The relationship between human sex ratios at birth and caloric availability per capita was examined across different countries. Significant positive correlations were obtained between the amount of food a country had available and the percentage of male births. Furthermore, increases or decreases in a country's caloric availability were related to corresponding changes in that country's sex ratio. These results provide evidence of adaptive sex ratio biasing in humans. The physiological mechanism by which this effect operates is probably higher mortality rates for male embryos and fetuses as a result of nutritional deficiencies and associated stressors.
PIP: Researchers applied data on caloric supply per capita and on number of female and male live births to the Pearson product-moment correlation to examine the relationship between various countries' caloric supply per capita and their sex ratio at birth. In 1979-80 and 1982-84, mean caloric availability and percentage of male births had a significant positive correlation (coeff. = .365 and .524, respectively; p .01 in both periods). Further, increases or decreases in a country's caloric availability (e.g., famine) in the 2 years following a stable 3-year-period were significantly correlated to corresponding changes in that country's percentage of male births (coeff. = .524; p .01). Even thought deviations from the mean 51.33% were minimal (SD = .55%), a small deviation equalled many 1000s of excess male or female births in countries with large populations. These findings suggested that adaptive sex ratio biasing occurs in humans. The physiological mechanism responsible for this biasing may be nutritional deficiencies during gestation causing increased mortality rates for male embryos and fetuses. The variance explained by changes in caloric supply was 28%, but several confounding factors may have reduced the extent of the correlations. For example, unavailability of birth and/or caloric supply data prevented the researchers from including many of the poorest countries. In fact, the 1980 World Tables indicated that 36% of all countries had a caloric supply per capita 100% of requirements while the countries meeting this definition in the study made up just 13% of all countries. Further, countries with caloric supplies 120% of requirements made up 54% of the countries in the study. Another concern was that food availability does not necessarily mean equal distribution or consumption of the food.