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

Abstract

Finnish investigators [Vartiainen et al. Environmental Chemicals and Changes 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 1751 to 1997 in order to evaluate whether Finnish long-term data are compatible with the hypothesis that the decrease in the ratio of male to female births 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 from 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 the trends in many other countries. The turning point of male proportion, however, preceded the period of industrialization and introduction of pesticides and hormonal drugs. Thus, a causal association between these environmental exposures and this decrease is unlikely. In addition, none of the various family parameters (e.g., paternal age, maternal age, age difference in parents, birth order) could explain the historical time trends. Vartiainen et al. concluded that at present it is unknown how these historical trends could be mediated. The postwar secular decline of the male:female ratio at birth is not an isolated phenomenon and parallels the decline of perinatal morbidity and mortality, congenital anomalies, and various constitutional diseases. This parallelism indicates a common etiology and may be caused by reduction of conceptopathology, as a correlate to increasing socioeconomic development. An inverted dose response or the dose-response fallacy due to vanishing male conceptuses explains the low sex ratios before World War I and World War II in newborns from black parents and from the lowest socioeconomic classes.

Striking variation in the sex ratio of pups born to mice according to whether maternal diet is high in fat or carbohydrate

“…Unfortunately, the group sizes have generally been small and the statistical significance of the data questionable (46,47). Similarly, much of the information available for sex ratio deviations in humans is also confounded by a variety of unresolved variables, including sample size, although some larger demographic studies have suggested an association of increased female births with suboptimal maternal nutrition (27,28,48,49). As in rodents, the shift in sex ratio in these human populations may be due the greater vulnerability of male fetuses to environmental stresses (27).…”

Acute undernutrition is not associated with excess of females at birth in humans: the Dutch Hunger Winter

“…In humans, the male : female ratio appears to be declining in some, but not all developed countries in the period since World War II ( Jongbloet et al 2001), and the role of environmental pollutants in this trend is hotly debated (Lummaa et al 1998;Vartiainen et al 1999), as are the relative importance of other factors such as polygyny, the age and condition of the mother, household wealth and frequency of coitus (Gaulin & Robbins 1991;Lindström & Kokko 1998;Gutierrez-Adan et al 2000;Koziel & Ulijaszek 2001). War induces a relative excess of male births (Graffelman & Hoekstra 2000); in both Germany (in World Wars I and II) and The Netherlands (in World War II, but not in World War I when The Netherlands was neutral), the proportion of males at birth was elevated (Bromen & Jockel 1997;Van den Broek 1997).…”

Secondary sex ratio in relation to exposures to polychlorinated biphenyls, dichlorodiphenyl dichloroethylene and methylmercury

“…The number of male live births exceeds the number of female live births [1], but in parts of the world the ratio between the two has decreased over the past decades [2]. Endocrine disruptive effects from persistent organic pollutants (POPs) on hormonal systems during conception and pregnancy have been suggested as explanations for sex ratio changes [3].…”