{{Short description|Ratio of males to females in a population}} {{Use dmy dates|date=November 2016}} [[File:Sex ratio total population per country 2020 (age 0-14).svg|thumb|300x300px|Sex ratio by country for the population below age 15. Blue represents more boys, and red represents more girls than the world average.]] [[File:Sex ratio total population per country 2020.svg|thumb|300x300px|Sex ratio by country for total population. Blue represents more men and boys, and red represents more women and girls than the world average of 1.01 males/female.]] [[File:Sex ratio total population per country 2020 (ages over 65).svg|thumb|300x300px|Sex ratio by country for the over-65 population. Blue represents more men, and red represents more women than the world average of 0.81 males/female.]] The '''human sex ratio''' is the [[ratio]] of [[male]]s to [[female]]s in a [[Population (human biology)|population]] in the context of [[anthropology]] and [[demography]]. In humans, the natural [[sex ratio]] at birth is slightly biased towards the male sex. It is estimated to be about 1.05 worldwide<ref name="CIA2024"/> or within a narrow range from 1.03 to 1.06<ref name="naturalratio">{{cite journal |last1=Chao |first1=Fengqing |last2=Gerland |first2=Patrick |last3=Cook |first3=Alex R. |last4=Alkema |first4=Leontine |title=Systematic assessment of the sex ratio at birth for all countries and estimation of national imbalances and regional reference levels |journal=Proceedings of the National Academy of Sciences |date=7 May 2019 |volume=116 |issue=19 |pages=9303–9311 |doi=10.1073/pnas.1812593116 |pmid=30988199 |pmc=6511063 |bibcode=2019PNAS..116.9303C |doi-access=free }}</ref> males per female at birth. The sex ratio for the entire [[world population]] including all ages is approximately 101 males to 100 females {{as of|2024|lc=y}}.<ref name="CIA2024"/>

The sex ratios at birth and of the total population are affected by various factors including natural factors, exposure to pesticides and environmental contaminants,<ref>{{cite web|url=https://www.stir.ac.uk/news/2014/06/boys-to-girls-birthrate/|title=How pollution may be changing the ratio of girls to boys|date=18 June 2014|website=Stir.ac.uk|access-date=6 January 2018}}</ref><ref>{{cite journal|pmid=9533502|url=http://www.precaution.org/lib/davis_sex_ratio_sentinel.19980601.pdf|year=1998|last1=Davis|first1=D. L.|title=Reduced ratio of male to female births in several industrial countries: A sentinel health indicator?|journal=JAMA|volume=279|issue=13|pages=1018–23|last2=Gottlieb|first2=M. B.|last3=Stampnitzky|first3=J. R.|doi=10.1001/jama.279.13.1018}}</ref> war casualties, [[returning soldier effect|effects of war on men]], [[sex-selective abortions]], infanticides,<ref>Very high sex ratios were common in even late medieval Europe, which may indicate sex-selective infanticide. Josiah Cox Russell, 1958, ''Late Ancient and Medieval Population,'' pp. 13–17.</ref> aging, [[gendercide]], problems with [[birth registration]] and [[Life expectancy#Sex differences|sex differences in life expectancy]].

Human sex ratios, either at birth or in the population as a whole, can be reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females. If there are 105,000 males and 100,000 females, the ratio of males to females is 1.05 and the proportion of males is 51.2%. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise.

==Demographic observations==

===Fisher's principle and biology=== [[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]].<ref name="Fisher">{{cite book |last=Fisher |first=R. A. |date=1930 |title=The Genetical Theory of Natural Selection |url=https://archive.org/details/geneticaltheoryo031631mbp/page/n164/mode/1up |location=Oxford |publisher=Clarendon Press |pages=141–143 |via=[[Internet Archive]]}}</ref><ref name="Hamilton">{{cite journal |last=Hamilton |first=W. D. |author-link=W. D. Hamilton |date=1967 |title=Extraordinary Sex Ratios: A Sex-ratio Theory for Sex Linkage and Inbreeding Has New Implications in Cytogenetics and Entomology |url=https://www.jstor.org/stable/1721222 |url-access=registration |journal=[[Science (journal)|Science]] |volume=156 |issue=3774 |pages=477–488 |doi=10.1126/science.156.3774.477 |pmid=6021675 |bibcode=1967Sci...156..477H |jstor=1721222}}</ref> Many species deviate from an even sex ratio, either periodically or permanently. Examples include [[parthenogenic]] species, periodically mating organisms such as [[aphid]]s, some [[Eusociality|eusocial]] [[wasp]]s, [[bee]]s, [[ant]]s, and [[termite]]s.<ref>{{cite journal | last1 = Kobayashi| first1 = Kazuya | last2 = Hasegawa | first2 = Eisuke | last3 = Yamamoto | first3 = Yuuka | last4 = Kazutaka | first4 = Kawatsu | last5 = Vargo | first5 = Edward L. | last6 = Yoshimura | first6 = Jin | last7 = Matsuura | first7 = Kenji | year = 2013 | title = Sex ratio biases in termites provide evidence for kin selection | journal = Nat Commun | volume = 4| article-number = 2048 | doi = 10.1038/ncomms3048 | pmid = 23807025 | bibcode = 2013NatCo...4.2048K| doi-access = free | hdl = 2123/11211 | hdl-access = free }}</ref>

In the [[evolutionary biology]] of [[sexual reproduction]] the [[operational sex ratio]], is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate,<ref name="CluttonBrock2007">{{Cite journal | last1 = Clutton-Brock | first1 = T. | title = Sexual Selection in Males and Females | doi = 10.1126/science.1133311 | journal = Science | volume = 318 | issue = 5858 | pages = 1882–1885 | year = 2007 | pmid = 18096798| bibcode = 2007Sci...318.1882C | s2cid = 6883765 }}</ref><ref name="Kvarnemo1996">{{cite journal | author = Kvarnemo, C. |author2=Ahnesjo, I. | year = 1996 | title = The dynamics of operational sex ratios and competition for mates | journal = Trends in Ecology & Evolution | volume = 11 | issue = 10 | pages = 404–408 | doi = 10.1016/0169-5347(96)10056-2 | pmid = 21237898|bibcode=1996TEcoE..11..404K }}</ref><ref name="Emlen1976">{{cite journal | author = Emlen, S.T. | year = 1976 | title = Lek organization and mating strategies in the bullfrog | journal = Behavioral Ecology and Sociobiology | volume = 1 | issue = 3 | pages = 283–313 |doi=10.1007/bf00300069 | jstor=4599103 | bibcode = 1976BEcoS...1..283E | s2cid = 10792384 }}</ref> or alternatively the local ratio of fertilizable females to sexually active males at any given time.<ref name="Emlen1977">{{cite journal | author = Emlen, S.T. |author2=Oring, L.W. | year = 1977| title = Ecology, Sexual Selection, and the Evolution of Mating Systems | journal = Science | volume = 197 | issue = 4300 | pages = 215–223 | pmid=327542 | doi=10.1126/science.327542|bibcode=1977Sci...197..215E }}</ref> This is different from the physical [[sex ratio]] because it does not take into account sexually inactive or non-competitive individuals (individuals that do not compete for mates).

