Editing Biology and sexual orientation (section)

==Homosexuality and evolution==
===General===
{{See also|Gene-centered view of evolution|Evolutionary psychology|Homosexual behavior in animals}}

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of [[Reproductive success|successful reproduction]], and for this reason, they would appear to be [[adaptation|maladaptive]] in an [[evolution]]ary context following a simple Darwinian model (competition amongst individuals) of natural selection—on the assumption that homosexuality would reduce this frequency. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility.<ref name="MacIntyre_1993">{{cite journal | vauthors = MacIntyre F, Estep KW | title = Sperm competition and the persistence of genes for male homosexuality | journal = Bio Systems | volume = 31 | issue = 2–3 | pages = 223–33 | year = 1993 | pmid = 8155854 | doi = 10.1016/0303-2647(93)90051-D | bibcode = 1993BiSys..31..223M }}</ref><ref name="Buss 2016 pp. 20–22" />

===Kin selection===
{{See also|Altruism (biology)|l1=Biological altruism|Inclusive fitness in humans}}

The "gay uncle hypothesis" posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (e.g., food, supervision, defense, shelter) to the offspring of their closest relatives.<ref name="Moskowitz_2010">{{cite web | date = 11 February 2010 | vauthors = Moskowitz C |title=How Gay Uncles Pass Down Genes |url=https://www.livescience.com/6106-gay-uncles-pass-genes.html |website=livescience.com |access-date=22 July 2020 |language=en}}</ref>

This hypothesis is an extension of the theory of kin selection, which was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by [[J. B. S. Haldane]] in 1932 and later elaborated by many others including [[John Maynard Smith]], [[W. D. Hamilton]], [[Mary Jane West-Eberhard]], and [[E. O. Wilson]].<ref name="Mayr_1982">{{cite book | vauthors = Mayr E |title=The growth of biological thought : diversity, evolution, and inheritance |date=1982 |publisher=Belknap Press |location=Cambridge, Mass. |isbn=978-0-674-36446-2 | page = [https://books.google.com/books?id=pHThtE2R0UQC&lpg=PA1061 598] }}</ref><ref>{{cite book|last=Wilson|first=Edward O.|year=1975|title=Sociobiology: The New Synthesis|title-link=Sociobiology: The New Synthesis|place=Cambridge, MA|publisher=Belknap Press|page=279|isbn=978-0674816213}}</ref> This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive. Conversely, social psychologist [[David Buss]] has argued that there is no empirical evidence that supports the hypothesis.<ref>{{cite book|last=Buss|first=David|year=2016|orig-year=1994|title=The Evolution of Desire: Strategies of Human Mating|title-link=The Evolution of Desire|place=New York|publisher=Basic Books|edition=3rd|page=97|isbn=978-0465097760}}</ref>

