page_title: Human sex ratio
section_title: Natural ratio
hierarchical_section_title: Human sex ratio / Natural ratio
caption_attribution_description: Don't abandon baby girls: the characters in red on the roadside sign in Danshan Township, Sichuan Party Committee and government reads "It is forbidden to discriminate against, mistreat or abandon baby girls." Photographed September 2005.
context_page_description: In anthropology and demography, the human sex ratio is the ratio of males to females in a population. More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult.
  Like most sexual species, the sex ratio in humans is close to 1:1. In humans, the natural ratio between males and females at birth is slightly biased towards the male sex, being estimated to be about 1.05 or 1.06 or within a narrow range from 1.03 to 1.06 males/per female born. Sex imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants, war casualties, sex-selective abortions, infanticides, aging, gendercide and problems with birth registration.
  The sex ratio for the entire world population is 101 males to 100 females. Depending upon which definition is used, between 0.018% and 1.7% of live births are intersex.
context_section_description: In a study around 2002, the natural sex ratio at birth was estimated to be within a narrow range of 1.03 to 1.06
  males/female. Some scholars suggest that countries considered to have significant practices of prenatal sex-selection are those with birth sex ratios of 1.08 and above (selection against females) and 1.02 and below (selection against males). This assumption has been questioned by some scholars.
  Infant mortality is significantly higher in boys than girls in most parts of the world. This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death. 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. 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), due to both natural causes such as heart attacks and strokes, which account for by far the majority of deaths, and also 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. Consequently, the sex ratio tends to reduce as age increases, and among the elderly there is usually a greater proportion of females. For example, the male to female ratio falls from 1.05 for the group aged 15 to 65 to 0.70 for the group over 65 in Germany, from 1.00 to 0.72 in the United States, from 1.06 to 0.91 in mainland China, and from 1.07 to 1.02 in India.
  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. Among Western European countries around 2001, the ratios ranged from 1.04 in Belgium to 1.07 in Switzerland, Italy, Ireland and Portugal. In the aggregated results of 56 Demographic and Health Surveys in African countries, the ratio is 1.03, albeit with considerable country-to-country variation.
  There is controversy about whether sex ratios outside the 1.03–1.07 range are due to sex selection, as suggested by some scholars, or due to natural causes. Some scholars argue that strong socioeconomic factors such as the dowry system in India and the one child policy of China are responsible for prenatal sex selection. In a widely cited article, Amartya Sen supported such views. Other researchers argue that an unbalanced sex ratio should not be automatically held as evidence of prenatal sex selection; Michel Garenne reports that many African nations have, over decades, witnessed birth sex ratios below 1.00, that is more girls are born than boys. Angola, Botswana and Namibia have reported birth sex ratios between 0.94 and 0.99, which is quite different from the presumed "normal" sex ratio, meaning that significantly more girls have been born in such societies.
  In an extensive study, carried out around 2005, of sex ratio at birth in the United States from 1940 over 62 years, statistical evidence suggested the following:
  For mothers having their first baby, the total sex ratio at birth was 1.06 overall, with some years at 1.07.
  For mothers having babies after the first, this ratio consistently decreased with each additional baby from 1.06 towards 1.03.
  The age of the mother affected the ratio:
  the overall ratio was 1.05 for mothers aged 25 to 35 at the time of birth
  mothers who were below the age of 15 or above 40 had babies with a sex ratio ranging between 0.94 and 1.11, and a total sex ratio of 1.04
caption: A roadside sign in rural Sichuan: "It is forbidden to discriminate against, mistreat or abandon baby girls."
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