In
anthropologyAnthropology is the study of human beings, everywhere and throughout time....
and
demographyDemography is the statistical study of all populations. It can be a very general science that can be applied to any kind of dynamic population, that is, one that changes over time or space...
, the
human sex ratio is the
sex ratioSex 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....
for
Homo sapiens (i.e. the
ratioA ratio is an expression that compares quantities relative to each other. The most common examples involve two quantities, but any number of quantities can be compared. Ratios are represented mathematically by separating each quantity with a colon – for example, the ratio 2:3, which is read as the...
of
maleMale refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
s to
femaleFemale is the sex of an organism, or a part of an organism, which produces mobile ova .- Defining Characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
s in a
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
). 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
boyA boy is a young male human , as contrasted to its female counterpart, girl, or an adult male, a man.The term "boy" is primarily used to indicate biological sex distinctions, cultural gender role distinctions or both...
s to 100
girlA girl is any female human from birth through childhood and adolescence to attainment of adulthood. The term may also be used to mean a young woman.-Etymology:...
s (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 to be close to 1.1 males/female. Due to the generally higher
life expectancyLife expectancy is the expected number of years of life remaining at a given age. It is denoted by e
x, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience...
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
GermanyThe Demographics of Germany were determined also by a series of full Census in Germany, with the most recent held in 1987. Since reunification, German authorities rely on a micro census....
, 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 selectionParents frequently prefer their offspring to be of a particular sex for a variety of reasons. Social sex selection or human sex selection is the attempt to control the sex of the offspring to achieve a desired sex. It can be accomplished in several ways, both pre- and post-implantation of an...
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.
DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
, in his
The Descent of Man, and Selection in Relation to SexThe Descent of Man, and Selection in Relation to Sex is a book on evolutionary theory by English naturalist Charles Darwin, first published in 1871. It was Darwin's second great book on evolutionary theory, following his 1859 work, On The Origin of Species...
, cites a sex ratio of 120 boys to 100 girls for Jewish communities in 19th century
LivoniaLivonia was once the land of the Finnic Livonians inhabiting the principal ancient Livonian County Metsepole with its center at Turaida...
, 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 Western 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.
Fisher's principle
Fisher's principle is an explanation of why the sex ratio of most species is approximately 1:1. Outlined by Ronald Fisher in his 1930 book, it is an argument in terms of parental expenditure. Essentially he argues that the 1:1 ratio is the evolutionarily stable strategy.
Environmental factors
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
InuitInuit is a general term for a group of culturally similar indigenous peoples inhabiting the Arctic regions of Canada, Greenland, and Alaska...
villages in
GreenlandGreenland is an autonomous constituent country within the Kingdom of Denmark located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago...
and
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
, and attributed this imbalance to high levels of
endocrine disruptorEndocrine disruptors are exogenous substances that act like hormones in the endocrine system and disrupt the physiologic function of endogenous hormones. They are sometimes also referred to as hormonally active agents or endocrine disrupting chemicals/compounds ...
s in the blood of inhabitants, including
PCBsPolychlorinated biphenyls are a class of organic compounds with 1 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx, where x = 1-10. PCBs were widely used for many...
and
DDTDDT is one of the most well-known synthetic pesticides. It is a chemical with a long, unique, and controversial history....
. 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.
A 2008 report demonstrates further evidence of effects of feminizing chemicals on male development in each class of
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
species as a wordwide phenomenon possibly leading to a decline in the sex ratio in humans and a possible decline in
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
counts. Ninety-nine percent of over 100,000 recently introduced chemicals are poorly regulated.
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 at conception.
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 birthA live birth occurs when a fetus, whatever its gestational age, exits the maternal body and subsequently shows any sign of life, such as voluntary movement, heartbeat, or pulsation of the umbilical cord, for however brief a time and regardless of whether the umbilical cord or placenta are...
or
infant deathInfant mortality is defined as the number of infant deaths per 1000 live births. The most common cause worldwide has traditionally been due to dehydration from diarrhea...
may vary from one society to another. For example, for most of the 20th century in Russia (and the
Soviet UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
), 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 OrganizationThe World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health...