In 2001, ''[[Evolution and Human Behavior]]'' published a questionnaire survey of 57 heterosexual and 66 homosexual male subjects in the [[United States]] that found that homosexual subjects were no more likely to provide financial resources towards family members, heterosexual subjects were more likely give more financial resources to siblings than homosexual subjects, and homosexual subjects tended to be more estranged from family members.<ref>{{cite journal|last1=Bobrow|first1=David|last2=Bailey|first2=J. Michael|author-link2=J. Michael Bailey|year=2001|title=Is male homosexuality maintained via kin selection?|journal=Evolution and Human Behavior|publisher=Elsevier|volume=22|issue=5|pages=361–368|doi=10.1016/S1090-5138(01)00074-5|bibcode=2001EHumB..22..361B }}</ref> In 2005, ''[[Archives of Sexual Behavior]]'' published a replication study with 60 heterosexual and 60 homosexual male subjects in [[England]] that likewise found no significant differences between heterosexual and homosexual subjects in familial affinity or generosity towards family members.<ref>{{cite journal|last1=Rahman|first1=Qazi|last2=Hull|first2=Matthew S.|year=2005|title=An Empirical Test of the Kin Selection Hypothesis for Male Homosexuality|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=34|issue=4 |pages=461–467|doi=10.1007/s10508-005-4345-6|pmid=16010468 }}</ref> Vasey, Pocock, and VanderLaan (2007) and Vasey and VanderLaan (2010) tested the theory on the Pacific island of [[Samoa]], where they studied women, straight men, and the ''[[Faʻafafine|fa'afafine]]'', men who prefer other men as sexual partners and are accepted within the culture as a distinct third gender category. Vasey and VanderLaan found that the ''fa'afafine'' said they were significantly more willing to help kin, yet much less interested in helping children who are not family, providing the first evidence to support the kin selection hypothesis. The hypothesis is consistent with other studies on homosexuality, which show that it is more prevalent amongst both siblings and twins.<ref>{{cite journal|last1=Vasey|first1=Paul L.|last2=Pocock|first2=David S.|last3=VanderLaan|first3=Doug P.|year=2007|title=Kin selection and male androphilia in Samoan ''fa'afafine''|journal=Evolution and Human Behavior|publisher=Elsevier|volume=28|issue=3|pages=159–167|doi=10.1016/j.evolhumbehav.2006.08.004|bibcode=2007EHumB..28..159V }}</ref><ref name="VanderLaan_2011">{{cite thesis | url=https://www.uleth.ca/dspace/handle/10133/3159 | title=The development and evolution of male androphilia in Samoan fa'afafine| year=2011| degree = Ph.D. | vauthors = VanderLaan DP }}</ref><ref name="Vasey_2010">{{cite journal | vauthors = Vasey PL, VanderLaan DP | title = An adaptive cognitive dissociation between willingness to help kin and nonkin in Samoan Fa'afafine | journal = Psychological Science | volume = 21 | issue = 2 | pages = 292–7 | date = February 2010 | pmid = 20424059 | doi = 10.1177/0956797609359623 | s2cid = 16265819 }}; {{Lay source | vauthors =  Bolcer J | date = 5 February 2010 | url= http://www.advocate.com/news/daily-news/2010/02/05/study-supports-gay-super-uncles-theory | title=Study Supports Gay Super Uncles Theory | work = The Advocate }}</ref>

Anthropologist Raymond Hames notes that Vasey and VanderLaan's research on the ''fa'afafine'' identifies them as "transgendered androphilic males" as opposed to "sex-gender congruent androphiles" or "egalitarian homosexuals" in Western societies.<ref>{{cite book|last=Hames|first=Raymond|editor-last=Buss|editor-first=David M.|year=2016|orig-year=2005|chapter=19. Kin Selection|title=The Handbook of Evolutionary Psychology, Volume 1: Foundations|place=Hoboken, NJ|publisher=Wiley|edition=2nd|pages=512|isbn=978-1118755884}}</ref> Based on research conducted in [[Japan]] that found no evidence that homosexual Japanese men exhibited elevated avuncular tendencies compared to heterosexual counterparts, Vasey and VanderLaan (2011) provides evidence that if an adaptively designed avuncular male androphilic phenotype exists and its development is contingent on a particular social environment, then a collectivistic cultural context is insufficient, in and of itself, for the expression of such a phenotype.<ref>{{cite journal | vauthors = Vasey PL, VanderLaan DP | title = Sexual orientation in men and avuncularity in Japan: implications for the kin selection hypothesis | url = https://archive.org/details/sim_archives-of-sexual-behavior_2012-02_41_1/page/209 | journal = Archives of Sexual Behavior | volume = 41 | issue = 1 | pages = 209–15 | date = February 2012 | pmid = 21656333 | doi = 10.1007/s10508-011-9763-z | s2cid = 33348533 }}</ref> In 2011 and 2014, the ''Journal of Cognition and Culture'' published two studies that found that [[Canada|Canadian]] homosexual men exhibited significantly greater altruistic tendencies toward kin versus non-kin children relative to heterosexual men and women, but did not find that Canadian homosexual males exhibited significantly higher altruistic behavior towards nieces and nephews over geographic disconnect.<ref>{{cite journal|last1=Forrester|first1=Deanna L.|last2=VanderLaan|first2=Doug P.|last3=Parker|first3=Jessica L.|last4=Vasey|first4=Paul L.|year=2011|title=Male Sexual Orientation and Avuncularity in Canada: Implications for the Kin Selection Hypothesis|journal=Journal of Cognition and Culture|publisher=Brill|volume=11|issue=3–4|pages=339–352|doi=10.1163/156853711X591288}}</ref><ref>{{cite journal|last1=Abild|first1=Miranda L.|last2=VanderLaan|first2=Doug P.|last3=Vasey|first3=Paul L.|year=2014|title=Does Geographic Proximity Influence the Expression of Avuncular Tendencies in Canadian Androphilic Males?|journal=Journal of Cognition and Culture|publisher=Brill|volume=14|issue=1–2|pages=41–63|doi=10.1163/15685373-12342109}}</ref>

