We investigate whether heterogeneity can explain the differences in mortality between the United States and a more homogeneous country, i.e. Japan. The background of the analysis is the growing gap between life-expectancy in the United States and the world record leader since the 1980s. This development is mirrored in the decelerated improvements made against mortality during the last 25 years — especially for women.
Our hypothesis is that it is possible to have survival improvements at a steady pace across all population subgroups. The proportion of the population subgroups which perform relatively well decreased over time whereas poor performing subgroups became more important. The consequence of this compositional change since the 1980s would be the observed levelling off in survival improvements on the population level.
Since official mortality estimates in the US are of poor quality, we estimate mortality rates via the extinct cohort method using Medicare enrollment data from the Social Security Administration due to their higher quality.
If our hypothesis was true, we should see little differences in the development of mortality rates between the best performing subgroups in the United States and Japan. We compared mortality rates between Japanese women and white women in the US from states who (a) either had the highest life expectancy or (b) the highest median household income or (c) the largest gross state product per head. Regardless of the choice of indicator, mortality rates levelled-off in the United States for those best performance subgroups as for the whole population.
The preliminary conclusion is therefore that compositional changes (heterogeneity) in the US population can not explain the widening gap between US life expectancy and best practice life expectancy.