Sex ratio
Encyclopedia : S : SE : SEX : 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.Nancy S. Coney and W. C. Mackey. 1998. "The woman as final arbiter: a case for the facultative character of the human sex ratio," Journal of Sex Research 35 (May): 169-175. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls (which for convenience we shall refer to as 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. Even in the absence of such 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, though more extreme ratios documented in some populations should be attributed rather more to cultural preferences than to biological variation in the propensity to bear boys or girls. However, 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.T. J. Matthews, et al., "Trend Analysis of the Sex Ratio at Birth in the United States," National Vital Statistics Reports 53, No. 20 (June 2005). 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[link] in African countries, the ratio is 103, though there is also considerable country-to-country variation. Michel Garenne, "Sex ratios at birth in African populations: A review of survey data," Human Biology 74 (December, 2002): 889-900.
By contrast, in biology, sex ratio is defined as the proportion of males in the population.
Factors affecting sex ratio in humans
Genetic
-->Males and females are produced in approximately equal numbers in most species with separate sexes, regardless of the mechanism of sex determination.
R.A. Fisher, in his 1930 book The Genetical Theory of Natural Selection outlined a model that explains the (approximately) 1:1 sex ratio commonly seen.Fisher, R.A. (1930) The Genetical Theory of Natural Selection. Clarendon Press, Oxford. Fisher's argument goes as follows (cf. The Genetical Theory of Natural Selection, pp. 142-143): Parents incur some level of expenditure to raise offspring. The reproductive value of male offspring is equal to the reproductive value of female offspring, since each individual of the following generation has exactly one male and one female parent. It follows that parental expenditures for males and females are equal. For, if, say, the parental expenditure for males were less than the parental expenditure for females, then a parent with a congenital bias to generate male offspring would produce, for the same amount of parental expenditure, offspring with a higher reproductive value than a parent with a "female bias" would. Hence the sex ratio would be raised until parental expenditures for males and females become equal. It follows that, if parental expenditure for individual male and female offspring are roughly the same (as is the case, for example, in humans), then the sex ratio is approximately 1:1, or 1/2 by the definition commonly used in biology.
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. Devra L. Davis, Michelle B. Gottlieb, and Julie R. Stampnitzky. 1998. "Reduced Ratio of Male to Female Births in Several Industrial Countries: A Sentinel Health Indicator?" Journal of the American Medical Association,279 (April): 1018-1023. 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.Social
Data sources and data quality issues. It should be borne in mind that 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 that 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.Barbara A. Anderson and Brian D. Silver. 1986. "Infant Mortality in the Soviet Union: Regional Differences and Measurement Issues," Population and Development Review 12, No. 4: 705-737.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.For a study in China that revealed underreporting or delayed reporting of female births, see M. G. Merli and A. E. Raftery. 1990. "Are births underreported in rural China? Manipulation of statistical records in response to China's population policies," Demography 37 (February): 109-126.
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 decling 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 dividied 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." Daniel Smith and Julie Von Behren. 2005. "Trends in the sex ratio of California births, 1960–1996," Journal of Epidemiology and Community Health 59: 1047-1053.
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.Charnov, E. L. (1982) Sex allocation. Princeton University Press, Princeton, New Jersey. ISBN 0691083126 He defines five major questions, both for his book and the field in general (slightly abbreviated here):- For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
- For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
- For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
- Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
- 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?
Examples in non-human species
Environmental
The bacterium wolbachia causes skewed sex ratios in some arthropod species as it kills males.References
Some General sources
- Hamilton, W.D. (1967). Extraordinary sex ratios. Science 156: 477-488. [pubmed] [JSTOR]
- Trivers R. L. & Willard D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science 170 90-92.
See also
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]
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.