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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.Cold weather stressors simultaneously extend male longevity thereby raise human sex ratio in its older age bracket.Catalano team finds that a 1 °C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year. have studied 138 years worth of human birth sex ratio data, from 1865 to 2003.Consequently, the sex ratio tends to reduce as age increases and among the elderly there is usually a greater proportion of females.
Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such as climate change and global warming. report that cold weather is an environmental stressor, and women subjected to colder weather abort frail male fetuses in greater proportion thereby lowering birth sex ratios.This article uses the ratio of males to females, unless specified otherwise.Infant mortality is higher in boys than girls in most parts of the world.Recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female.It has been proposed that these environmental factors also explain sex differences in mortality.
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Outlined by Ronald Fisher in his 1930 book, it is an argument in terms of parental expenditure.