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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.The relationship between natural factors and human sex ratio at birth, and with aging, remains an active area of scientific research.Like most sexual species, the sex ratio in humans is approximately 1:1.Due to higher female fetal mortality, war casualties, gender-selective abortions and infanticides, aging, and deliberate gendercide.For example, in the United States, as of 2006, an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.5 to 3.5 times more likely to die in an accident than a female of the same age.Consequently, the sex ratio tends to reduce as age increases and among the elderly there is usually a greater proportion of females.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.

Recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female.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.These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress.Remarkably, the trends in human sex ratio are not consistent across countries at a given time, or over time for a given country.