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Fisher's principle is an explanation of why the sex ratio of most species is approximately 1:1.Outlined by Ronald Fisher in his 1930 book, it is an argument in terms of parental expenditure.Infant mortality is higher in boys than girls in most parts of the world.

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.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.They find an increased excess of male births during periods of the exogenous stress (World War II) and during warm years.A 1999 scientific paper published by Jacobsen reported the sex ratio for 815,891 children born in Denmark between 1980–1993.