Average age at death in infancy and infant mortality level: reconsidering the Coale-Demeny formulas at current levels of low mortality

The longterm historical decline in infant mortality has been accompanied by increasing concentration of infant deaths at the earliest stages of infancy. The influence of prenatal and neonatal conditions has become increasingly dominant relative to postnatal conditions as external causes of death such as infectious disease have been diminished. In the mid-1960s Coale and Demeny developed formulas describing the dependency of the average age of death in infancy on the level of infant mortality. Almost at the same time as Coale and Demeny’s analysis, as shown in this paper, in the more developed countries a steady rise in average age of infant death began. This paper demonstrates this phenomenon with several different data sources, including the linked individual birth and infant death datasets available from the US National Center for Health Statistics and the Human Mortality Database. A possible explanation for the increase in average age of death in infancy is proposed, and modifications of the Coale-Demeny formulas for practical application to contemporary low levels of mortality are offered. Introduction During the period from the 1920s to the 1970s infant mortality decline in Europe and other industrialized countries was accompanied by concentration of infant deaths in the neonatal period. The distribution of infant deaths became more and more highly skew. The average length of life for infants who died during the first year in low-mortality countries was less than 0.25, and exhibited systematic decline as the infant mortality rate (IMR) decreased. The eminent French demographer Jean Bourgeois-Pichat proposed an explanation of this phenomenon [1951a,b]. He maintained that there are two types of infant mortality: exogenous mortality due to the influence of postnatal conditions as infants become exposed to the external environment, and endogenous mortality due to conditions of the prenatal period, including congenital diseases. Endogenous mortality is concentrated in the first month of life and its level is relatively stable through time. In general, historical mortality decline has been connected with declining exogenous mortality, including in infancy. Thus, rapid infant mortality decline was observed at ages 1-11 months. Bourgeois-Pichat also derived a formula for the distribution of infant deaths by age in infancy as the level of infant mortality varied, together with other determinants of infant mortality. The average duration until death for infants who die is an important parameter in life table construction. In this connection the Bourgeois-Pichat formula leaves something to be desired. The formula appears rather complicated and demands additional data beyond what is needed to compute the infant mortality rate. For life table construction other purposes, such as calculation of infant mortality rates from infant death rates, simpler formulas (e.g. Chiang, 1978) have been preferred. A set of formulas known as the “Coale-Demeny formulas” that * Max Planck Institute for Demographic Research. Rostock, Germany. ** 1201 Belle View Boulevard, Alexandria, Virginia, 22303. USA. [email protected].