===Observed ratio=== {{owidslider |start = 2023 |list = Template:OWID/sex ratio at birth#gallery |location = commons |caption = |title = |language = |file = [[File:sex ratio at birth, World, 2023 (cropped).svg|link=|thumb|upright=1.6|Sex ratio at birth]] |startingView = World }}

The sex ratio at birth is estimated to be about 1.05 males/female worldwide {{as of|2024|lc=y}} according to the [[CIA]].<ref name="CIA2024"/>

In a study around 2002, the natural sex ratio at birth was estimated to be within a narrow range of 1.03 to 1.07 males/female.<ref name="naturalratio"/><ref>{{cite journal |title=Unexplained differences in sex ratios at birth in Europe and North America | pmc=102777 | pmid=11976243 |year=2002 |last1=Grech |first1=V |last2=Savona-Ventura |first2=C |last3=Vassallo-Agius |first3=P |volume=324 |issue=7344 |pages=1010–1 |journal=BMJ (Clinical Research Ed.) | doi=10.1136/bmj.324.7344.1010}}</ref> Some scholars suggest that countries considered to have significant practices of prenatal sex-selection are those with birth sex ratios of 1.08 and above (selection against females) and 1.02 and below (selection against males). This assumption has been questioned by some scholars.<ref name=James-JournalofEndocrinology>{{cite journal|author=James W.H.|title=Hypothesis:Evidence that Mammalian Sex Ratios at birth are partially controlled by parental hormonal levels around the time of conception|journal=Journal of Endocrinology|volume=198|pages=3–15|date=July 2008|doi= 10.1677/JOE-07-0446|issue=1|pmid=18577567|doi-access=free}}</ref>

Infant mortality is significantly higher in boys than girls in most parts of the world. Often this is explained as due to biological and genetic sex differences, with boys more biologically vulnerable to premature death and disease.<ref>{{cite journal | doi = 10.1007/s13524-012-0161-5 | volume=50 | title=Why Is Infant Mortality Higher in Boys Than in Girls? A New Hypothesis Based on Preconception Environment and Evidence From a Large Sample of Twins | year=2012 | journal=Demography | pages=421–444 | last1 = Pongou | first1 = Roland| issue=2 | pmid=23151996 | s2cid=24188622 | doi-access=free }}</ref><ref>{{cite journal |last1=Alkema |first1=Leontine | last2=Chao |first2=Fengqing |last3=You |first3=Danzhen |last4= Pedersen |first4=Jon |last5= Sawyer |first5=Cheryl C. |title=National, regional, and global sex ratios of infant, child, and under-5 mortality and identification of countries with outlying ratios: a systematic assessment |journal=The Lancet Global Health |date=1 Sep 2014 |volume=2 |issue=9 |pages=e521–e530 |doi=10.1016/S2214-109X(14)70280-3 | pmid= 25304419|doi-access=free }}</ref> Numerous preconception or prenatal environmental factors affect the probabilities of a baby being conceived male or female. It has been proposed that these environmental factors also explain sex differences in mortality.<ref>{{cite journal|last1=Pongou|first1=Roland|title=Why Is Infant Mortality Higher in Boys Than in Girls? A New Hypothesis Based on Preconception Environment and Evidence From a Large Sample of Twins|journal=Demography|date=14 November 2012|volume=50|issue=2|pages=421–444|doi=10.1007/s13524-012-0161-5|pmid=23151996|s2cid=24188622|doi-access=free}}</ref>

In the United States, the yearly sex ratios at birth ranged from 1.046 to 1.059 between 1940 and 2002, with an overall decreasing trend.<ref name="nvss-trends">{{cite journal |last1=Mathews |first1=TJ |last2=Hamilton |first2=Brady E |date=14 June 2005 |title=Trend analysis of the sex ratio at birth in the United States |pmid=15974501 |journal=National Vital Statistics Reports |volume=53 |issue=20 |pages=1–17 |publisher=U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics, National Vital Statistics System }}</ref> Among Western European countries around 2001, the ratios ranged from 1.04 in Belgium to 1.07 in Switzerland,<ref>{{cite web |title=Sex ratio in Switzerland|url=http://www.bfs.admin.ch/bfs/portal/en/index/themen/00/01/blank/02.html | publisher=Switzerland Federal Statistics Office}}</ref> Italy,<ref>{{cite web |title=UN Sex Ratio Statistics|url=https://www.un.org/esa/population/unpop.htm|publisher=United Nations Population Division}}</ref> Ireland<ref>{{cite web|title=Sex ratio at birth (per 100 female newborn)|url=http://data.un.org/Data.aspx?q=sex+ratio+birth&d=PopDiv&f=variableID%3a52|publisher=United Nations Data Division}}</ref> and Portugal. In the aggregated results of 56 demographic and health surveys<ref>{{cite web|url=http://www.measuredhs.com/|title=The Demographic and Health Survey Program – Quality information to plan, monitor and improve population, health, and nutrition programs|publisher=measuredhs.com}}</ref> in African countries, the ratio is 1.03, but with considerable country-to-country variation.<ref>{{cite journal |author=Garenne M |title=Sex ratios at birth in African populations: a review of survey data |journal=Hum. Biol. |volume=74 |issue=6 |pages=889–900 |date=December 2002 |pmid=12617497 |doi=10.1353/hub.2003.0003 |s2cid=12297795 }}</ref>

[[File:PRC family planning don't abandon girls.jpg|thumb|A roadside sign in rural [[Sichuan]]: "It is forbidden to discriminate against, abuse or abandon baby girls."]] There is controversy about whether sex ratios outside the 1.03–1.07 range are due to sex selection, as suggested by some scholars, or due to natural causes. Some scholars, including [[Amartya Sen]],<ref name="Sen1990">Sen, Amartya (1990), More than 100 million women are missing, New York Review of Books, 20 December, pp. 61–66</ref> argue that strong socioeconomic factors such as the [[dowry system in India]] and the [[one child policy]] of China are responsible for prenatal sex selection. Others argue that an unbalanced sex ratio should not be automatically held as evidence of prenatal sex selection; Michel Garenne reports that many African nations have, over decades, had birth sex ratios below 1.00<ref>Michel Garenne, Southern African Journal of Demography, Vol. 9, No. 1 (June 2004), pp. 91–96</ref> (that is, more girls are born than boys). Angola, Botswana and Namibia have reported birth sex ratios between 0.94 and 0.99, which is quite different from the presumed "normal" sex ratio, meaning that significantly more girls have been born in such countries.<ref>Michel Garenne, Southern African Journal of Demography, Vol. 9, No. 1 (June 2004), p. 95</ref>

===Data sources and data quality issues=== 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&nbsp;grams in weight, or less than 35 centimeters in length) were not counted as a live birth until they had survived for seven days; if that infant died in those first 168 hours it, would not be counted as an infant death. This led to serious underreporting of the [[infant mortality rate]] (by 22% to 25%) relative to standards recommended by the [[World Health Organization]].<ref>{{cite journal |last1=Anderson |first1=Barbara A. |first2=Brian D. |last2=Silver |title=Infant Mortality in the Soviet Union: Regional Differences and Measurement Issues |journal=Population and Development Review |volume=12 |issue=4 |pages=705–37 |year=1986 |doi=10.2307/1973432 |jstor=1973432}}</ref>

[[File:China 1982 happy family with one child.jpg|thumb|A poster from 1982 showing a Chinese family with one child]]

