Human sex ratio

[4] The sex ratios at birth and of the total population are affected by various factors including natural factors, exposure to pesticides and environmental contaminants,[5][6] war casualties, effects of war on men, sex-selective abortions, infanticides,[7] aging, gendercide, problems with birth registration and sex differences in life expectancy.

[10][11] Recent studies have found that numerous preconception or prenatal environmental factors affect the probabilities of a baby being conceived male or female.

In the aggregated results of 56 demographic and health surveys[17] in African countries, the ratio is 1.03, not[clarification needed] with considerable country-to-country variation.

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.

[21] 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:[22] The human sex ratio at birth has been an object of study since early in the history of statistics, as it is easily recorded and a large number for sufficiently large populations.

Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.

[35][36] The scientific studies are based on extensive birth and death records in Africa, the Americas, Asia, Australia, and Europe.

An analysis study from 2015 showed that human sex ratio at conception is roughly 50%, but rises due to total mortality surplus of female embryos.

A 2009 research paper published by Branum et al. reports the sex ratio derived from data in United States birth records over a 25-year period (1981–2006).

A research group led by Ein-Mor reported that sex ratio does not seem to change significantly with either maternal or paternal age.

For example, James suggested[41] that Ein-Mor results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend effects of length of gestation than those reported by Ein-Mor.

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.

Cold weather stressors also extend male longevity, thereby raising the human sex ratio at older ages.

[43] The Catalano team found that a 1 °C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year.

They find an increased excess of male births during periods of exogenous stress (World War II) and during warm years.

[44] Causes of stress during gestation, such as maternal malnutrition,[45] generally appear to increase fetal deaths, particularly among males,[43] resulting in a lower sex ratio at birth.

A higher incidence of Hepatitis B virus in a population is believed to increase the sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect.

[46] The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports.

[47] A 2007 survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canada, and attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT.

[54] This scientific team evaluated whether Finnish long-term data are compatible with the hypothesis that the decrease in the ratio of male to female births in industrial countries is caused by environmental factors.

[clarification needed] Moreover, these scientists claim that the trends they found in Finland are similar to those observed in other countries with higher levels of pollution and much greater pesticide use.

[55][56] As an example of how the social composition of a human population may produce unusual changes in sex ratios, in a study in several counties of California where declining sex ratios had been observed, Smith and Von Behren observe "In the raw data, the male birth proportion is indeed declining.

These studies suggest that social factors such as early marriage and males siring their children at a young age may play a role in raising birth sex ratios in certain societies.

[62] Catalano has examined the hypothesis that population stress induced by a declining economy reduces the human sex ratio.

According to Catalano's study, assuming women in East Germany did not opt to abort male fetuses more than female fetuses, the best hypothesis is that a collapsing economy lowers the human birth sex ratio, while a booming economy raises the birth sex ratio.

Since their independence from the Soviet Union, the birth sex ratio in these Caucasus countries has risen sharply, to between 1.11 and 1.20, among the world's highest.

[71] Gender imbalance may result in the threat of social unrest, especially in the case of an excess of low-status young males unable to find spouses,[72] and being recruited into the service of militaristic political factions.

[73] One study[74] found that the male-to-female sex ratio in the German state of Bavaria fell as low as 0.60 after the end of World War II for the most severely affected age cohort (those between 21 and 23 years old in 1946).

For example, the native population of Cusco, Peru at the time of the Spanish conquest was stressed by an imbalance in the sex ratio between men and women.

[79] Analyses of how sex ratio imbalances affect personal consumption and intra-household distribution were pioneered by Gary Becker, Shoshana Grossbard-Shechtman,[80][81] and Marcia Guttentag and Paul Secord.