.
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 ChinaMainland China, Continental China, the Chinese mainland or simply the mainland, is a geopolitical term that refers to the area under the jurisdiction of the People's Republic of China , excluding Hong Kong and Macau, which are under the jurisdiction of the PRC but run on different economic and...
in the last 25 years in part to the underreporting of the births of female children after the implementation of the
one-child policyThe one-child policy is the population control policy of the People's Republic of China . The Chinese government refers to it under the official translation of family planning policy...
, though alternative explanations are now generally more widely accepted, including above all the use of ultrasound technology and
sex-selective abortionSex-selective abortion are methods of sex-selection which are practiced in areas where male children are valued over female children...
of female fetuses and, probably to a more limited degree, neglect or in some cases
infanticideInfanticide is the practice of someone intentionally killing an infant. Often it is the mother who commits the act, but criminology recognizes various forms of non-maternal child murder. In many past societies, certain forms of infanticide were considered permissible...
of females. In the case of China, because of deficiencies in the
vital statisticsVital statistics are the information maintained by a government, recording the birth and death of individuals within that government's jurisdiction. These data are used by public health programs to evaluate how effective their programs are...
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 abortionSex-selective abortion are methods of sex-selection which are practiced in areas where male children are valued over female children...
and
infanticideInfanticide is the practice of someone intentionally killing an infant. Often it is the mother who commits the act, but criminology recognizes various forms of non-maternal child murder. In many past societies, certain forms of infanticide were considered permissible...
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. Sons are preferred as an 'asset', since they can earn and support the family, daughters are a 'liability' since she is to be married off to another family, and will not be able to contribute financially to her parents. Also, rearing a girl child would mean arranging a huge
dowryA dowry is the money, goods, or estate that a woman brings to her husband in marriage. Compare bride price, which is paid to the bride's parents, and dower, which is property settled on the bride herself by the groom at the time of marriage. The same culture may simultaneously practice both dowry...
for her marriage. The patriarchal structure of the society is the single most important factor which skews the sex ratio in favor of the males. 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
warWar is a reciprocated, armed conflict, between two or more non-congruous entities, aimed at reorganising a subjectively designed, geo-politically desired result...
fare (excess of females, notably in the wake of
WWIThe total number of casualties in World War I, both military and civilian, were about 37 million: 16 million deaths and 21 million wounded.The total number of deaths includes 9.7 million military personnel and about 6.8 million civilians...
in western Europe, and
WWIIWorld War II was the deadliest military conflict in history. Over 60 million people were killed. The tables below give a detailed country-by-country count of human losses.-Total dead:...
, particularly in
the Soviet UnionThis articles details the demographics of the Soviet Union.According to data from the 1989 Soviet census, the majority of the population of Soviet Union was atheist, ethnic Russian and lived in Eastern Europe and in the Russian Soviet Federated Socialist Republic, the Soviet Republic which...
) or of sex-selective abortion and infanticide (excess of males, notably in
mainland ChinaThe demographics of the People's Republic of China are characterized by a large population with a relatively small youth cohort, which is partially a result of the People's Republic of China's one-child policy. The population policies implemented in China since 1979 have helped to prevent an extra...
as a result of the
one-child policyThe one-child policy is the population control policy of the People's Republic of China . The Chinese government refers to it under the official translation of family planning policy...
, or in
IndiaThe demographics of India is remarkably diverse. India's population of approximately 1.17 billion people consists of approximately one-sixth of the world's population...
), or of large-scale immigration, such as that of male labourers unable to bring their families with them (as in
QatarQatar , also known as the State of Qatar or locally ', is an Arab emirate in the Middle East, occupying the small Qatar Peninsula on the northeasterly coast of the larger Arabian Peninsula...
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.
One study 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 IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
for the most severely affected age cohort (those between 21 and 23 years old in 1946). This same study found that out-of-wedlock births spiked from approximately 10-15% during the inter-war years up to 22% at the end of the war. This increase in out-of-wedlock births was attributed to a change in the marriage market caused by the decline in the sex-ratio.