Some researchers have, in part, attributed the high sex ratios reported in [[mainland China]] in the last 25 years 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 (government records)|vital statistics]] registration system, studies of sex ratios at birth have relied either on special fertility surveys, whose accuracy depends on full reporting of births and survival of both male and female infants, or on the national [[population census]] from which both birth rates and death rates are calculated from the household's reporting of births and deaths in the 18 months preceding the census.<ref>For example, the number of births reported to family planning and other administrative agencies has been significantly lower than the number determined in population surveys and the census. See Zhang, Guangyu (April 2004) [http://paa2004.princeton.edu/papers/40283 "Very Low Fertility in China in the 1990s: Reality or An Illusion Arising from Birth Underreporting?"] {{Webarchive|url=https://web.archive.org/web/20210705080752/https://paa2004.princeton.edu/papers/40283 |date=5 July 2021 }} Paper presented at the annual meeting of the Population Association of America.</ref> To the extent that this underreporting of births or deaths is sex-selective, both fertility surveys and censuses may inaccurately reflect the actual sex ratios at birth.<ref>For studies reveal underreporting or delayed reporting of female births in China, see {{cite journal |vauthors=Merli MG, Raftery AE |title=Are births underreported in rural China? Manipulation of statistical records in response to China's population policies |journal=Demography |volume=37 |pages=109–26 |date=February 1990 |doi=10.2307/2648100 |pmid=10748993 |issue=1 |jstor=2648100|s2cid=41085573 |doi-access=free }} and {{cite journal |author1=Cai, Yong |author2=Lavely, William |title=China's Missing Girls: Numerical Estimates and Effects on Population Growth |journal=The China Review |volume=3 |issue=2 |pages=13–29 |date=2003|jstor=23461902|url=http://www.excellentfuture.ca/sites/default/files/China's%20Missing%20Girls_0.pdf}}</ref>

===History=== The human sex ratio at birth has been an object of study since early in the [[history of statistics]], as it is easily recorded and a large number for sufficiently large populations.<ref>{{cite book |title=The Descent of Human Sex Ratio at Birth |url=https://archive.org/details/descenthumansexr00bria |url-access=limited |first1=Éric |last1=Brian |first2=Marie |last2=Jaisson |chapter=Physico-Theology and Mathematics (1710–1794) |pages=[https://archive.org/details/descenthumansexr00bria/page/n17 1]–25 |year=2007 |publisher=Springer Science & Business Media |isbn=978-1-4020-6036-6}}</ref> An early researcher was [[John Arbuthnot]] (1710),<ref>{{cite journal|author=John Arbuthnot |title=An argument for Divine Providence, taken from the constant regularity observed in the births of both sexes|journal=[[Philosophical Transactions of the Royal Society of London]] | volume=27| pages=186–190 | year=1710 | url=http://www.york.ac.uk/depts/maths/histstat/arbuthnot.pdf|doi=10.1098/rstl.1710.0011|issue=325–336|s2cid=186209819|doi-access=free}}</ref><ref name="Conover1999">{{Citation |last=Conover |first=W.J. |title=Practical Nonparametric Statistics |edition=Third |year=1999 |publisher=Wiley |isbn=0-471-16068-7 |pages=157–176 |chapter=Chapter 3.4: The Sign Test }} </ref><ref name="Sprent1989">{{Citation |last=Sprent |first=P. |title=Applied Nonparametric Statistical Methods |edition=Second |year=1989 |publisher=Chapman & Hall |isbn=0-412-44980-3 }} </ref><ref name="Stigler1986">{{cite book |title=The History of Statistics: The Measurement of Uncertainty Before 1900 |first=Stephen M. |last=Stigler |publisher=Harvard University Press |year=1986 |isbn=0-67440341-X |pages=[https://archive.org/details/historyofstatist00stig/page/225 225–226]|bibcode=1986hsmu.book.....S }}</ref> who in modern terms, performed [[statistical hypothesis testing]], computing the [[p-value]] (via a [[sign test]]), interpreted it as [[statistical significance]], and rejected the [[null hypothesis]].<ref name="Bellhouse2001">{{Citation |last=Bellhouse |first=P. |title=in Statisticians of the Centuries |editor1=C. C. Heyde |editor1-link=Chris Heyde |editor2-link=Eugene Seneta |editor2=E. Seneta |year=2001 |publisher=Springer |isbn=0-387-95329-9 |pages=39–42 |chapter=John Arbuthnot}} </ref>

Human sex at birth was also analyzed and used as an example by [[Jacob Bernoulli]] in ''[[Ars Conjectandi]]'' (1713), in which an unequal sex ratio is a natural example of a [[Bernoulli trial]] with uneven odds. [[Willem 's Gravesande]] (1774) also studied it.<ref name="Stigler1986" /> [[Pierre-Simon Laplace]] (1778) used human sex ratio as an example in his development of [[probability theory]]. He considered the statistics of almost half a million births; the statistics showed an excess of boys compared to girls.<ref name="Laplace 1778">{{cite journal| last=Laplace| first=P.| title=Mémoire sur les probabilités| journal=Mémoires de l'Académie royale des sciences de Paris| year=1778| volume=9| pages=227–332| url=http://cerebro.xu.edu/math/Sources/Laplace/memoir_probabilities.pdf| access-date=13 May 2018| archive-date=27 April 2015| archive-url=https://web.archive.org/web/20150427142452/http://cerebro.xu.edu/math/Sources/Laplace/memoir_probabilities.pdf| url-status=dead}}</ref><ref name="Laplace 1878">{{cite book| last=Laplace| first=P.| title=Oeuvres complètes de Laplace |chapter=Mémoire sur les probabilités (XIX, XX)|journal=Mémoires de l'Académie royale des sciences de Paris|year=1778| volume=9 | pages=429–438| chapter-url=http://gallica.bnf.fr/ark:/12148/bpt6k77597p/f386}}</ref> He concluded by calculation of a p-value that the excess was a real, but unexplained, effect.<ref>{{cite book|last=Stigler|first=Stephen M.|title=The History of Statistics: The Measurement of Uncertainty before 1900|publisher=Belknap Press of Harvard University Press|location=Cambridge, Mass|year=1986|isbn=0-674-40340-1|page=[https://archive.org/details/historyofstatist00stig/page/134 134]|bibcode=1986hsmu.book.....S|url=https://archive.org/details/historyofstatist00stig/page/134}}</ref>

==Factors affecting sex ratio in humans==

===Natural factors=== Scientific studies on human sex ratio are based on extensive birth and death records in Africa, the Americas, Asia, Australia, and Europe. A few of these studies extend to over 100 years of yearly human sex ratio data for some countries. These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, [[multiple birth]]s, [[birth order]], gestation weeks, race, parent's health history, and parent's psychological stress. The trends in human sex ratio are not consistent across countries at a given time, or over time for a given country. In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0.94 and 1.15 for natural reasons.<ref name=review1>{{cite journal |author= James WH |year= 1987 |title= The human sex ratio. Part 1: A review of the literature |journal= Human Biology |volume= 59 |issue= 5 |pages= 721–752 |pmid= 3319883 |url= http://bases.bireme.br/cgi-bin/wxislind.exe/iah/online/?IsisScript=iah/iah.xis&nextAction=lnk&base=MEDLINE&exprSearch=3319883&indexSearch=UI&lang=i}}</ref><ref>{{cite journal |author= James WH |year= 1987 |title= The human sex ratio. Part 2: A hypothesis and a program of research |journal= Human Biology |volume= 59 |issue= 6 |pages= 873–900 |pmid= 3327803 |url= http://bases.bireme.br/cgi-bin/wxislind.exe/iah/online/?IsisScript=iah/iah.xis&nextAction=lnk&base=MEDLINE&exprSearch=3319883&indexSearch=UI&lang=i}}</ref> While considering the race, human sex ratio at birth is about 1.03 for [[Blacks]] and 1.06 for [[Whites]].<ref name=review1/><ref>{{cite journal |last1=Khoury |first1=M. J. |last2=Erickson |first2=J. D. |last3=James |first3=L. M. |title=Paternal effects on the human sex ratio at birth: evidence from interracial crosses |journal=American Journal of Human Genetics |date=September 1984 |volume=36 |issue=5 |pages=1103–1111 |url=https://pmc.ncbi.nlm.nih.gov/articles/PMC1684514/ |issn=0002-9297}}</ref><ref name="review2">{{cite journal |last1=Terrell |first1=Metrecia L. |last2=Hartnett |first2=Kathleen P. |last3=Marcus |first3=Michele |title=Can environmental or occupational hazards alter the sex ratio at birth? A systematic review |journal=Emerging Health Threats Journal |date=20 April 2011 |volume=4 |pages=7109 |url=https://pubmed.ncbi.nlm.nih.gov/24149027/ |issn=1752-8550}}</ref>

In a scientific paper published in 2008, the author states that conventional assumptions have been:<ref name=James-JournalofEndocrinology /> * there are equal numbers of X and Y chromosomes in mammalian sperm * X and Y stand equal chance of achieving conception * therefore equal numbers of male and female zygotes are formed * therefore any variation of sex ratio at birth is due to sex selection between conception and birth.

