Sex ratio

Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms.^[1] In humans the secondary sex ratio 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.

In biology, sex ratio is defined as the proportion of males in the population.

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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^[2]. 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.

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.^[3] 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^[4] in African countries, the ratio is 103, though there is also considerable country-to-country variation.^[5]

Factors affecting sex ratio in humans

Genetic

Further information: Parental expenditure

Males and females are produced in approximately equal numbers in most sexually reproducing species, regardless of the mechanism of sex determination. At first, this seems to make sense, if one considers a socially-monogamous species such as humans. However, in a harem-based system such as that of elephant seals where 4 percent of males account for 88 percent of all copulations, the actual sex ratio of 50:50 seems to produce an excess of males who consume resources but end up leaving no offspring. Furthermore, these excess males typically weigh about two to three times the weight of females. The puzzle of 50:50 sex ratio in the face of great evolutionary waste troubled Charles Darwin. In the first edition of Origin of Species he provided a verbal explanation similar to that which R.A. Fisher would provide in 1930, but in subsequent editions Darwin backed off and instead wrote that it would be safer to leave this problem for the future. Richard Dawkins in his book, River out of Eden, described how such seemingly wasteful production of males was solved by the concept of parental expenditure by R.A. Fisher and the gene-centered view of evolution.^[6][7][8]

The 50:50 sex ratio is known to be evolutionarily stable. Every child has exactly one father and one mother. If reproductive success is measured by number of children produced, then the total reproductive success of all living males must equal that of all living females. If the sex ratio remains at 50:50, then males and females enjoy equal average reproductive success per individual. However, in a population of animals, the sex ratio could theoretically change over time, resulting in differences in average reproductive success between males and females. For instance, a sex ratio of 25:50 would make an average male twice as reproductively successful as an average female. Such inequality in reproductive success would give parents (in evolutionary speak) an incentive to produce more boys, assuming that parental expenditure is the same for boys and girls, and that both of them will have equal opportunities at mating in the future. Over time, such negative feedback by parents will bring the sex ratio towards the 50:50 equilibrium. Parents' choice of producing children belonging to the minority gender is said to be an evolutionarily stable strategy.^[9]

The same logic based on average reproductive success can be applied again to solve the harem problems in polygynous species, even though males in an elephant seal population have wildly different opportunities at mating. This is because the average reproductive success does not change, as the total reproductive success of males remains pegged to that of female. As a result, the evolutionarily stable sex ratio in a harem system is still 50:50.^[7]

Fisher's theory of parental expenditure posits that it is the parental expenditure on male and female children which is held by nature at a constant 50:50, not the actual sex ratio. For instance, if it takes twice the amount of energy and effort to give birth to a male child compared to a female child, one would expect a sex ratio of 25:50, because for the price (parental expenditure) of one male child, a parent can afford to produce two female children.^[7]

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.^[10] 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. ^[11] 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^[12][1][2]
• Whether the mother smokes [3]
• Maternal malnutrition [4]
• Whether the mother has a partner or other support network [5]

Social

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.^[13]

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.^[14] 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.^[15]

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."^[16]

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 social unrest, especially in the case of an excess of young males unable to find a spouse. But it is equally a problem when there are significantly more women than men, especially for young women trying to find a spouse.

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 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.

List of sex ratios by country/region

Main article: List of countries by sex ratio

Sex ratio theory

The theory of sex ratio is a field of study concerned with the accurate prediction of sex ratios in all sexual species, based on a consideration of their natural history. The field continues to be heavily influenced by Eric Charnov's 1982 book, Sex Allocation.^[17] He defines five major questions, both for his book and the field in general (slightly abbreviated here):

1. For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
2. For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
3. For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
4. Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
5. When does selection favour the ability of an individual to alter its allocation to male versus female function, in response to particular environmental or life history situations?

Biological research mostly concerns itself with sex allocation rather than sex ratio, sex allocation denoting the allocation of energy to either sex. Common research themes are the effects of local mate and resource competition (often abbreviated LMC and LRC, respectively).

Examples in non-human species

Environmental

Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategy: if the general population deviates from this equilibrium by favoring one sex, one can obtain higher reproductive success with less effort by producing more of the other. For species where the cost of successfully raising one offspring is roughly the same regardless of its sex, this translates to an approximately equal sex ratio.

The bacterium wolbachia causes skewed sex ratios in some arthropod species as it kills males. For the copepods the sex ratio is seldom close to 1, but generally in favour of the females.

Dioecious plants secondary sex ratio and amount of pollen

It was found that the amount of fertilizing pollen can influence secondary sex ratio in dioecious plants. Increase in pollen amount leads to decrease in number of male plants in the progeny. This relationship was confirmed on four plant species from three families – Rumex acetosa (Polygonaceae),^[18][19] Melandrium album (Cariophyllaceae),^[20][21] Cannabis sativa^[22] and Humulus japonicus (Cannabinaceae).^[23]

Individual

In birds, mothers can influence the gender of their chicks. In peafowl, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.^[24]

Economical

Traditionally, farmers have discovered that the most economically efficient community of animals will have a large number of females and a very small number of males. A herd of cows and a few prize bulls or a flock of chickens and one rooster are the most economical sex ratios for domesticated livestock.

Notes and citations

Some general sources

• Coale, Ansley J. (1996). "Five Decades of Missing Females in China," Proceedings of the American Philosophical Society 140 (4): 421-450.
• Hamilton, W.D. (1967). Extraordinary sex ratios. Science 156: 477-488. pubmed JSTOR
• Nishimura, K. & G. C. Jahn 1996. "Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: sex ratio change with host quality and local mate competition," Journal of Ethology 14 (1): 27-34.
• Trivers R. L. & Willard D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science 170 90-92.

Further reading

• Stone, E.A., Shackelford, T.K. & Buss, D.M. (2007). Sex ratio and mate preferences: A cross-cultural investigation. European Journal of Social Psychology, 37, 288-296. Full text

See also

• Sex allocation
• Sex selection
• Sex-selective abortion and infanticide
• Demographic transition
• Youth Bulge
• Evolution of sex
• XY sex-determination system