What is wrong with season-of-birth studies?

Important alert on studies of season-of-birth effects:

"While discussing studies of month-of-birth effects, it is important to be aware of methodological problems and pitfalls.

In some cases a simplistic approach is applied to study the effects of month of birth on human lifespan: mean ages at death are calculated for people born in different months using cross-sectional data (i.e., death certificates collected during a relatively short period of time [38]).

This methodology is flawed and can produce both false positive and false negative findings.

For example, if the seasonality of births and infant mortality were more expressed in the past, then the month-of-birth distribution of people would differ in different age groups of the population, thus producing a spurious month-of-birth effect on lifespan (if erroneously estimated through mean age at death). This mistake happens because the mean age at death depends on the age distribution of living people, which may differ depending on month-of-birth. Thus, even if the month of birth does not affect adult lifespan, nevertheless a false positive finding may occur, simply because the effects of population age structure are not taken into account.

On the other hand, month-of-birth effects could be overlooked by this cross-sectional method if the seasonal effects on age-specific mortality rates are proportional. This false negative finding happens because proportional changes in death rates produce a proportional changes in the numbers of deaths in all age groups, and such proportional changes in numbers have no effect on the mean age at death. Thus, a false negative finding may occur, because cross-sectional analysis of death records is blind to proportional changes in age-specific death rates.

In our study we avoided this simplistic cross-sectional analysis of death records as a flawed methodology. Instead we applied a cohort approach by following people born in the same calendar years until the last person died (method of extinct generations)."

Cited from pages 34 -36 in:

Gavrilov, L.A., Gavrilova, N.S.
Early-life factors modulating lifespan.
In: Rattan, S.I.S. (Ed.).Modulating Aging and Longevity.
Kluwer Academic Publishers, Dordrecht, The Netherlands, 2003, 27-50.


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