The crossover between life expectancies at birth and at age one: The imbalance in the life table

The single most used demographic measure to describe population health is life expectancy at birth, but life expectancies at ages other than zero are also used in the study of human longevity. Our intuition tells us that the longest life expectancy is that of a newborn. However, historically, the expectation of life at age one (e₁) has exceeded the expectation of life at birth (e₀). The crossover between e₀ and e₁ only occurred in the developed world in the second half of the twentieth century. Life tables for populations that have not achieved this crossing between life expectancy at birth and at age one are referred to here as imbalanced. This crossover occurs when infant mortality is equal to the inverse of life expectancy at age one. This simple relation between mortality at age zero and mortality after age one divides the world into countries that have achieved the crossover in life expectancies and those that have not. It is a within-population comparison of mortality at infancy and after age one. However, results of these within-population comparisons can be used for comparison between populations. For countries that have already achieved this crossing in life expectancies, the sex differential in the timing of the crossing is marked: Females attain the crossing before males for every single population and in some cases by up to 18 years earlier. However, for most developing countries, life expectancy at age one is still higher than life expectancy at birth, in some cases by several years. Subpopulation comparisons for the US show that black Americans are near to transitioning out of the imbalanced life table situation while the white population has already done so.