The author cautions that available scientific evidence stands against the above assumptions and conclusions. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.02 and 1.08. However, the ratio may deviate significantly from this range for natural reasons.

A 2015 study showed that human sex ratio at conception is roughly 50%, but rises due to total mortality surplus of female embryos.<ref name=concepbirth>{{cite journal |last1=Orzack |first1=Steven Hecht |last2=Stubblefield |first2=J. William |last3=Akmaev |first3=Viatcheslav R. |last4=Colls |first4=Pere |last5=Munné |first5=Santiago |last6=Scholl |first6=Thomas |last7=Steinsaltz |first7=David |last8=Zuckerman |first8=James E. |title=The human sex ratio from conception to birth |journal=Proceedings of the National Academy of Sciences |date=21 April 2015 |volume=112 |issue=16 |pages=E2102-11 |doi=10.1073/pnas.1416546112 |pmid=25825766 |pmc=4413259 |language=en |issn=0027-8424|doi-access=free |bibcode=2015PNAS..112E2102O }}</ref> A dataset of 139,704 embryos derived from [[assisted reproductive technology]] showed a male sex ratio of 50.2%.<ref name=concepbirth/> A dataset of 4,999 embryos from [[induced abortion]]s showed a rate of 51.1% for the first [[Trimester (pregnancy)|trimester]] and 55.9% for the last two trimesters.<ref name=concepbirth/>

====Multiple birth==== A 1999 study reported the sex ratio for 815,891 children born in Denmark between 1980 and 1993. They studied the birth records to identify the effects of [[multiple birth]], birth order, age of parents and the sexes of preceding siblings on the proportion of males using contingency tables, [[chi-squared test]]s and regression analysis. The secondary sex ratio decreased with increasing number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors.<ref>{{cite journal |year= 1999 |title= Natural variation in the human sex ratio |journal= Human Reproduction |volume= 14 |issue= 12 |pages= 3120–3125 |doi= 10.1093/humrep/14.12.3120 |pmid= 10601107 |last1= Jacobsen |first1= R |last2= Møller |first2= H |last3= Mouritsen |first3= A |doi-access= free}}</ref>

====Length of gestation==== A 2009 study reported the sex ratio derived from data in United States birth records over a 25-year period (1981–2006). This paper reports that the sex ratio at birth for the white ethnic group in the United States was 1.04 when the gestational age was 33–36 weeks, but 1.15 for gestational ages of less than 28 weeks, 28–32 weeks, and 37 or more weeks. This study also found that the sex ratios at birth in the United States, between 1981 and 2006, were lower in both black and Hispanic ethnic groups when compared with non-Hispanic white ethnic group.<ref>{{cite journal |year= 2009 |title= Trends in US sex ratio by plurality, gestational age and race/ethnicity |journal= Reproductive Epidemiology|volume= 24 |issue= 11 |pages= 2936–2944 (see p. 2941 Figure 2) |doi= 10.1093/humrep/dep255 |last1= Branum |first1= A. M. |last2= Parker |first2= J. D. |author2-link=Jennifer D. Parker |last3= Schoendorf |first3= K. C. |url= https://zenodo.org/record/1234335 |pmid= 19654108 |doi-access= free }}</ref>

A 2010 study found that sex ratio does not seem to change significantly with either maternal or paternal age. Neither [[gravidity]] nor [[parity (medicine)|parity]] seem to affect the male-to-female ratio.<ref name=Ein-Mor /> However, there is a significant association of sex ratio with the length of [[gestation]].<ref name=Ein-Mor>{{cite journal |vauthors=Ein-Mor E, Mankuta D, Hochner-Celnikier D, Hurwitz A, Haimov-Kochman R |title=Sex ratio is remarkably constant |journal=Fertil. Steril. |volume=93 |issue=6 |pages=1961–5 |date=April 2010 |pmid=19159875 |doi=10.1016/j.fertnstert.2008.12.036 |doi-access=free }}</ref> These conclusions have been disputed, due to suggestions that the results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend effects of length of gestation than those reported.<ref name=James-FertilSterilJournal>{{cite journal |author=James W.H.|title=The Inconstancy of Human Sex Ratios at Birth |journal=Fertil. Steril. |volume=94 |issue=3 | doi =10.1016/j.fertnstert.2010.05.044 |pmid=20598299 |date=May 2010 |url=http://fertstert.wordpress.com/2010/05/27/the-inconstancy-of-human-sex-ratios-at%C2%A0birth/ |pages=e53}}</ref>

===Environmental factors===

====Effects of climate change==== High temperature raises proportion of male births, but the reasons for this are disputed.<ref name=Helle_2009/> Women subjected to colder weather abort frail male fetuses in greater proportion, thereby lowering birth sex ratios. Cold weather stressors also extend male longevity, thereby raising the human sex ratio at older ages.<ref name=Catalano /> A 1&nbsp;°C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year.<ref name=Catalano /><ref>Helle et al., 2009, p. 1228: "an increase of 1&nbsp;°C in ambient temperature anomaly was related to a 0.06% increase in annual birth sex ratio". Sex ratio in this work is defined as boys/(boys+girls); this increase corresponds to 1.2 additional males for every 1,000 females.</ref> A 2009 study on 138 years of human birth sex ratio data, from 1865 to 2003, found an increased excess of male births during periods of exogenous stress (World War II) and during warm years. In the warmest period over the 138 years, the birth sex ratio peaked at about 1.08 in Northern Europe.<ref name=Helle_2009>{{cite journal|journal=Journal of Animal Ecology|volume=78|issue=6|pages=1226–1233|date=November 2009|doi=10.1111/j.1365-2656.2009.01598.x|title=Evolutionary ecology of human birth sex ratio under the compound influence of climate change, famine, economic crises and wars|last1=Helle|first1=Samuli|last2=Helama|first2=Samuli|last3=Lertola|first3=Kalle|pmid=19719518|doi-access=free|bibcode=2009JAnEc..78.1226H }}</ref>

====Effects of gestation environment==== Causes of stress during gestation, such as maternal malnutrition,<ref>{{cite journal |vauthors=Andersson R, Bergström S |title=Is maternal malnutrition associated with a low sex ratio at birth? |journal=Hum. Biol. |volume=70 |issue=6 |pages=1101–6 |date=December 1998 |pmid=9825599 }}</ref> generally appear to increase fetal deaths, particularly among males,<ref name=Catalano>{{cite journal |vauthors=Catalano R, Bruckner T, Smith KR |title=Ambient temperature predicts sex ratios and male longevity |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=6 |pages=2244–7 |date=February 2008 |pmid=18250336 |pmc=2538905 |doi=10.1073/pnas.0710711104 |bibcode=2008PNAS..105.2244C |doi-access=free }}</ref> resulting in a lower sex ratio at birth. A higher incidence of [[Hepatitis B]] virus in a population is believed to increase the sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect.<ref>{{cite journal |vauthors=Davis DL, Gottlieb MB, Stampnitzky JR |title=Reduced ratio of male to female births in several industrial countries: a sentinel health indicator? |journal=JAMA |volume=279 |issue=13 |pages=1018–23 |date=April 1998 |pmid=9533502|doi=10.1001/jama.279.13.1018}}</ref>

