The secular trends in male : female ratio at birth in postwar industrialized countries

Finnish investigators [Vartiainen et al. Environmental Chemicals and Change s in Sex Ratio: Analysis Over 250 Years in Finland. Environ Health Perspect 107:813-815 (1999)] presented the sex ratio of all newborn babies from 175 1 to 1997 in order to evaluate whether Finnish long-term data are compatibl e with the hypothesis that the decrease in the ratio of male to female birt hs after World War I and World War II in industrial countries is caused by environmental factors. They found an increase in the proportion of males fr om 1751 to 1920, which was interrupted by peaks in male births during World War I and World War II and followed by a decrease thereafter, similar to t he trends in many other countries. The turning point of male proportion, ho wever, preceded the period of industrialization and introduction of pestici des and hormonal drugs. Thus, a causal association between these environmen tal exposures and this decrease is unlikely. In addition, none of the vario us family parameters (e.g., paternal age, maternal age, age difference in p arents, birth order) could explain the historical time trends. Vartiainen e t al. concluded that at present it is unknown how these historical trends c ould be mediated. The postwar secular decline of the male:female ratio at b irth is not an isolated phenomenon and parallels the decline of perinatal m orbidity and mortality, congenital anomalies, and various constitutional di seases. This parallelism indicates a common etiology and may be caused by r eduction of conceptopathology, as a correlate to increasing socioeconomic d evelopment. An inverted dose response or the dose-response fallacy due to v anishing male conceptuses explains the low sex ratios before World War I an d World War II in newborns from black parents and from the lowest socioecon omic classes.