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Outlined by Ronald Fisher in his 1930 book, it is an argument in terms of parental expenditure.
Essentially he argues that the 1:1 ratio is the evolutionarily stable strategy.
This paper reports that the sex ratio at birth for the white ethnic group in the United States was 1.04 when the gestational age was 33–36 weeks, but 1.15 for gestational ages of less than 28 weeks, 28–32 weeks, and 37 or more weeks.
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.
Recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female.
Scientific literature often uses the proportion of males.
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, 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.
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.
Fisher's principle is an explanation of why the sex ratio of most species is approximately 1:1.