In 2016, the ''[[Journal of Sex Research]]'' published a study comprising 278 homosexual (or ''[[kathoey]]'') and heterosexual male subjects in [[Italy]] and [[Spain]] and from the [[Urak Lawoi]] of [[Thailand]] that found no greater kin altruism or avuncularity among homosexual subjects in any of the three cultures and that kin altruism and avuncularity was associated with societal differences in cultural norms about general altruism toward non-kin children.<ref>{{cite journal|last1=Camperio Ciani|first1=Andrea|last2=Battaglia|first2=Umberto|last3=Liotta|first3=Marina|year=2016|title=Societal Norms Rather Than Sexual Orientation Influence Kin Altruism and Avuncularity in Tribal Urak-Lawoi, Italian, and Spanish Adult Males|journal=Journal of Sex Research|publisher=Routledge|volume=53|issue=2|pages=137–148|doi=10.1080/00224499.2014.993748|pmid=26132515}}</ref> In 2017, ''Evolutionary Psychological Science'' published a [[logistic regression]] analysis of the results of 17,295 female subjects across 58 countries on [[World Values Survey]] questionnaires about attitudes toward homosexuality that found that subjects that were potentially most in need of [[Alloparenting|alloparental]] support exhibited significantly more positive attitudes towards homosexuals, which the researchers suggested was circumstantial evidence in support of the hypothesis on a global scale.<ref>{{cite journal|last1=Playà|first1=Eduard|last2=Vinicius|first2=Lucio|last3=Vasey|first3=Paul L.|year=2017|title=Need for Alloparental Care and Attitudes Toward Homosexuals in 58 Countries: Implications for the Kin Selection Hypothesis|journal=Evolutionary Psychological Science|publisher=Springer|volume=3|issue=4 |pages=345–352|doi=10.1007/s40806-017-0105-9|doi-access=free}}</ref> In 2018, ''Archives of Sexual Behavior'' published a study comparing avuncular tendencies between heterosexual and homosexual men on [[Java]] in [[Indonesia]] that found that homosexual men reported an increased willingness to transfer resources and money toward nephews and nieces but only reduced the direct reproductive cost to homosexual men by 20%, with the researchers concluding that kin selection alone was an insufficient explanation of male homosexuality.<ref>{{cite journal|last1=Nila|first1=Sarah|last2=Barthes|first2=Julien|last3=Crochet|first3=Pierre-Andre|last4=Suryobroto|first4=Bambang|last5=Raymond|first5=Michel|year=2018|title=Kin Selection and Male Homosexual Preference in Indonesia|journal=Archives of Sexual Behavior|publisher=Springer|volume=47|issue=8 |pages=2455–2465|doi=10.1007/s10508-018-1202-y|pmid=29797146 |url=https://hal.umontpellier.fr/hal-01827409/file/2018_Nila_Kin%20selection%20and%20male%20homosexual%20preference%20in%20Indonesia.pdf }}</ref>

===Antagonistic pleiotropy===
{{See also|Neolithic|Neolithic Revolution|Recent human evolution}}

