Differential mortality exists in the United States both between racial/ethnic groups and along gradients of socioeconomic status. The specification of statistical models for processes underlying these observed disparities has been hindered by the fact that social and economic quantities are distributed in a highly nonrandom manner throughout the population. We sought to provide a substantive foundation for model development by representing the shape of the income-mortality relation by racial/ethnic group.
We used data on black and white men and women from the longitudinal component of the National Health Interview Survey (NHIS), 1986–1990, which provided 1,191,824 person-years of follow-up and 12, 165 mortal events. To account for family size when considering income, we used the ratio of annual family income to the federal poverty line for a family of similar composition.
To avoid unnecessary categorizations and prior assumptions about model form, we employed kernel smoothing techniques and calculated the continuous mortality surface across dimensions of adjusted income and age for each of the gender and racial/ethnic groups. Representing regions of equal mortality density with contour plots, we observed interactions that need to be accommodated by any subsequent statistical models. We propose two general theories that provide a foundation for more elaborate and testable hypotheses in the future.