Results Conslusion
4 Discussion
This study is the first comparative analysis of the relationship between month of birth and longevity on the basis of data for total populations. The life span of 681,677 Austrians and 1,371,003 Danes was studied. The Austrian data set is cross-sectional and includes only information about those who have died but lacks information about the risk-population. This is due to the fact that in Austria no population register exists. Therefore, for inter-census years it is not possible to sub-divide the Austrian population according to month of birth. Because of the restrictions of the Austrian data set I refrained from estimating an event-history model which would be conditional on the outcome of death and would therefore be biased. The Danish data is based on the population register established in 1968: it is longitudinal and includes both the population at risk and those who have died. Thus, an event-history model is the proper statistical method for the analysis of this data.

Both data sets reveal that the mean age at death of people born in spring is lower than that of those born in winter. The differences in Austria seem to be greater than in Denmark. Neither the seasonal distribution of deaths nor the ‘birthday effect’ can explain the differences in life span.

Following I will discuss the evidence for and against three hypotheses that explain the relationship between month of birth and longevity.

The first hypothesis is that the differences in life expectancy are the result of selective survival in infancy. In the last century it was a well-known fact that the likelihood of infants to survive their first year of life depended to some extent on their month of birth. This knowledge is documented in the extensive data sets on infant mortality in the central statistical offices of many countries (for an overview see [6] ). Here, there is data on infant mortality by single month age groups and by month of death. For example, between 1841 and 1850, Belgian infants born in spring experienced lower mortality during their first three months of life than infants born in other seasons but were exposed to a greater mortality risk between their third and sixth months [6, 10] .

A recent study on infant mortality by age and season of birth [6] shows that for five countries in the last century, the mortality risk in the first two years of life differed according to the season of birth. The pattern was different in the various countries. The authors explain this by the interaction between climate and socio-cultural behaviour peculiar to the given country, such as breast-feeding and weaning practices. For example, in Italy the summer cohorts were advantaged because they went through the summer with the full protection afforded by breast-feeding and reached winter at an age when they were less vulnerable to viral infections of the respiratory tract. The winter cohorts, on the other hand, were exposed to the impact of the cold season on respiratory diseases in their first months of life. This was then followed by the hot summers and the accompanying viral infections of the digestive tract, at a time when the protection of breast-feeding was diminished. In Switzerland the pattern was just the opposite: the mortality risk was highest for those born in spring and summer; it was lowest for infants born in autumn.

An unpublished analysis of Danish data on infant mortality for the years 1911–1915 conducted by the author reveals that in their first year of life, infants who were born in spring and early summer had a noticeably increased mortality risk. This pattern runs counter to the notion that at older ages the difference in life expectancy according to month of birth is due to selective survival during infancy. Selective survival implies that those cohorts live longer that are born in a season where it is more difficult to survive the first year of life. At older ages these cohorts will then consist of relatively more robust individuals, since frailer members of the cohort already died during infancy. It seems that the opposite is the case: the seasonal pattern in infant death suggests that for those born during the more harmful period of a year, some trait is fixed which makes them more susceptible to diseases later in life.

The explanation of the debilitating effect of early life events, in particular of viral infections early in life, is supported by a large number of studies on seasonality in schizophrenia. A review by Torrey et al. [19] consists of more than 250 studies covering 29 countries in the northern and 5 in the southern hemisphere. For the northern hemisphere most of these studies find a significant excess of births in winter and spring (December to May) for schizophrenia. A study on schizophrenia in Queensland, Australia, suggests for those born in the southern Hemisphere an excess birth in their winter (July to September), while those born in the northern Hemisphere had a March–April birth excess [13] . A recent study in Denmark on the effects of family history and place and season of birth on the risk of schizophrenia [16] found an excess of spring-births. The authors come to the conclusion that, although in most or all cases genetic factors are a necessary cause of schizophrenia, they are modulated by environmental factors such as the season of birth.

The second possible explanation for the differences in life expectancy according to month of birth is prenatal influence. A series of studies by Barker (e.g. [4] ) suggest that the susceptibility to circulatory heart diseases later in life may, among other things, also be determined by the nutrition of the mother during pregnancy. At the beginning of this century the food supply in general, and the availability of fresh fruits and vegetables in particular, differed from season to season. Mothers who gave birth in late autumn and early winter had access to fresh fruits and vegetables throughout most of the time of their pregnancy; those who gave birth in spring and early summer may have experienced relatively longer periods of inadequate nutrition. However, a study on the effect of the great famine in Finland in the years 1866-1868 showed that cohorts born shortly before or during the famine did not have a higher mortality risk later in life than those born after the famine [11] .

The third hypothesis is that social factors that are closely related to an individual’s birthday could be responsible for the differences in life span. Age at first school attendance is one example. Children who are born after a certain deadline have to wait one more year before they can enrol. In this case they are about a year older than the youngest children in their class. At the turn of the century school started on 1 October in Austria. Those children who had not turned six before this date had to wait another year. Thus, children born in autumn and winter experienced an age advantage over their classmates who were born at the beginning of the year. Research suggests that this age advantage affects scholastic aptitude [1, 7, 8] which may translate into a lifelong advantage in various ways [18] . However, at the turn of the century most schools, especially in rural areas, consisted of only one or two classes. Thus, children of all ages were instructed together. It is questionable whether a one year age advantage would also influence the development of children in a class where the youngest and the oldest pupils are more than four years apart in age.

Results Conslusion

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Longevity and Month of Birth:
Evidence from Austria and Denmark

Gabrielle Doblhammer
1999 - 2000 Max-Planck-Gesellschaft ISSN 1435-9871
http://www.demographic-research.org/Volumes/Vol1/3