The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports. For example, a 2008 study found no effect on birth sex ratio from hepatitis B presence in either mothers or fathers in Chinese populations.<ref name="revision">{{cite web | last1=Oster | first1=Emily | author-link=Emily Oster | last2=Chen | first2=Gang | last3=Yu | first3=Xinsen | last4=Lin | first4=Wenyao | title=Hepatitis B Does Not Explain Male-Biased Sex Ratios in China | year=2008 | url=http://home.uchicago.edu/~eoster/hbvnotecon.pdf | access-date=19 May 2009 | archive-date=18 January 2010 | archive-url=https://web.archive.org/web/20100118130724/http://home.uchicago.edu/~eoster/hbvnotecon.pdf | url-status=dead }}</ref>

====Effects of chemical pollution==== [[Image:NonylphenolEstradiol.svg|250px|thumb|A comparison of the structures of the natural hormone [[estradiol]] (left) and one of the [[nonylphenol|nonyl-phenols]] (right), an endocrine disruptor]] A 2007 survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios <!-- in which direction? --> in Russian Arctic villages and [[Inuit]] villages in [[Greenland]] and Canada, and attributed this imbalance to high levels of [[endocrine disruptor]]s in the blood of inhabitants, including [[Polychlorinated biphenyl|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.<ref>[https://www.theguardian.com/gender/story/0,,2167005,00.html Man-made chemicals blamed many more girls than boys are born in Arctic], The Guardian, 12 September 2007.</ref> Some studies have found that certain kinds of environmental [[pollution]], specifically [[Dioxins and dioxin-like compounds|dioxins]], are associated with a lower sex ratio.<ref>{{Cite journal|last=Check|first=Erika|date=2005-10-21|title=Pollution makes for more girls|url=http://www.nature.com/news/2005/051017/full/news051017-16.html|journal=Nature News|pages=news051017–16|language=en|doi=10.1038/news051017-16|url-access=subscription}}</ref><ref>{{Cite news|url=https://www.theglobeandmail.com/technology/science/pollution-debate-born-of-chemical-valleys-girl-baby-boom/article18253243/|title=Pollution debate born of Chemical Valley's girl-baby boom|last=Mittelstaedt|first=Martin|date=2005-11-15|work=The Globe and Mail|access-date=2017-08-07|language=en-ca}}</ref>

Other scientific studies suggest that environmental effects on human sex ratio at birth are either limited or not properly understood. A 1999 study from Finland's National Public Health Institute reports the effect of environmental chemicals and changes in sex ratio over 250 years in Finland. It evaluated whether Finnish long-term data are compatible with the hypothesis that the decrease in the ratio of male to female births in industrial countries is caused by environmental factors. They analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from 1751 to 1997. They found an increase in the proportion of males from 1751 to 1920; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II. None of the natural factors such as paternal age, maternal age, age difference of parents or birth order could explain the time trends. They found that the peak sex ratio precedes the period of industrialization and the introduction of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely. Moreover, these scientists claim that the trends they found in Finland are similar to those observed in other countries with higher levels of pollution and much greater pesticide use.<ref>{{cite journal |year= 1999|title= Environmental chemicals and changes in sex ratio: analysis over 250 years in finland|journal= Environmental Health Perspectives|volume= 107|issue= 10 |pages= 813–815|pmid= 10504147 |pmc= 1566625 | doi= 10.1289/ehp.99107813|last1= Vartiainen|first1= T|last2= Kartovaara|first2= L|last3= Tuomisto|first3= J|doi-broken-date= 21 January 2026|bibcode= 1999EnvHP.107..813V}}</ref>

{| class="wikitable" |+ Summary of environmental exposures and the sex ratio at birth<ref name=Terrell>{{cite journal |last1=Terrell |first1=Metrecia L. |last2=Hartnett |first2=Kathleen P. |last3=Marcus |first3=Michele |title=Can environmental or occupational hazards alter the sex ratio at birth? A systematic review |journal=Emerging Health Threats Journal |date=January 2011 |volume=4 |issue=1 |pages=7109 |doi=10.3402/ehtj.v4i0.7109}}</ref> |- ! Chemical !! Over father !! Over mother !! Over both parents !! Reference |- | [[2,3,7,8-Tetrachlorodibenzodioxin|Dioxin]]s || some evidence for more girls || || limited evidence for more girls || <ref name=Terrell/> |- | [[Polychlorinated biphenyl|PCB]]s || some evidence for more boys || || limited evidence for more boys || <ref name=Terrell/> |- | [[DBCP]] || limited evidence for more girls || || || <ref name=Terrell/> |- | [[Lead]] || limited evidence for more girls || || || <ref name=Terrell/> |- | [[Methylmercury]] || limited evidence for more girls || || || <ref name=Terrell/> |- | [[Boron]] || limited evidence for more girls || || || <ref name=Terrell/> |}

<!-- |- | [[Non-ionizing radiation]] || - || || || <ref name=Terrell/> |- | Childhood cancer survivors || - || || || <ref name=Terrell/> |- | G–forces experienced by astronauts and military pilots || - || || || <ref name=Terrell/> -->

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, known to be a factor in influencing the sex ratio of certain animals such as swine,<ref>{{cite journal |vauthors=Mendl M, Zanella AJ, Broom DM, Whittemore CT |title=Maternal social status and birth sex ratio in domestic pigs: an analysis of mechanisms |journal=Animal Behaviour |volume=50 |issue=5 |pages=1361–70 |year=1995 |doi=10.1016/0003-3472(95)80051-4 |bibcode=1995AnBeh..50.1361M |s2cid=53173184 }}</ref> but apparently not in humans.<ref>{{cite journal |vauthors=Ellis L, Bonin S |title=Social status and the secondary sex ratio: new evidence on a lingering controversy |journal=Soc Biol |volume=49 |issue=1–2 |pages=35–43 |year=2002 |pmid=14652908 |doi=10.1080/19485565.2002.9989047|s2cid=39347627 }}</ref> * Whether the mother has a partner may have a small effect on sex ratio, with a 2004 study of 84,500 births finding 51.5% male births among women living with a partner, and 49.9% in women who were not.<ref>{{Cite journal |author=Norberg, Karen |title=Partnership Status and the Human Sex Ratio at Birth |journal=Proceedings. Biological Sciences |series=NBER Working Paper Series |id=w10920 |date=November 2004 |volume=271 |issue=1555 |pages=2403–10 |doi=10.1098/rspb.2004.2857 |pmid=15556894 |pmc=1691865 |ssrn=622634}}</ref> * [[Latitude]], with countries near the equator producing more females than near the poles.<ref>{{cite journal |author=Navara KJ |title=Humans at tropical latitudes produce more females |journal=Biol. Lett. |volume=5 |issue=4 |pages=524–7 |date=August 2009 |pmid=19364717 |doi=10.1098/rsbl.2009.0069|pmc=2781905}}</ref>