Some scholars<ref name="MacIntyre_1993"/> have suggested that homosexuality is indirectly adaptive, by conferring a reproductive advantage on heterosexual siblings or their children. By way of analogy, the [[allele]] (a particular version of a gene) which causes [[sickle cell disease]] when two copies are present, also confers an [[Adaptation|adaptive advantage]] when one copy is present by providing resistance to [[malaria]] with non-symptomatic [[sickle cell trait]]—which is known as "heterozygote advantage".<ref name="Baker_1996">{{cite book | vauthors = Baker R |title = Sperm Wars: The Science of Sex |date=1996 |publisher=BasicBooks |location=New York |isbn=978-0-465-08180-6 |page=241 |edition=1st}}</ref>

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.<ref>{{cite news |url=http://www.economist.com/science/displaystory.cfm?story_id=12465295 |title=Gender bending |newspaper=The Economist |date=2008-10-23}}</ref>

In a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them". Their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population".<ref name="Zietsch_2008">{{cite journal | vauthors = Zietsch B, Morley K, Shekar S, Verweij K, Keller M, Macgregor S |date=November 2008 |title=Genetic factors predisposing to homosexuality may increase mating success in heterosexuals |journal=Evolution and Human Behavior |volume=29 |issue=6 |pages=424–433 |doi=10.1016/j.evolhumbehav.2008.07.002|bibcode=2008EHumB..29..424Z |display-authors=etal}}</ref> However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" (and thus seek out a greater number of sexual partners) as an example of one alternative explanation. The study acknowledges that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success, either in the present or in our evolutionary past".<ref name="Zietsch_2008"/>

The heterosexual advantage hypothesis was given strong support by the 2004 Italian study demonstrating increased [[fecundity]] in the female matrilineal relatives of gay men.<ref name="Camperio-Ciani_2004"/><ref name="Camperio_2008"/> As originally pointed out by Dean Hamer,<ref>{{cite book | vauthors = Hamer D, Copeland P | title = The Science of Desire: The Search for the Gay Gene and the Biology of Behavior | publisher = Simon and Schuster | date = 1994 | isbn = 978-0-684-80446-0 }}</ref> even a modest increase in reproductive capacity in females carrying a "gay gene" could easily account for its maintenance at high levels in the population.<ref name="Camperio_2008"/>

In 2004, Italian researchers conducted a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. The researchers concluded that there was genetic material being passed down on the X chromosome which both promote [[fertility]] in the mother and homosexuality in her male offspring. The connections discovered would explain about 20% of the cases studied, indicating that it being a highly significant factor to account for, but not the sole genetic factor determining sexual orientation.<ref name="Camperio-Ciani_2004">{{cite journal | vauthors = Camperio-Ciani A, Corna F, Capiluppi C | title = Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity | journal = Proceedings. Biological Sciences | volume = 271 | issue = 1554 | pages = 2217–21 | date = November 2004 | pmid = 15539346 | pmc = 1691850 | doi = 10.1098/rspb.2004.2872 }}</ref><ref name="Camperio_2008">{{cite journal | vauthors = Camperio Ciani A, Cermelli P, Zanzotto G | title = Sexually antagonistic selection in human male homosexuality | journal = PLOS ONE | volume = 3 | issue = 6 | pages = e2282 | date = June 2008 | pmid = 18560521 | pmc = 2427196 | doi = 10.1371/journal.pone.0002282 | bibcode = 2008PLoSO...3.2282C | doi-access = free }}</ref> A 2008 follow-up study using a comparative survey design found that 147 white and non-white homosexual men had a significantly more relatives in their maternal lines than 155 heterosexual men but not in the paternal line, and while the maternal aunts of white homosexual men had significantly elevated fecundity as compared to white heterosexual men, every class of relative for non-white heterosexual men showed elevated fecundities as compared to non-white homosexual men.<ref>{{cite journal|last1=Rahman|first1=Qazi|last2=Collins|first2=Anthony|last3=Morrison|first3=Martine|last4=Orrells|first4=Jennifer Claire|last5=Cadinouche|first5=Khatija|last6=Greenfield|first6=Sherene|last7=Begum|first7=Sabina|year=2008|title=Maternal Inheritance and Familial Fecundity Factors in Male Homosexuality|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=37|issue=6 |pages=962–969|doi=10.1007/s10508-007-9191-2|pmid=17665299 }}</ref>

