Cohort and Age Effects

Abstract

The last chapters provide ample evidence that life span and month of birth are related. Two questions arise: first, does this influence also exist for the ages 1 to 49? Second, is the magnitude of the excess mortality of the spring-born similar in all age groups? There are at least two different hypotheses concerning these questions. A first hypothesis is that the excess mortality of the spring-born is age-specific because of the changing distribution of causes of death with age. The analysis of the US death certificates presented in Chapter 5 shows that the month-of-birth effect exists in all major groups of causes of death and that its magnitude differs, however. The excess mortality of the spring-born is largest for heart disease and considerably smaller for the group of malignant neoplasms. Thus, the month-of-birth pattern might be comparatively small at younger ages and increase its magnitude at middle and old ages when heart disease becomes an important cause of death. A second hypothesis is that the differences decrease with age due to mortality selection. Among both the spring- and the autumn-born, the frailer will die first, resulting into an ever more homogeneous and “robust” population at higher ages. In practice, it is most probably the case that both forces work simultaneously.