===Social factors=== ====Population composition==== A 2005 study of California, where declining sex ratios had been observed, observed "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-American, Native American, other), with little or no change (Black American), or an increase (Chinese-American). 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."<ref>{{cite journal |vauthors=Smith D, Von Behren J |title=Trends in the sex ratio of California births, 1960–1996 |journal=J Epidemiol Community Health |volume=59 |issue=12 |pages=1047–53 |date=December 2005 |pmid=16286492 |pmc=1732975 |doi=10.1136/jech.2005.036970 }}</ref>

====Effects of war==== Increased sex ratio during and after a war is called the [[returning soldier effect]]. There is still no clear explanation of its mechanics.<ref>{{cite thesis |last1=Sävfors |first1=Ragnar |title=Great Britain after World War I & II: Studying the post-war sex-ratio imbalance |date=2016 |degree=Master's|publisher=Stockholm University, Department of Sociology |url=https://www.diva-portal.org/smash/record.jsf?pid=diva2%3A947036&dswid=-2207 |access-date=1 February 2022}}</ref>

====Early marriage and parents' age==== A 1985 study of 1.67 million births in 33 states in the United States and a 1999 study of 820,000 births in Denmark found that maternal age has no statistically significant role on the human birth sex ratio. However, they report a significant effect of paternal age. Significantly more male babies were born per 1000 female babies to younger fathers than to older fathers.<ref>{{cite journal|title=Paternal-Age and Birth-Order Effect on the Human Secondary Sex Ratio|author=Ruder, Avima |journal=American Journal of Human Genetics|volume=37|pages=362–372|year=1985f|pmc=1684568|pmid=3985011|issue=2}}</ref><ref>{{cite journal|title=Natural Variation in the Human Sex Ratio|author=Jacobsen|journal=Human Reproduction|volume=14|issue=12|pages=3120–3125|year=1999|doi=10.1093/humrep/14.12.3120|pmid=10601107|display-authors=etal|doi-access=free}}</ref> These studies suggest that social factors such as [[Child marriage|early marriage]] and males siring their children at a young age may play a role in raising birth sex ratios in certain societies.<ref>{{cite journal|title=Studies in The Human Sex Ratio 5. A Genetic Explanation of the Wartime Secondary Sex Ratio|journal=American Journal of Human Genetics|volume=10|pages=68–70|year=1958|pmc=1931860|pmid=13520702|issue=1|last1=Bernstein|first1=M. E.}}</ref>

====Partnership status==== A 2004 study of 86,436 human births from a US population-based survey showed that 51.4% boys were born among married parents living together, 52.2% among unmarried parents living together and only 49.9% boys among parents living apart.<ref>{{cite journal |last1=Norberg |first1=K. |title=Partnership status and the human sex ratio at birth |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |date=22 November 2004 |volume=271 |issue=1555 |pages=2403–2410 |doi=10.1098/rspb.2004.2857 |pmid=15556894 |pmc=1691865 }}</ref>

===Economic factors=== A 2003 study examined the hypothesis that population stress induced by a declining economy reduces the human sex ratio. It compared the sex ratio in East and West Germany for the years 1946 to 1999, with genetically similar populations. The population stressors theory predicts that the East German sex ratio should have been lower than expected in 1991, when East Germany's economy collapsed, than in previous years. The hypothesis further suggests that, over time, East German birth sex ratios should generally be lower than the observed sex ratios found in West Germany for the same years. According to the study, the birth sex ratio data from East Germany and West Germany over 45 years support the hypothesis. The sex ratio in East Germany was also at its lowest in 1991. According to the study, assuming women in East Germany did not opt to abort male fetuses more than female fetuses, the best hypothesis is that a collapsing economy lowers the human birth sex ratio, while a booming economy raises the birth sex ratio. The publication notes that these trends may be related to the observed trend of an elevated occurrence of very [[low birth-weight]] babies from maternal stress, during certain macroeconomic circumstances.<ref>{{cite journal|title=Sex ratios in the two Germanies: a test of the economic stress hypothesis|author=Catalano, Ralph |journal=Human Reproduction| volume=18|issue=9|pages=1972–1975|year=2003|doi=10.1093/humrep/deg370|pmid=12923159 |doi-access=free}}</ref>

===Sex-selective abortion and infanticide=== [[File:Sex determination ban.JPG|thumb|left|Sign in an Indian hospital stating that prenatal sex determination is not done there and is illegal]] [[Sex-selective abortion]] and [[infanticide]] are thought to significantly skew the naturally occurring ratio in some populations, such as China, where the introduction of ultrasound scans in the late 1980s has led to a birth sex ratio (males to females) of 1.181 (2010 official census data for China).<ref>[https://web.archive.org/web/20111103075517/http://news.xinhuanet.com/english2010/china/2011-08/16/c_131052436.htm China's sex ratio declines for two straight years] Xinhua, China (16 August 2011) – "Li Bin, director of the National Population and Family Planning Commission, made the remarks at a press conference (...). China's sex ratio at birth was 118.08 males for every 100 females in 2010, according to census data, Li said."</ref> The 2011 India census reports India's sex ratio in the 0–6 age bracket at 1.088.<ref>[http://www.census2011.co.in/p/glance.php India at Glance – Population Census 2011 – Final] Census of India, Government of India (2013)</ref>

The 2011 birth sex ratios for China and India are significantly above the mean ratio recorded in the United States from 1940 through 2002 (1.051); however, their birth sex ratios are within the 0.98–1.14 range observed in the United States for major ethnic groups over the same time period.<ref name="nvss-trends"/>{{rp|10}}{{synthesis inline|date=October 2025|reason=The cited source only notes data for the US; it does not indicate whether China and India's ratios are unusual.}} Along with Asian countries, a number of European, Middle Eastern, and Latin American countries have reported high birth sex ratios in the 1.06 to 1.14 range. High birth sex ratios, according to some studies, can be caused in part by social factors.{{citation needed|date=October 2025|reason=Sources exist for the data about various countries and the impact of social factors.}}

Another hypothesis has been inspired by the persistent high birth sex ratios observed in [[Georgia (country)|Georgia]] and [[Armenia]]—both predominantly Orthodox Christian societies—and [[Azerbaijan]], a predominantly Muslim society. Since their independence from the Soviet Union, the birth sex ratio in these Caucasus countries has risen sharply, to between 1.11 and 1.20, among the world's highest. Mesle et al. consider the hypothesis that the high birth sex ratio may be because of the social trend of more than two children per family, and birth order possibly affects the sex ratio in this region of the world. They also consider the hypothesis that sons are preferred in these countries of the Caucasus, the spread of scans and there being a practice of sex-selective abortion; however, the scientists admit that they do not have definitive proof that sex-selective abortion is actually happening or that there are no natural reasons for the persistently high birth sex ratios.<ref>{{cite book|title=A Sharp Increase in Sex Ratio at Birth in the Caucasus. Why? How?|author1=Mesle, France |author2=Vallin, Jacques |author3=Badurashvili, Irina |year=2007|pages=73–89|isbn=978-2-910053-29-1|publisher=Committee for International Cooperation in National Research in Demography|url=http://www.cicred.org/Eng/Publications/pdf/BOOK_singapore.pdf#page=85}}</ref>

===Disparate gendered access to resources=== There is evidence that some of the variation in global sex ratios is due to disparate access to resources. As MacPherson (2007) notes, there can be significant differences in gender violence and access to food, healthcare, immunizations between male and female children. This leads to high infant and childhood mortality among girls, which causes changes in sex ratio.<ref name=macpherson>{{cite journal | vauthors = MacPherson Y |title=Images and icons: harnessing the power of the media to reduce sex-selective abortion in India |journal=Gender & Development |date=November 2, 2007 |volume=15 |issue=3 |pages=413–423 |doi=10.1080/13552070701630574 |doi-access=free }}</ref>

