Human sex ratio as it relates to caloric availability.

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.