Abstract
Month of birth influences adult life expectancy at ages 50+. Why? In two countries of the Northern Hemisphere–Austria and Denmark–people born in autumn (October–December) live longer than those born in spring (April–June). Data for Australia show that, in the Southern Hemisphere, the pattern is shifted by half a year. The lifespan pattern of British immigrants to Australia is similar to that of Austrians and Danes and significantly different from that of Australians. These findings are based on population data with more than a million observations and little or no selectivity. The differences in lifespan are independent of the seasonal distribution of deaths and the social differences in the seasonal distribution of births. In the Northern Hemisphere, the excess mortality in the first year of life of infants born in spring does not support the explanation of selective infant survival. Instead, remaining life expectancy at age 50 appears to depend on factors that arise in utero or early in infancy and that increase susceptibility to diseases later in life. This result is consistent with the finding that, at the turn of the last century, infants born in autumn had higher birth weights than those born in other seasons. Furthermore, differences in adult lifespan by month of birth decrease over time and are significantly smaller in more recent cohorts, which benefited from substantial improvements in maternal and infant health.
Remarkable reductions in old-age mortality over the past half century have fueled rapid growth of the elderly population and have led to a substantial increase in life expectancy (1). Yet we still have only limited knowledge about the factors that affect mortality and survival in old age (2). Recent research highlights the role of early-life factors that affect late-life mortality (3). In particular, environmental conditions during the prenatal and early postnatal period have been found to influence adult health and mortality significantly (4, 5) although these results are still controversial (6, 7).
We conjectured that the month of birth may be an indicator for environmental factors that are linked to the seasons of the year. If this conjecture is true, then the patterns of two geographically close populations should resemble each other, and the pattern in the Northern Hemisphere should be mirrored in the Southern Hemisphere. Furthermore, lifespans of people who were born in the Northern Hemisphere but who died in the Southern Hemisphere should resemble the pattern of the Northern Hemisphere.
We obtained data on the populations of Denmark, Austria, and Australia to test our conjecture. For Denmark, the longitudinal data are based on the population register, which follows every person living in Denmark from 1968 to the present. For Austria and Australia, we used information from death certificates for all deaths that occurred in 1988–1996 and 1993–1997, respectively.
We have found that month of birth and remaining life expectancy at age 50 are related. We tested four hypotheses to explain the relationship. The first hypothesis assumes that the interaction between age and the seasons of mortality causes the differences in lifespan by month of birth. For example, people born in April are older than people born in November when the high mortality of winter strikes them. The second hypothesis tests whether the differences are due to unobserved social factors that influence or result from the seasonal timing of births. The third hypothesis explains the differences in adult lifespan by differential survival in the first year of life, whereas the fourth hypothesis assumes that debilitation in utero or in the first year of life increases the infant’s susceptibility to diseases at adult ages.
