Sex ratio by country for total population. Blue represents more women, red more men than the world average of 1.01 males/female.

Sex ratio by country population aged below 15. Blue represents more women, red more men than the world average of 1.06 males/female.

Sex ratio by country for population above 65. Blue represents more women, red more men than the world average of 0.79 males/female.

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. Image File history File links Download high-resolution version (1357x628, 32 KB) Sex ratio of total population per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... Image File history File links Download high-resolution version (1357x628, 32 KB) Sex ratio of total population per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... Image File history File links Download high-resolution version (1338x621, 29 KB) Sex ratio of population aged below 15 years per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... Image File history File links Download high-resolution version (1338x621, 29 KB) Sex ratio of population aged below 15 years per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... Image File history File links Download high-resolution version (1341x618, 33 KB) Sex ratio of population aged over 65 years per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... Image File history File links Download high-resolution version (1341x618, 33 KB) Sex ratio of population aged over 65 years per country (2006 CIA World Factbook) smooth scale from blue to red: Blue: below 0. ... This article is about the mathematical concept. ... This article is about the Male sex. ... For other uses, see Female (disambiguation). ... For other uses, see Boy (disambiguation). ... For other uses, see Girl (disambiguation). ... Issues of discussion Sex-selective abortion is the targeted abortion of a fetus based upon its sex. ...

In biology, sex ratio is defined as the proportion of males in the population. For the song by Girls Aloud see Biology (song) Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: βίος, bio, life; and λόγος, logos, speech lit. ...

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). This article is about the measure of remaining life. ...

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, though this may be apocryphal^{[dubious – discuss]}. Parents frequently prefer their offspring to be of a particular sex for a variety of reasons. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Title page of the first edition of Charles Darwins The Descent of Man, and Selection in Relation to Sex. ... Baltic Tribes, ca 1200 CE This article is about the region in Europe. ... In Judeo-Christian theologies, apocrypha refers to religious Sacred text that have questionable authenticity or are otherwise disputed. ...

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: Fisher's principle

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] Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. ... Faithfulness redirects here. ... The term polygyny (neo-Greek: poly+gune Many + Wives) is used in related ways in social anthropology and sociobiology. ... Species M. leonina M. angustirostris Elephant seals are large, oceangoing mammals in the genus Mirounga, in the earless seal family (Phocidae). ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was an English statistician, evolutionary biologist, and geneticist. ... Clinton Richard Dawkins, FRS (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer who holds the Charles Simonyi Chair for the Public Understanding of Science at the University of Oxford. ... River out of Eden (subtitled A Darwinian View of Life) is a 1995 popular science book by Richard Dawkins. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was an English statistician, evolutionary biologist, and geneticist. ... The gene-centered view of evolution, gene selection theory or selfish gene theory holds that natural selection acts through differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation. ...

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] In game theory, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which if adopted by a population cannot be invaded by any competing alternative strategy. ... Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. ... In mathematics, an average or central tendency of a set (list) of data refers to a measure of the middle of the data set. ... This article does not cite any references or sources. ... A standard definition of mechanical equilibrium is: A system is in mechanical equilibrium when the sum of the forces, and torque, on each particle of the system is zero. ... In game theory, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which if adopted by a population cannot be invaded by any competing alternative strategy. ...

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] The term polygyny (neo-Greek: poly+gune Many + Wives) is used in related ways in social anthropology and sociobiology. ...

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. “HBV” redirects here. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... For other uses, see Inuit (disambiguation). ... Endocrine disruptors are substances which interfere with the endocrine system by mimicking, blocking or otherwise disrupting the function of hormones. ... Labelling transformers containing PCBs. ... For other uses, see DDT (disambiguation). ...

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^[12][13], but apparently not in humans ^[14]
• Whether the mother smokes ^[15]
• Miscarriages caused by maternal malnutrition, since they occur slightly more often to male foeti than to female ones ^[16]
• Whether the mother has a partner or other support network^[17], although this correlation is widely considered the effect of an unknown third factor
• Environmental temperature during pregnancy ^[18]

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.^[19] A live birth of a human being occurs when a fetus is expelled and separated from the mothers body and subsequently shows some sign of life, such as voluntary movement, heartbeat, or pulsation of the umbilical cord, but for however brief thistime. ... is the death of infants in the first year of life. ... Infant mortality is the death of infants in the first year of life. ... WHO redirects here. ...