Noting subsequent research by the original researchers that found that female relatives in the maternal line of homosexual men have higher fertility,<ref>{{cite journal|last1=Camperio Ciani|first1=Andrea|last2=Pellizzari|first2=Elena|year=2012|title=Fecundity of Paternal and Maternal Non-Parental Female Relatives of Homosexual and Heterosexual Men|journal=PLOS ONE|publisher=PLOS|volume=7|issue=12|page=e51088|doi=10.1371/journal.pone.0051088|doi-access=free |pmid=23227237|pmc=3515521|bibcode=2012PLoSO...751088C }}</ref> anthropologist [[Ruth Mace]] suggests that homosexuality is maintained by antagonistic pleiotropy and cites a cross-cultural study spanning 48 societies that found that male homosexuality was more prevalent in [[social stratification|stratified societies]], did not appear to be a [[cultural universal]], and is possibly maintained by [[Hypergamy|hypergyny]].<ref>{{cite book|last=Mace|first=Ruth|editor-last=Buss|editor-first=David M.|year=2016|orig-year=2005|chapter=22. The Evolutionary Ecology of the Family|title=The Handbook of Evolutionary Psychology, Volume 1: Foundations|place=Hoboken, NJ|publisher=Wiley|edition=2nd|pages=572–573|isbn=978-1118755884}}</ref><ref>{{cite journal|last1=Barthes|first1=Julien|last2=Godelle|first2=Bernard|last3=Raymond|first3=Michel|year=2013|title=Human social stratification and hypergyny: toward an understanding of male homosexual preference|journal=Evolution and Human Behavior|publisher=Elsevier|volume=34|issue=3|pages=155–163|doi=10.1016/j.evolhumbehav.2013.01.001|bibcode=2013EHumB..34..155B }}</ref> While the original cross-cultural study's empirical findings, conceptual basis, and methodology has been disputed by other researchers,<ref>{{cite journal|last1=VanderLaan|first1=Doug P.|last2=Garfield|first2=Zachary H.|last3=Garfield|first3=Melissa J.|last4=Leca|first4=Jean-Baptiste|last5=Vasey|first5=Paul L.|last6=Hames|first6=Raymond B.|year=2014|title=The 'female fertility–social stratification–hypergyny' hypothesis of male homosexual preference: Factual, conceptual and methodological errors in Barthes et al. [Commentary]|journal=Evolution and Human Behavior|publisher=Elsevier|volume=35|issue=5|pages=445–447|doi=10.1016/j.evolhumbehav.2014.06.002}}</ref><ref>{{cite journal|last1=Barthes|first1=Julien|last2=Godelle|first2=Bernard|last3=Raymond|first3=Michel|year=2014|title=Response to comment on "Human social stratification and hypergyny: toward an understanding of male"|journal= Evolution and Human Behavior|publisher=Elsevier|volume=35|issue=5|pages=448–450|doi=10.1016/j.evolhumbehav.2014.06.001|bibcode=2014EHumB..35..448B }}</ref> a subsequent cross-cultural study of 107 societies across 6 continents replicated the previous study's findings.<ref>{{cite journal|last1=Barthes|first1=Julien|last2=Crochet|first2=Pierre-André|last3=Raymond|first3=Michel|year=2015|title=Male Homosexual Preference: Where, When, Why?|journal=PLOS ONE|publisher=PLOS|volume=10|issue=8|pages=e0134817|doi=10.1371/journal.pone.0134817|doi-access=free |pmid=26267276 |pmc=4534200 |bibcode=2015PLoSO..1034817B }}</ref>

