Social class and the black/white crossover in the age-specific incidence of breast cancer: a study linking census-derived data to population-based registry records.

An unexplained racial crossover in breast cancer rates in the United States has been apparent since 1969: among women over age 40 years, the rate is higher among white compared with black women, while among women under age 40, the reverse is true. To explore the contribution of social class to this phenomenon, the present study used 1980 census block-group data to generate population-based age-specific breast cancer rates stratified by race and class. Cancer data included all black women (n = 405) and white women (n = 4,259) diagnosed with primary breast cancer during 1979-1981 in the San Francisco Bay Area Resource for Cancer Control registry. Among women under age 40, the black/white incidence ratio among women from predominantly working class block-groups was 1.08 (95% confidence interval (CI) 0.74-1.56), but among those from non-working class block-groups, the black/white ratio equaled 1.96 (95% CI 1.17-3.26). Among women age 40 years or over, the black/white incidence ratio among women from working class block-groups was 0.78 (95% CI 0.68-0.89), but was 0.98 (95% CI 0.78-1.23) among women from non-working class block-groups. These findings underscore the class-based nature of the observed crossover and pose a challenge to accepted theories of breast cancer etiology. Possible risk factors contributing to these findings are discussed, including induced abortion and oral contraceptive use prior to first full-term pregnancy.