Survival of the Weakest? Culling Evidence from the 1918 Flu Pandemic

Abstract

When a negative shock affects a cohort in utero, two things may happen: first, the population suffers detrimental consequences in later life; and second, some will die as a consequence of the shock, either in utero or early in life. The latter effect, often referred to as culling, may induce a bias in estimates of later life outcomes. When the health shock disproportionately affects a positively selected subpopulation, the longterm effects are overestimated. The 1918 flu pandemic was plausibly more harmful to mothers of high socioeconomic status, as a suppressed immune system in mothers of low socioeconomic status may have been protective against the most severe consequences of infection. Using historical birth records from the city of Bern, Switzerland, we assess this concern empirically and document that a careful consideration of culling is paramount for the evaluation of the 1918 flu pandemic and other fetal health shocks.

Health of neonates born in the maternity hospital in Bern, Switzerland, 1880–1900 and 1914–1922

“…These records in both datasets include the following individual information for all deliveries that occurred at the hospital in the corresponding years: maternal age, place of residence (categorized into “urban” (the cities of Bern, Biel, and Thun) and “rural”), parity (categorized into 1, 2, 3 and more than 3), birth date of the newborn, infant sex, stillbirth (yes or no), and the infant’s birth weight [ 52 ]. In the dataset for the years 1880–1900, the following variables are also provided: qualitative information on maternal body height (categorized into short, medium, tall), maternal body shape (categorized into weak, medium, strong) and nutritional status of the mother (categorized into undernourished or normal), occupation of the mother, and age at menarche.…”