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It has been proposed that these environmental factors also explain sex differences in mortality.
In most populations, adult males tend to have higher death rates than adult females of the same age (even after allowing for causes specific to females such as death in childbirth), both due to natural causes such as heart attacks and strokes, which account for by far the majority of deaths, and also to violent causes, such as homicide and warfare, resulting in higher life expectancy of females.
For example, the male to female ratio falls from 1.05 for the group aged 15 to 65 to 0.70 for the group over 65 in Germany, from 1.00 to 0.72 in the United States, from 1.06 to 0.91 in mainland China, and from 1.07 to 1.02 in India.
In the United States, the sex ratios at birth over the period 1970–2002 were 1.05 for the white non-Hispanic population, 1.04 for Mexican Americans, 1.03 for African Americans and Indians, and 1.07 for mothers of Chinese or Filipino ethnicity.
A 1999 scientific paper published by Jacobsen reported the sex ratio for 815,891 children born in Denmark between 1980–1993.
They studied the birth records to identify the effects of multiple birth, birth order, age of parents and the sexes of preceding siblings on the proportion of males using contingency tables, chi-squared tests and regression analysis.
Essentially he argues that the 1:1 ratio is the evolutionarily stable strategy.
They find an increased excess of male births during periods of the exogenous stress (World War II) and during warm years.
Human sex ratios, either at birth or in the population as a whole, are 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.