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The gender ratio is the ratio of males to females in a population. In most sexually reproducing species, the ratio tends to be close to 1:1. Reasons for the tendency are discussed in Fisher's principle. For various reasons however many species deviate from anything like an even gender ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as ''Polistes fuscatus'' and ''Polistes exclamans'', bees, ants, and termites.〔Kobayashi, Kazuya; Hasegawa, Eisuke; Yamamoto, Yuuka; Kawatsu, Kazutaka; Vargo, Edward L.; Yoshimura, Jin; Matsuura, Kenji. "Sex ratio biases in termites provide evidence for kin selection" Nat Commun, 2013/06/28/ http://dx.doi.org/10.1038/ncomms3048 10.1038/ncomms3048〕 The human gender ratio is of particular interest to anthropologists and demographers. In human societies, however, gender ratios at birth may be considerably skewed by factors such as the age of mother at birth,〔(【引用サイトリンク】url=http://www.cdc.gov/nchs/data/nvsr/nvsr53/nvsr53_20.pdf )〕 and by gender-selective abortion and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well.〔Davis, Devra Lee; Gottlieb, Michelle and Stampnitzky, Julie; "Reduced Ratio of Male to Female Births in Several Industrial Countries" in ''Journal of the American Medical Association''; April 1, 1998, volume 279(13); pp. 1018-1023〕 As of 2014, the global gender ratio at birth is estimated at 107 boys to 100 girls (1000 boys per 934 girls).〔(【引用サイトリンク】 url=https://www.cia.gov/library/publications/the-world-factbook/fields/2018.html )〕 == Types == In most species, the sex ratio varies according to the age profile of the population. It is generally divided into four subdivisions: * primary sex ratio — ratio at fertilization * secondary sex ratio — ratio at birth * tertiary sex ratio — ratio in sexually mature organisms * * Also called ''adult sex ratio'' and abbreviated to ''ASR''. ASR is defined as the proportion of adults in a population that are male. 〔Wilson, K. & Hardy, I.C.W. (2002) “Statistical analysis of sex ratios: an introduction”; in Hardy, Ian C.W. (editor), ''Sex Ratios: Concepts and Research Methods'', pp. 48–92. ISBN 0521665787〕 * *''Operational sex ratio'' abbreviated as ''OSR'' is the proportion of adults in the sexually active population that are males. 'OSR' has often been confused with 'ASR' although these are conceptually different. 〔Székely, T., Weissing, F. J. and Komdeur, J. (2014), Adult sex ratio variation: implications for breeding system evolution. Journal of Evolutionary Biology, 27: 1500–1512. doi: 10.1111/jeb.12415〕 * quaternary sex ratio — ratio in post-reproductive organisms Measuring these requires sophisticated mathematics since they lack clear boundaries. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sex ratio」の詳細全文を読む スポンサード リンク
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