The
Northern Mariana IslandsThe Northern Mariana Islands , officially the Commonwealth of the Northern Mariana Islands , is a commonwealth in political union with the United States, occupying a strategic region of the western Pacific Ocean. It consists of fifteen islands about three-quarters of the way from Hawaii to the...
have the highest female ratio with 0.77 males/female.
QatarQatar , also known as the State of Qatar or locally ', is an Arab emirate in the Middle East, occupying the small Qatar Peninsula on the northeasterly coast of the larger Arabian Peninsula...
has the highest male ratio, with 2.87 males/female. For the group aged below 15,
Sierra LeoneSierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea in the north, Liberia in the southeast, and the Atlantic Ocean in the southwest. Sierra Leone covers a total area of and has a population estimated at 6.4 million...
has the highest female ratio with 0.96 males/female, and the
Republic of GeorgiaGeorgia Georgia Georgia is a country in the Caucasus region of Eurasia. Situated at the juncture of Western Asia and Eastern Europe, it is bounded to the west by the Black Sea, to the north by Russia, to the south by Turkey and Armenia, and to the east by Azerbaijan...
and the
People's Republic of ChinaThe People's Republic of China , commonly known as China, is the largest country in East Asia and the most populous in the world with over 1.3 billion people, approximately one-fifth of the world's population...
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 populationThe term world population commonly refers to the total number of living humans on Earth at a given time. As of , the Earth's population is estimated by the United States Census Bureau to be billion. The world population has been growing continuously since the end of the Black Death around 1400...
is 1.01 males/female, with 1.07 at birth, 1.06 for those under 15, 1.02 for those between 15 and 64, and 0.78 for those over 65.
The "
First WorldThe terms First World, Second World, and Third World were used to divide nations into three broad categories. The three terms did not arise simultaneously. After World War II, people began to speak of the NATO and Warsaw Pact countries as two major blocs, often using such terms as the "Western...
" 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 peninsulaThe Arabian Peninsula , Arabia, Arabistan, and the Arabian subcontinent is a peninsula in Southwest Asia at the junction of Africa and Asia...
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 EuropeEastern Europe is a region lying in the Eastern part of Europe. The term is highly context-dependent and there are "almost as many definitions of Eastern Europe as there are scholars of the region"...
(the Baltic states,
BelarusBelarus is a landlocked country in Eastern Europe, bordered by Russia to the north and east, Ukraine to the south, Poland to the west, and Lithuania and Latvia to the north. Its capital is Minsk; other major cities include Brest, Grodno , Gomel , Mahilyow and Vitebsk...
,
UkraineUkraine is a country in Eastern Europe. It is bordered by Russia to the east; Belarus to the north; Poland, Slovakia, and Hungary to the west; Romania and Moldova to the southwest; and the Black Sea and Sea of Azov to the south. The city of Kiev is both the capital and the largest city of...
,
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
) 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,
ArmeniaArmenia , officially the Republic of Armenia , is a landlocked mountainous country in the Caucasus region of Eurasia...
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
emigrationHuman migration is movement by humans from one district to another, sometimes over long distances or in large groups....
and higher male mortality as result of higher Soviet era deaths.
See also
- Bride kidnapping
Bride kidnapping, also known as marriage by abduction or marriage by capture, is a practice throughout history and around the world in which a man abducts the woman he wishes to marry...
- Demographic transition
The Demographic transition model is a model used to represent the process of explaining the transformation of countries 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...
- Lost boys (polygamy)
- Missing women of Asia
The phenomenon of the missing women of Asia is a shortfall in the number of women in Asia relative to the number that would be expected if there was no sex-selective abortion or female infanticide or if the newborn of both sexes received similar levels of health care and nutrition.The phenomenon...
- Sex selection
Parents frequently prefer their offspring to be of a particular sex for a variety of reasons. Social sex selection or human sex selection is the attempt to control the sex of the offspring to achieve a desired sex. It can be accomplished in several ways, both pre- and post-implantation of an...
- Sex-selective abortion and infanticide
- XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex...
- Youth Bulge