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.^[20] 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.^[21] ... The phrase one-child policy is commonly used in English to refer to the population control policy (or Planned Birth policy) of the Peoples Republic of China (PRC). ... Sex-selective abortion is the practice of aborting a fetus after a determination (usually by ultrasound but also rarely by amniocentesis or another procedure) that the fetus is an undesired sex, typically female. ... In sociology and biology, infanticide is the practice of intentionally causing the death of an infant of a given species, by members of the same species - often by the mother. ... Vital statistics are the information maintained by a government, recording the birth and death of individuals within that governments jurisdiction. ... A census is the process of obtaining information about every member of a population (not necessarily a human population). ...

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. Sex-selective abortion is the practice of aborting a fetus after a determination (usually by ultrasound but also rarely by amniocentesis or another procedure) that the fetus is an undesired sex, typically female. ... In sociology and biology, infanticide is the practice of intentionally causing the death of an infant of a given species, by members of the same species - often by the mother. ...

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."^[22]

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^[23], and being recruited into the service of militaristic political factions. A demographic or demographic profile is a term used in marketing and broadcasting, to describe a demographic grouping or a market segment. ... For other uses, see War (disambiguation). ... Pie chart showing deaths by alliance and military/civilian. ... Military and civilian deaths during World War II for the Allied and the Axis Powers. ... Issues of discussion Sex-selective abortion is the targeted abortion of a fetus based upon its sex. ... Demographics of China, Data of FAO, year 2005 ; Number of inhabitants in thousands. ... The phrase one-child policy is commonly used in English to refer to the population control policy (or Planned Birth policy) of the Peoples Republic of China (PRC). ...

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). Motto ძალა ერთობაშია(Georgian) Strength is in Unity Anthem Tavisupleba Freedom Capital (and largest city) Tbilisi Official languages Georgian1 Demonym Georgian Government Semi-presidential unitary republic  -  President Mikheil Saakashvili  -  Prime Minister Zurab Noghaideli Consolidation  -  Georgian kingdoms of Colchis and Iberia c. ... World Factbook 2004 cover The World Factbook is an annual publication by the Central Intelligence Agency of the United States with basic almanac-style information about the various countries of the world. ...

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. Map of countries by population — China and India, the only two countries to have a population greater than one billion, together possess more than a third of the worlds population. ...

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. The terms First World, Second World, and Third World were used to divide the nations of Earth into three broad categories. ... G7 or G-7 or Group of Seven may be: Group of Seven (G7), a group of seven industrialized nations of the world, formed in 1976 when Canada joined the Group of Six (United States of America, France, Germany, Italy, Japan, United Kingdom); now known as G8 (with Russia) Group...

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. Arabia redirects here. ... A foreign worker (cf expatriate), is a person who works in a country other than the one of which he or she is a citizen. ... Statistical regions of Europe as delineated by the United Nations (UN definition of Eastern Europe marked red):  Northern Europe  Western Europe  Eastern Europe  Southern Europe Pre-1989 division between the West (grey) and Eastern Bloc (orange) superimposed on current borders: Russia (dark orange), other countries formerly part of the USSR... The three Baltic states: Estonia, Latvia, and Lithuania. ... Net migration rates for 2006: positive (blue), negative (orange) and stable (green). ...

List of sex ratios by country/region

Main article: List of countries by sex ratio

Sex ratio by country for total population. ...

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.^[24] He defines five major questions, both for his book and the field in general (slightly abbreviated here): Table of natural history, 1728 Cyclopaedia Natural history is an umbrella term for what are now often viewed as several distinct scientific disciplines of integrative organismal biology. ...

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). In biology, Dioecious is an adjective which indicates the exisistence of separate sexes in a species of organisms. ... A clownfish Sex change in animals Some species are known to change sex, including reproductive functions, in special circumstances, such as the clownfish. ...

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. In game theory, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which if adopted by a population cannot be invaded by any competing alternative strategy. ...