Disparate, gendered access to resources appears to be strongly linked to socioeconomic status. Specifically, poorer families are sometimes forced to ration food, with daughters typically receiving less priority than sons.<ref name= "Klausen_2003">{{cite journal | vauthors = Klausen S, Wink C | year = 2003 | title = Missing Women: Revisiting the Debate | doi = 10.1080/1354570022000077999 | journal = Feminist Economics | volume = 9 | issue = 2–3| pages = 263–299 | s2cid = 154492092 }}</ref> However, Klasen's 2001 study revealed that this practice is less common in the poorest families, but rises dramatically in the slightly less poor families.<ref name= "Klausen_2003" /> Klasen and Wink's 2003 study suggests that this is "related to greater female economic independence and fewer cultural strictures among the poorest sections of the population". In other words, the poorest families are typically less bound by cultural expectations and norms, and women tend to have more freedom to become family breadwinners out of necessity.<ref name= "Klausen_2003" />

Increased sex ratios can be caused by disparities in aspects of life other than vital resources. According to Sen (1990), differences in wages and job advancement also have a dramatic effect on sex ratios. This is why high sex ratios are sometimes seen in nations with little sex-selective abortion.<ref name= "Sen_1990">{{cite book | vauthors = Sen A | chapter = More than 100 million women are missing. | title = Gender and Justice | date = July 2017 | pages = 219–222 | publisher = Routledge }}</ref> Additionally, high female education rates are correlated with lower sex ratios (World Bank 2011).<ref>World Bank, Engendering Development, The World Bank, (2001)</ref>

Lopez and Ruzikah (1983) found that, when given the same resources, women tend to outlive men at all stages of life after infancy. However, globally, resources are not always allocated equitably. Thus, some scholars argue that disparities in access to resources such as healthcare, education, and nutrition play at least a small role in the high sex ratios seen in some parts of the world.<ref name= "Klausen_2003" /> For example, Alderman and Gerter (1997) found that unequal access to healthcare is a primary cause of female death in developing nations, especially in Southeast Asia. Moreover, in India, lack of equal access to healthcare has led to increased disease and higher rates of female mortality in every age group until the late thirties (Sen 1990). This is particularly noteworthy because, in regions of the world where women receive equal resources, women tend to outlive men (Sen 1990). Women outlive men in all but 2 countries.<ref>{{cite journal | vauthors = Austad SN, Fischer KE | title = Sex Differences in Lifespan | journal = Cell Metabolism | volume = 23 | issue = 6 | pages = 1022–1033 | date = June 2016 | pmid = 27304504 | pmc = 4932837 | doi = 10.1016/j.cmet.2016.05.019 }}</ref>

Economic disadvantage alone may not always lead to increased sex ratio, claimed Sen in 1990. For example, in sub-Saharan Africa, one of the most economically disadvantaged regions of the world, there is an excess of women. So, if economic disadvantage is uncorrelated with sex ratio in Africa, some other factor(s) may be at play.<ref name= "Sen_1990" /> More detailed analysis of African demographics, in 2002, suggests that Africa too has wide variation in birth sex ratios (from 1.01 in Bantu populations of East Africa to 1.08 in Nigeria and Ethiopia).<ref name=pmid12617497>{{cite journal | vauthors = Garenne M | title = Sex ratios at birth in African populations: a review of survey data | journal = Human Biology | volume = 74 | issue = 6 | pages = 889–900 | date = December 2002 | pmid = 12617497 | doi = 10.1353/hub.2003.0003 | s2cid = 12297795 }}</ref> Thus economic disadvantage remains a possible unresolved hypothesis for Africa as well.

==Consequences of different sex ratios== [[File:Schnorr von Carolsfeld Bibel in Bildern 1860 083.png|thumb|right|[[Tribe of Benjamin|Benjaminites]] seize wives from Shiloh in this 1860 woodcut by [[Julius Schnorr von Karolsfeld]]. There were not enough women available for marriage due to the high losses in the [[Battle at Gibeah]].]] There are several social consequences of an imbalanced sex ratio. It may also become a factor in societal and demographic collapse. For example, the native population of [[Cusco]], Peru at the time of the Spanish conquest was stressed by an imbalance in the sex ratio between men and women.<ref>{{cite journal |last1=Covey |first1=R. Alan |last2=Childs |first2=Geoff |last3=Kippen |first3=Rebecca |title=Dynamics of Indigenous Demographic Fluctuations: Lessons from Sixteenth-Century Cusco, Peru |journal=Current Anthropology |date=1 June 2011 |volume=52 |issue=3 |pages=335–360 |doi=10.1086/660010 |s2cid=15702434 |url=https://www.journals.uchicago.edu/doi/abs/10.1086/660010 |issn=0011-3204|url-access=subscription }}</ref> Analyses of how sex ratio imbalances affect personal consumption and intra-household distribution were pioneered by [[Gary Becker]], [[Shoshana Grossbard-Shechtman]],<ref>{{cite journal | year = 1981 | title = The Impact of the Female Marriage Squeeze and the Contraceptive Revolution on Sex Roles and the Women's Liberation Movement in the United States, 1960 to 1975 | journal = Journal of Marriage and the Family | volume = 43 | issue = 1| pages = 49–65 | doi=10.2307/351416| jstor = 351416 | last1 = Heer | first1 = David M. | last2 = Grossbard-Shechtman | first2 = Amyra }}</ref><ref>Shoshana Grossbard-Shechtman (1993), ''On the Economics of Marriage – A Theory of Marriage, Labor and Divorce''. Boulder, CO: Westview Press.</ref> and Marcia Guttentag and Paul Secord.<ref>Guttentag, Marcia and Secord, Paul F. (1983), ''Too Many Women: The Sex Ratio Question''. Beverly Hills: Sage Publications</ref>

High ratios of males have a positive effect on marital fertility and women's share of household consumption, and negative effects on non-marital cohabitation and fertility and women's labor supply. It has been shown that the labor supply of married women in the U.S., over time, varies inversely with the sex ratio.<ref>Grossbard-Shechtman, Shoshana and Granger, Clive W. (September 1998). [https://www.researchgate.net/publication/4805034_Women's_Jobs_and_Marriage_--_Baby-Boom_Versus_Baby-Bust "Women's Jobs and Marriage, Baby-Boom versus Baby-Bust,"] ''Population'', 53: 731–52 (in French)</ref><ref>{{cite journal | year = 2007 | title = Marriage Markets and Women's Labor Force Participation | journal = Review of Economics of the Household | volume = 5 | issue = 3| pages = 249–278 | doi=10.1007/s11150-007-9014-1| last1 = Amuedo-Dorantes | first1 = Catalina |author-link1=Catalina Amuedo-Dorantes | last2 = Grossbard | first2 = Shoshana | hdl = 10419/34677 | s2cid = 189952913 | url = https://econpapers.repec.org/RePEc:iza:izadps:dp2722 | hdl-access = free }}</ref>

===Case of missing women===

{{main|Sex-selective abortion#Societal effects}}

* Societies with fewer women tend to have low rates of literacy and female workforce participation.<ref name="hvi">{{Cite Q|Q131827562}}</ref> * The lack of women in Taiwan has led many Taiwanese men to procure brides from other countries such as Vietnam via matchmaking agencies. Ill-treatment of the brides led the Vietnamese government to crack down on the trade in the 2000s, pushing it underground.<ref name="hvi"/> * Following the [[one-child policy]], the lack of women in China has helped prostitution to flourish. Many women were purchased or kidnapped in other countries for that purpose.<ref name="hvi"/> * Surplus men constitute a bridging population for the propagation of the [[AIDS]] pandemic, transferring HIV from high-risk to low-risk people. This has helped the virus spread in Africa and Asia.<ref name="hvi"/>

