A biologically based explanation for mortality crossover in human populations.

We consider the impact of a genetically predetermined maximum life span (or upper bound on life span of a species) on patterns of human mortality. In particular, we consider the implication of a predetermined maximum life span on two distinct population subgroups: one group advantaged, the other group disadvantaged. We show that the existence of a genetically predetermined maximum life span imposes the condition that the two subgroup mortality rates must cross. We further observe that proportional mortality must be associated with divergent mortality differences over the life course. We illustrate these arguments via some simulated examples.