Human sex ratio

In anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens (i.e. the ratio of males to females in a population). Like most sexual species, the sex ratio is approximately 1:1. In humans the secondary sex ratio (i.e. at birth) is commonly assumed to be 105 boys to 100 girls (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth or among infants may be considerably skewed by sex-selective abortion and infanticide. 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.

Natural ratio
The natural sex ratio at birth is estimated close to 1.05 males/female. Due to the generally higher life expectancy of females, sex ratio tends to even out in adult population, and result in an excess of females among the elderly (e.g., 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 USA, from 1.06 to 0.91 in mainland China and from 1.07 to 1.02 in India).

Even in the absence of sex selection practices, a range of "normal" sex ratios at birth of between 103 to 107 boys per 100 girls has been observed in different societies, and among different ethnic and racial groups within a given society. Darwin, in his The Descent of Man, and Selection in Relation to Sex, cites a sex ratio of 120 boys to 100 girls for Jewish communities in 19th century Livonia, where infanticide is not historically documented, and the means for pre-natal sex determination did not exist, though this may be apocryphal.

In the United States, the sex ratios at birth over the period 1970-2002 were 105 for the white non-Hispanic population, 104 for Mexican Americans, 103 for African Americans and Indians, and 107 for mothers of Chinese or Filipino ethnicity. Among European countries ca. 2001, the ratios ranged between 104 in Belgium and 107 in Portugal. In the aggregated results of 56 Demographic and Health Surveys in African countries, the ratio is 103, though there is also considerable country-to-country variation.

Environmental
Higher incidence of Hepatitis B virus in populations is believed to increase the sex ratio while some unexplained environmental health hazards are thought to have the opposite effect. 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. These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets. However, as noted in the Social Factors section below, it is important to exclude alternative explanations, including social ones, when examining large human populations whose composition by ethnicity and race may be changing.

Other factors that could possibly affect the sex ratio include:
 * Social status of the mother, known to be a factor in influencing the sex ratio of certain animals such as swine, but apparently not in humans
 * Whether the mother smokes
 * Miscarriages caused by maternal malnutrition, since they occur slightly more often to male foeti than to female ones
 * Whether the mother has a partner or other support network, although this correlation is widely considered the effect of an unknown third factor
 * Environmental temperature during pregnancy

Data sources and data quality issues
Sex ratios at birth for human societies may be unusual not only because of cultural preferences and social practices that favor the birth or survival of one sex over the other (more often favoring males than favoring females) but also because of incomplete or inaccurate reporting or recording of the births or the survival of infants. Even what constitutes a live birth or infant death may vary from one society to another. For example, for most of the 20th century in Russia (and the Soviet Union), extremely premature newborns (less than 28 weeks gestational age, or less than 1000 grams in weight, or less than 35 centimeters in length) were not counted as a live birth until they had survived 7 days; and if that infant died in those first 168 hours it would not be counted as an infant death. Such a practice led to serious underestimation of the Infant mortality rate (by 22 to 25 percent) relative to standards recommended by the World Health Organization.

When unusual sex ratios at birth (or any other age) are observed, it is important to examine misreporting, misrecording, or underregistration of births or deaths as a possible explanatory factor. Some researchers have attributed the highly masculine sex ratios observed in mainland China in the last 25 years in part to the underreporting of the births of female children after the implementation of the one-child policy, though alternative explanations are now generally more widely accepted, including above all the use of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide of females. In the case of China, because of deficiencies in the vital statistics registration system studies of sex ratios at birth have relied either on special fertility surveys, whose accuracy depends on whether the respondents fully report the births and survival of both male and female infants, or on the national population census for which both birth rates and death rates are calculated from the household’s reporting of births and deaths that occurred in the 18 months preceding the census. To the extent that household underreporting of births or deaths is sex-selective, both fertility surveys and censuses may inaccurately reflect the actual sex ratios at birth.

Illustrations of social factors at work
Sex-selective abortion and infanticide are thought to significantly skew the naturally occurring ratio in some populations. These practices are said to be based on a strong cultural preference for one sex -- typically males -- over the other. Reported sex ratios at birth -- outside the typical range of 103:100 to 107:100 -- thus call for an explanation of some kind. In other populations that have witnessed declining sex ratios, researchers have suggested that ecological factors may be at work. In all such reports, it is important to exclude plausible alternative explanations.

As an example of how the social composition of a human society may produce seemingly unusual changes in sex ratios, we can take a study in several counties of California where declining sex ratios had been observed. Smith and Von Behren observe that: "In the raw data, the male birth proportion is indeed declining. However, during this period, there were also shifts in demographics that influence the sex ratio. Controlling for birth order, parents’ age, and race/ethnicity, different trends emerged. White births (which account for over 80%) continued to show a statistically significant decline, while other racial groups showed non-statistically significant declines (Japanese, Native American, other), little or no change (black), or an increase (Chinese). Finally, when the white births were divided into Hispanic and non-Hispanic (possible since 1982), it was found that both white subgroups suggest an increase in male births." They concluded "that the decline in male births in California is largely attributable to changes in demographics."

Gender imbalance
Gender imbalance is a demographic effect that may arise either as a consequence of warfare (excess of females, notably in the wake of WWI in western Europe, and WWII, particularly in the Soviet Union) or of Sex-selective abortion and infanticide (excess of males, notably in mainland China as a result of the one-child policy, or in India), or of large-scale immigration, such as that of male labourers unable to bring their families with them (as in Qatar and other Gulf countries.) 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 a spouse, and being recruited into the service of militaristic political factions.

The Northern Mariana Islands have the highest female ratio with 0.77 males/female. Qatar has the highest male ratio, with 1.87 males/female. For the group aged below 15, Sierra Leone has the highest female ratio with 0.96 males/female, and the Republic of Georgia and the People's Republic of China are tied for the highest male ratio with 1.13 males/female (according to the 2006 CIA World factbook).

The value for the entire world population is 1.01 males/female, with 1.06 at birth, still 1.06 for those under 15, 1.03 for those between 15 and 64, and 0.79 for those over 65.

The "First World" G7 members all have a gender ratio in the range of 0.95–0.98 for the total population, of 1.05–1.07 at birth, of 1.05–1.06 for the group below 15, of  1.00–1.04 for the group aged 15–64, and of  0.70–0.75 for those over 65.

Countries on the Arabian peninsula tend to have a 'natural' ratio of ca. 1.05 at birth but a very high ratio of males for those over 65 (Saudi Arabia 1.13, Arab Emirates 2.73, Qatar 2.84), indicating either an above average mortality rate for females or a below average mortality for males, or, more likely in this case, a large population of aging male guest workers. Conversely, countries of Eastern Europe (the Baltic states, Belarus, Ukraine, Russia) tend to have a 'normal' ratio at birth but a very low ratio of males for those over 65 (Russia 0.46, Latvia 0.48, Ukraine 0.52); similarly, Armenia has a far above average male ratio at birth (1.17), and a below average male ratio above 65 (0.67). This effect may be caused by emigration of males as well as by higher male mortality.