Human sex ratios, either at birth or in the population as a whole, can be reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females.
Various metric measurements of the skull were collected using a 3D measuring tool between 18 standard points of measurement of the cranium and mandible largest bone in the skull.
None of the natural factors such as paternal age, maternal age, age difference of parents, birth order could explain the time trends.
Mémoires de l'Académie royale des sciences de Paris.
Second, there is pressure for women to give birth to sons, often because of cultural preferences for male heirs.
Some scholars argue that strong socioeconomic factors such as the and the of China are responsible for prenatal sex selection.
These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets.
They find an increased excess of male births during periods of exogenous stress World War II and during warm years.