systems to cope with endemic diseases of sub-Saharan Africa. "Although these characteristics are advantageous for women living in Africa," the researchers wrote, "the very strong immune response may be disadvantageous in other situations and could lead to an increased risk of aggressive breast tumors following pregnancy without lactation because of the inflammatory process accompanying involution."

The findings "suggest that breastfeeding can reduce risk of developing the aggressive, difficult-to-treat breast cancers that disproportionately affect African American women," Palmer said.