===Missing women and crime===

A 2022 study used 2021 National Archive of Criminal Justice data and 2016 U.S. Census Bureau data to show the impact of different gender ratios on [[violence against women|men's violence against women]] in 3,165 U.S. cities and counties. The sex ratio ranged from 40% men to 60% men in the data. Rates of violence were lowest in places with a 51% male ratio. For lower and higher proportions of males, the more the ratio deviated from the average, the higher was the violence.<ref>{{cite journal |last1=Pabst |first1=Jennifer |last2=Walfield |first2=Scott M. |last3=Schacht |first3=Ryan |title=Patterning of Sexual Violence against Women across US Cities and Counties |journal=Social Sciences |date=May 2022 |volume=11 |issue=5 |pages=208 |doi=10.3390/socsci11050208 |language=en |issn=2076-0760|doi-access=free }}</ref>

A study of Chinese provinces found that an increase of the human sex ratio by 1% resulted in an increase of the crime rate by 5-6%.<ref name="hvi"/>

==Countries with imbalances in sex ratio== [[File:Pyramide Bahrein.PNG|thumb|Sex imbalance in [[Bahrain]] caused by policies that restrict female spouses and children of immigrant workers]] Some countries have a significant disparity between males and females in their population. As stated above, males usually exceed females at birth but subsequently experience different mortality rates due to many possible causes such as differential natural death rates, and increased accidental and violent deaths (including war).

Countries with significant imbalances tend to have three characteristics in common. First, a rapid decline in fertility, either because of preference for smaller families or to comply with their nation's population control measures. Second, there is [[social pressure]] for women to give birth to sons, often because of a cultural preference for male heirs. Third, families have widespread access to technology to selectively abort female foetuses.<ref>{{Cite journal |last=Guilmoto |first=Christophe Z. |date=2009 |title=The Sex Ratio Transition in Asia |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1728-4457.2009.00295.x |journal=Population and Development Review |language=en |volume=35 |issue=3 |pages=519–549 |doi=10.1111/j.1728-4457.2009.00295.x |issn=1728-4457}}</ref><ref>{{cite web|title=A Single Man: One Chinese Bachelor's Search for Love|date=15 November 2011|author1=Jian Lee, Deborah|author2=Subramanian, Sushma|url=http://pulitzercenter.org/reporting/china-gao-po-bachelor-women-marriage-economy|publisher=Pulitzer Center on Crisis Reporting|access-date=19 December 2011|archive-date=30 November 2016|archive-url=https://web.archive.org/web/20161130190334/http://pulitzercenter.org/reporting/china-gao-po-bachelor-women-marriage-economy|url-status=dead}}</ref>

Some of the factors suggested as causes of this sex imbalance are [[sex-selective abortion]] and infanticide, large-scale migration, and behavioral factors statistically linked with sex ratio, such as excessive drinking and violence.<ref>{{cite news|url = http://qz.com/335183/heres-why-men-on-earth-outnumber-women-by-60-million/|title = A story of drinkers, genocide and unborn girls|last = Bauer|first = David|date = January 2015|work = Quartz}}</ref> 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 spouses,<ref>[[Valerie M. Hudson|Hudson, Valerie M.]] and den Boer, Andrea M. (2004) ''Bare Branches: The Security Implications of Asia's Surplus Male Population''. The MIT Press. {{ISBN|0262582643}}.</ref> and being recruited into the service of militaristic political factions. [[Economic]] factors such as male-majority industries and activities, such as the [[petrochemical]], [[agriculture]], [[engineering]], [[military]], and [[technology]] industries, have also contributed to an imbalance toward males in some areas dependent on these industries.<ref>{{cite news| url=http://archive.boston.com/bostonglobe/ideas/articles/2008/03/30/a_singles_map_of_the_united_states_of_america/ | work=The Boston Globe | title=A singles map of the United States of America | first=Richard | last=Florida | date=30 March 2008}}</ref>

A 2007 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 II]] 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.<ref>{{Cite web |first1=Michael |last1=Kvasnicka |first2=Dirk |last2=Bethmann |title=World War II, Missing Men, and out-of-wedlock childbearing |id=07-30 |publisher=The Institute of Economic Research, Korea University |date=1 October 2007 |url=http://econ.korea.ac.kr/~ri/WorkingPapers/W0730.pdf }}</ref>

[[File:Pyramide Estonie.PNG|thumb|left|Sex imbalance in [[Estonia]]]] The value for the entire [[world population]] is 1.01 males/female, with 1.05 at birth, 1.05 for those under 15, 1.03 for those between 15 and 64, and 0.81 for those over 65 {{as of|2024|lc=y}}.<ref name="CIA2024">{{cite web |title=CIA Fact Book | date=30 April 2025 | url=https://www.cia.gov/the-world-factbook/countries/world/#people-and-society | archive-url=https://web.archive.org/web/20210126032610/https://www.cia.gov/the-world-factbook/countries/world/#people-and-society | url-status=dead | archive-date=26 January 2021 | publisher=The Central Intelligence Agency of the United States}}</ref>

[[Qatar]] has the highest male-to-female ratio amongst the whole population, with 3.32 males/female {{as of|2024|lc=y}}. For the group aged below 15, [[British Virgin Islands]], [[Malawi]] and [[Collectivity of Saint Martin|Saint Martin]] have the lowest male-to-female ratio with 0.98–0.99 males/female, and [[Liechtenstein]] has the highest male-to-female ratio with 1.25 males/female in that age group.<ref>{{cite web |title=Sex Ratio – CIA Fact Book | date=2024 | url=https://www.cia.gov/the-world-factbook/field/sex-ratio/ | archive-url=https://web.archive.org/web/20210108060622/https://www.cia.gov/the-world-factbook/field/sex-ratio | url-status=dead | archive-date=8 January 2021 | publisher=The Central Intelligence Agency of the United States}}</ref>

Countries on the [[Arabian Peninsula]] tend to have a 'natural' ratio of about 1.05 at birth but a very high ratio of males for those over 65 (Saudi Arabia 1.14, 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]].{{citation needed|date=November 2017}} Conversely, countries of [[Northern Europe|Northern]] and [[Eastern Europe]] (the [[Baltic states]], [[Belarus]], Ukraine, Russia) tend to have a 'normal' ratio at birth but a very low ratio of males among those over 65 (Russia 0.46, Latvia 0.48, Ukraine 0.52); similarly, [[Armenia]] has an above average male ratio at birth (1.17), and a below-average male ratio above 65 (0.67). The latter may be caused by [[human migration|emigration]] and higher male mortality as a result of higher Soviet-era deaths; it may also be related to the enormous (by western standards) rate of [[Alcohol consumption in Russia|alcoholism in the former Soviet states.]]{{citation needed|date=November 2017}} Another contributory factor is an aging population, given that due to higher differential mortality rates, the ratio of males to females declines with age.

==See also== * [[Gender parity]] * [[Operational sex ratio]] * [[Returning soldier effect]] * [[Sex selection]] * [[Sex-selective abortion]] * [[Surplus women]] * [[Missing women]]

'''Countries:''' * [[List of sovereign states by sex ratio]] * [[Lost boys (Mormon fundamentalism)]] * [[Sex-ratio imbalance in China]] * [[Sex ratio in India]]

==References== {{reflist}}

{{Population}}

[[Category:Human sex ratio| ]] [[Category:Human populations|sex ratio]] [[Category:Demography]]