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He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.02 and 1.08.However the ratio may deviate significantly from this range for natural reasons.In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0.94 and 1.15 for natural reasons.In a scientific paper published in 2008, James cautions that available scientific evidence stands against the above assumptions and conclusions.
They find an increased excess of male births during periods of the exogenous stress (World War II) and during warm years.
In the warmest period over the 138 years, the birth sex ratio peaked at about 1.08 in northern Europe. Also, higher incidence of Hepatitis B virus in populations is believed to increase the male to female sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect.
The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports. examined a data set of 67,000 births in China, 15 percent of whom were Hepatitis B carriers.
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.
If there are 108,000 males and 100,000 females the ratio of males to females is 1.080 and the proportion of males is 51.9%.
This study also found that the sex ratios at birth in the United States, between 1981–2006, were lower in both black and Hispanic ethnic groups when compared with white ethnic group.