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Introduction
The sex ratio at birth, defined as the number of male births per 100 female births, has been investigated for centuries in human populations (Süssmilch, 1761; Geissler, 1889; Gini, 1951; Edwards, 1958; Garenne, 2009; Fellman, 2015). It averages about 105.5 in European populations, with variations from about 104 to 107 even in the same country over time, and presents larger variations in other populations, from 100 or below to 110 and above (Ciocco, 1938; Aubenque, 1989). The highest value recorded in vital registration statistics of national populations (113) seems to be that of Korea in the 1920s (Visaria, 1967). Lowest values are found in African populations, and in particular in southern Africa (James, 1984; Garenne, 2002, 2004).
Many factors could explain variations in sex ratios among populations. Firstly, genetic factors have been identified, in particular among African populations. African-Americans living in the USA or in the UK have lower sex ratios than Europeans living in the same country, and this seems to be due to genetic characteristics of the males, since offspring of mixed couples have the characteristics of the father, and not those of the mother (lower sex ratio if father is African, higher if he is European) (Khoury et al., 1984). Numerous environmental factors affecting the sex ratio have been identified, such as age of mother, birth order, socioeconomic status, stress and exposure to chemicals and occupational hazards (Teitelbaum, 1970; James, 1987, 2000, 2009; Chahnazarian, 1988; Fukuda et al., 1998; Graffelman & Hoekstra, 2000; Ansari-Lari & Saadat, 2002; Zorn et al., 2002; Saadat & Ansari-Lari, 2004; Catalano et al., 2005, 2006; Mackenzie et al., 2005; Polasek et al., 2005; Polasek, 2006; Saadat, 2006). Of particular interest for this study is the controversial issue of nutritional status. Very low fat mass of father and mother seems to have an effect on the sex ratio of the offspring.
In a review of survey data from sub-Saharan Africa, sex ratios at birth have been found to vary from close to, or below, 100 in southern Africa, to about 102 in East Africa, and about 104 in West Africa, with local variations. However, one country stood out with outstandingly high...