Mortality Crossovers from Dynamic Subpopulation Reordering

Mortality crossovers are often understood to be the result of differential mortality selection. Models of mortality selection commonly assume a single dimension of heterogeneity, which stratifies populations into homogenous frail and robust subpopulations with proportional hazards. We propose a more realistic mortality selection model in which black and white populations are stratified by multiple crosscutting dimensions of heterogeneity, resulting in heterogeneous subpopulations. In the multidimensional model, in contrast to the conventional unidimensional model, the rank order of subpopulation mortalities is dynamic over age. As a result, a crossover can arise in either of two ways: from a change in the share of subpopulations in the black and white populations (analogous to the crossover in the standard, unidimensional mortality selection model), or alternatively, from a change in the rank order of subpopulation mortalities, regardless of subpopulation shares. The latter possibility has no analogue in the standard, unidimensional model. Our results therefore identify a new mechanism by which mortality selection can create mortality crossovers.