In 2005, ''Archives of Sexual Behavior'' published a study comparing family size and fecundity among family relatives between 301 homosexual men and 404 heterosexual men and found that the mean family size for homosexual men was significantly larger than heterosexual men as well as increased fecundity among the relatives of homosexual men.<ref>{{cite journal|last1=King|first1=Michael|last2=Green|first2=John|last3=Osborn|first3=David P. J.|last4=Arkell|first4=Jamie|last5=Hetherton|first5=Jacqueline|last6=Pereira|first6=Elizabeth|year=2005|title=Family Size in White Gay and Heterosexual Men|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=34|issue=1 |pages=117–122|doi=10.1007/s10508-005-1006-8|pmid=15772775 }}</ref> In 2007, the ''[[Proceedings of the Royal Society|Proceedings of the Royal Society: Series B]]'' published a study comparing the number of siblings of a sample of Samoan heterosexual males to ''fa'afafine'' counterparts that found that the ''fa'afafine'' tend to have more siblings, and specifically, a greater number of older brothers, older sisters, and younger brothers,<ref>{{cite journal|last1=Vasey|first1=Paul L.|last2=VanderLaan|first2=Doug P.|year=2007|title=Birth order and male androphilia in Samoan fa'afafine|journal=Proceedings of the Royal Society B|publisher=Royal Society|volume=274|issue=1616|pages=1437–1442|doi=10.1098/rspb.2007.0120|pmid=17412683|pmc=2176197}}</ref> and ''Archives of Sexual Behavior'' published a replication study in 2011 that found that likewise showed that the ''fa'afafine'' had a greater number of siblings than heterosexual males, which the researchers suggested indicates that a maternal fecundity effect exists in Samoa.<ref>{{cite journal|last1=VanderLaan|first1=Doug P.|last2=Vasey|first2=Paul L.|year=2011|title=Male sexual orientation in Independent Samoa: Evidence for fraternal birth order and maternal fecundity effects|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=40|issue=3|pages=495–503|doi=10.1007/s10508-009-9576-5|pmid=20039114}}</ref> In 2009, ''Archives of Sexual Behavior'' published a study that examined the [[proband]]s of 152 homosexual men and 98 heterosexual men that found a significant fecundity increase in mothers (including [[Gravidity and parity|primiparous]] mothers) but no evidence of increased paternal fecundity, which the researchers concluded suggested the existence of a sexually antagonistic inheritance partly linked to the X-chromosome promoting fecundity in females and homosexual sexual orientation in males.<ref>{{cite journal|last1=Iemmola|first1=Francesca|last2=Camperio Ciani|first2=Andrea|year=2009|title=New Evidence of Genetic Factors Influencing Sexual Orientation in Men: Female Fecundity Increase in the Maternal Line|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=38|issue=3 |pages=393–399|doi=10.1007/s10508-008-9381-6|pmid=18561014 }}</ref>