The bacterium wolbachia causes skewed sex ratios in some arthropod species as it kills males. Sex-ratio of adult populations of pelagic copepods is usually skewed towards dominance of females. However, there are differences in adult sex ratios between families: in families which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); and in families which females can produce eggs continuously after only one mating, sex ratios are strongly skewed towards females.^[25]. Phyla/Divisions Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Omnibacteria Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Bacteria (singular, bacterium) are a major group of living organisms. ... Wolbachia is a genus of inherited bacterium that infects arthropod species, including a high proportion of all insects. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc. ... Orders Calanoida Cyclopoida Gelyelloida Harpacticoida Misophrioida Monstrilloida Mormonilloida Platycopioida Poecilostomatoida Siphonostomatoida Copepods are small, aquatic animals living in the sea and nearly every freshwater habitat, a form of plankton, specifically zooplankton, some copepods are parasitic. ...

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),^[26][27] Melandrium album (Cariophyllaceae),^[28][29] Cannabis sativa^[30] and Humulus japonicus (Cannabinaceae).^[31] Genera See text The Polygonaceae, or the Knotweed Family, are a group of dicots including buckwheat, sorrel (but not wood sorrel), rhubarb, and knotgrass. ... Binomial name Linnaeus Subspecies L. subsp. ... Binomial name Humulus japonicus Siebold & Zucc. ... Genera Cannabis -- Hemp Humulus - Hops Cannabaceae is a family of flowering plants consisting of the genera Cannabis and Humulus. ...

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%.^[32] Peacock redirects here. ...

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

The Journal of Sex Research (or JSR, ISSN 0022-4499) is published by the Society for the Scientific Study of Sexuality registered in New York City. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was an English statistician, evolutionary biologist, and geneticist. ... The Genetical Theory of Natural Selection is a book by Ronald Fisher. ... Clinton Richard Dawkins, FRS (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer who holds the Charles Simonyi Chair for the Public Understanding of Science at the University of Oxford. ... Gods utility function is a phrase coined by Richard Dawkins in his book River Out of Eden. ... River out of Eden (subtitled A Darwinian View of Life) is a 1995 popular science book by Richard Dawkins. ... Clinton Richard Dawkins, FRS (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer who holds the Charles Simonyi Chair for the Public Understanding of Science at the University of Oxford. ... The Ancestors Tale cover The Ancestors Tale (subtitled A Pilgrimage to the Dawn of Life) is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins research assistant Yan Wong. ...

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.

Ansley Johnson Coale (1917-2002), was one of Americas foremost demographers. ... W. D. Hamilton William Donald Bill Hamilton, F.R.S. (1 August 1936 — 7 March 2000) was a British evolutionary biologist, considered one of the greatest evolutionary theorists of the 20th century. ... Science is the academic journal of the American Association for the Advancement of Science and is considered one of the worlds most prestigious scientific journals. ... Robert L. Trivers, (born 19 February 1943) is an American evolutionary biologist and sociobiologist, most noted for proposing the theories of reciprocal altruism (1971), parental investment (1972), and parent-offspring conflict (1974). ... Science is the academic journal of the American Association for the Advancement of Science and is considered one of the worlds most prestigious scientific journals. ...

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

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

Wikinews has related news:

India subsidizes girls' education to offset gender imbalance

Sex allocation is allocation of resources to male versus female reproduction. ... Parents frequently prefer their offspring to be of a particular sex for a variety of reasons. ... Issues of discussion Sex-selective abortion is the targeted abortion of a fetus based upon its sex. ... Demographic transition occurs in societies that transition from high birth rates and high death rates to low birth rates and low death rates as part of the economic development of a country from a pre-industrial to an industrialized economy. ... A population pyramid is two back-to-back bar graphs, one showing the number of males and one showing females in a particular population in five-year age groups (also called cohorts). ... The evolution of sex is a major puzzle in modern evolutionary biology. ... Drosophila sex-chromosomes The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila) and some plants (Ginkgo). ... Bride kidnapping, also known as marriage by abduction or marriage by capture, is a form of forced marriage practiced in a few traditional cultures, in countries including Kyrgyzstan, Turkmenistan, Kazakhstan, the Caucasus region, Ethiopia and Rwanda. ... Image File history File links WikiNews-Logo. ... Wikinews is a free-content news source and a project of the Wikimedia Foundation. ...

External links

• Declining sex ratios in industrialized countries - environmental hazards
• Sex ratio in communities with high Hepatitis B favours boys
• CIA listing of sex ratios for individual countries (including age divisions)
• A review of sex ratio theory
• Sex ratio of Marine Planktonic Copepods
• The Evolutionary Theory of Sex: Sex Ratio