Selection and Adaptation Components of Infant Mortality

We test the selection hypothesis of infant mortality against the adaptation hypothesis by decomposing the mortality age pattern over the first year of life into an adaptation- and a selection component. We show that the population level decline in mortality over the first hour of life is significantly influenced by mortality selection, i.e.~the frailest infants leaving the population shortly after birth. The subsequent mortality decline predominantly results from mortality changes observed in homogeneous sub-populations. This confirms the common view of the infant mortality age pattern being caused by adaptation on an individual level. The analysis is informed by detailed micro-data on births and infant deaths in the United States including more than 25 million births and 162,546 deaths. No parametric assumptions were necessary.