In 2009, the ''Journal of Sexual Medicine'' published a study of 151 homosexual or bisexual men and 88 exclusively heterosexual men that found that significantly higher fecundity of female relatives of the maternal line (including mothers, maternal grandparents, and maternal aunts) for both bisexuals and homosexuals compared to the corresponding relatives of heterosexual subjects, which the researchers argued provided evidence for an association between X-chromosomal genetic factors with bisexuality in men and fecundity promotion in female carriers.<ref>{{cite journal|last1=Camperio Ciani|first1=Andrea|last2=Iemmola|first2=Francesca|last3=Blecher|first3=Stan R.|year=2009|title=Genetic Factors Increase Fecundity in Female Maternal Relatives of Bisexual Men as in Homosexuals|journal=The Journal of Sexual Medicine|publisher=Elsevier|volume=6|issue=2|pages=449–455|doi=10.1111/j.1743-6109.2008.00944.x|pmid=18637994 }}</ref> In 2010, ''Archives of Sexual Behavior'' published a study comparing the pedigree sizes of 694 homosexual men and 894 heterosexual men sampled at [[pride parade]]s that found that homosexual men had more relatives, especially paternal relatives, but no evidence that male sexual orientation is transmitted predominantly through the maternal line—which the researchers noted was contrary to previous research.<ref>{{cite journal|last1=Schwartz|first1=Gene|last2=Kim|first2=Rachael M.|last3=Kolundzija|first3=Alana B.|last4=Rieger|first4=Gerulf|last5=Sanders|first5=Alan R.|year=2010|title=Biodemographic and physical correlates of sexual orientation in men|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=39|issue=1|pages=93–109|doi=10.1007/s10508-009-9499-1|pmid=19387815}}</ref> In 2012, ''Archives of Sexual Behavior'' published a study that compared the probands of 4,784 firstborn homosexual men and 40,197 first-born heterosexual men across 6 datasets that found that the homosexual probands had significantly fewer siblings or a statistically insignificant difference in the number of siblings—which the researcher concluded was a direct contradiction of an antagonistic pleiotropy explanation of homosexuality, but also that such an explanation could not be tested solely by comparing the number of siblings of heterosexual and homosexual subjects because of the [[confounding]] impact of the fraternal birth order effect.<ref>{{cite journal|last=Blanchard|first=Ray|year=2012|title=Fertility in the mothers of firstborn homosexual and heterosexual men|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=41|issue=3|pages=551–556|doi=10.1007/s10508-011-9888-0|pmid=22187029}}</ref><ref>{{cite journal|last1=Rieger|first1=Gerulf|last2=Blanchard|first2=Ray|last3=Schwartz|first3=Gene|last4=Bailey|first4=J. Michael|last5=Sanders|first5=Alan R.|title=Further data concerning Blanchard's (2011) 'Fertility in the mothers of firstborn homosexual and heterosexual men' [Letter to the Editor]|journal=Archives of Sexual Behavior|date=2012 |publisher=Springer Science+Business Media|volume=41|issue=3|pages=529–531|doi=10.1007/s10508-012-9942-6|pmid=22399055}}</ref>

In 2012, ''[[PLOS One]]'' published a study that compared the fecundity of the paternal and maternal line grandmothers, aunts, and uncles of 86 Samoan heterosexual males and 86 ''fa'afafine'' that found elevated fecundity in the paternal and maternal line grandmothers of ''fa'afafine'' but not their aunts or uncles.<ref>{{cite journal|last1=VanderLaan|first1=Doug P.|last2=Forrester|first2=Deanna L.|last3=Petterson|first3=Lanna J.|last4=Vasey|first4=Paul L.|year=2012|title=Offspring production among the extended relatives of Samoan men and fa'afafine|journal=PLOS ONE|publisher=PLOS|volume=7|issue=4|pages=e36088|doi=10.1371/journal.pone.0036088|doi-access=free |pmid=22558342|pmc=3338633|bibcode=2012PLoSO...736088V }}</ref> In 2012, the ''Journal of Sexual Medicine'' published a study examining the [[Phenotype|phenotypic expression]] of the genetic factors that influence increased fecundity in female relative carriers that found that as compared with corresponding female relatives of heterosexual men, mothers and maternal aunts of homosexual men had fewer [[Gynaecology|gynecological disorders]] and [[Complications of pregnancy|complicated pregnancies]], had [[Dysfunctional family|reduced family stability]] and a [[Divorce demography|higher divorce and spousal separation rate]], and self-reported less interest in having children, less emphasis on romantic love within couples, and less importance on their social life, but scored higher on the [[Extraversion and introversion|extraversion]] scale of a [[Big Five personality traits]] questionnaire.<ref>{{cite journal|last1=Camperio Ciani|first1=Andrea S.|last2=Fontanesi|first2=Lilybeth|last3=Iemmola|first3=Francesca|last4=Giannella|first4=Elga|last5=Ferron|first5=Claudia|last6=Lombardi|first6=Luigi|year=2012|title=Factors Associated with Higher Fecundity in Female Maternal Relatives of Homosexual Men|journal=The Journal of Sexual Medicine|publisher=Elsevier|volume=9|issue=11|pages=2878–2887|doi=10.1111/j.1743-6109.2012.02785.x|pmid=22616723 }}</ref>

In 2017, ''Archives of Sexual Behavior'' published a study investigating differences in fertility rates of the paternal and maternal biological relatives of 191 ''fa'afafine'' and 191 homosexual Samoan male subjects that found that the mothers and maternal grandmothers of ''fa'afafine'' showed elevated fertility as compared with homosexual men but not for paternal grandmothers or aunts, which led the researchers to conclude that their findings in Samoa for an antagonistic pleiotropy explanation of homosexuality provided only equivocal support.<ref>{{cite journal|last1=Semenyna|first1=Scott W.|last2=Petterson|first2=Lanna J.|last3=VanderLaan|first3=Doug P.|last4=Vasey|first4=Paul L.|year=2017|title=A Comparison of the Reproductive Output Among the Relatives of Samoan Androphilic ''Fa'afafine'' and Gynephilic Men|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=46|issue=1 |pages=87–93|doi=10.1007/s10508-016-0765-8|pmid=27785648 }}</ref> In 2018, ''[[Human Nature (journal)|Human Nature]]'' published a study investigating the fecundity among 30,203 relatives of 650 homosexual or bisexual women as compared with 808 heterosexual women that found that the direct fitness of homosexual females was four times lower than that of heterosexual females, but that the [[Pedigree chart|pedigree size]] and relative average fecundity in both the paternal and maternal sides of the families of the homosexual women were significantly higher than the families of the heterosexual women—which the researchers suggested appeared to offset the loss in fitness.<ref>{{cite journal|last1=Camperio Ciani|first1=Andrea|last2=Battaglia|first2=Umberto|last3=Cesare|first3=Linda Cesare|last4=Camperio Ciani|first4=Giorgia|last5=Capiluppi|first5=Claudio|year=2018|title=Possible Balancing Selection in Human Female Homosexuality|journal=Human Nature|publisher=Springer Science+Business Media|volume=29|issue=1 |pages=14–32|doi=10.1007/s12110-017-9309-8|pmid=29204792 |hdl=11577/3300828|hdl-access=free}}</ref>

In 2020, ''Archives of Sexual Behavior'' published a study that investigated fecundity among the relatives of 115 transgendered men and 112 homosexual men in the [[Isthmus Zapotec]] culture of [[Mexico]] (whom the Isthmus Zapotec refer to as ''muxe gunaa'' and the ''muxe nguiiu'' respectively) as compared with 171 heterosexual men and found that the mothers and paternal aunts of the transgendered men had higher fecundity than those of homosexual and heterosexual men, but found no such differences between the families of the heterosexual men and the homosexual men.<ref>{{cite journal|last1=Gómez Jiménez|first1=Francisco R.|last2=Semenyna|first2=Scott W.|last3=Vasey|first3=Paul L.|year=2020|title=Offspring Production Among the Relatives of Istmo Zapotec Men and ''Muxes''|journal=Archives of Sexual Behavior|publisher=Springer Science+Business Media|volume=49|issue=2 |pages=581–594|doi=10.1007/s10508-019-01611-y|pmid=31897830 }}</ref> In 2020, the ''Proceedings of the Royal Society: Series B'' published a meta-analysis of 10 studies comprising 5,390 homosexual and heterosexual subjects across 14 samples that found evidence for a fraternal birth order effect but no evidence for a female fecundity effect (i.e. an association between higher maternal fertility and homosexual orientation among male offspring) using a mathematical model proposed by mathematician [[Tanya Khovanova]],<ref>{{ Cite journal | issue = 2 | publisher = [[Springer Science and Business Media LLC]] | year = 2019 | volume = 49 | last = Khovanova | first = Tanya | author-link = Tanya Khovanova | title = On the Mathematics of the Fraternal Birth Order Effect and the Genetics of Homosexuality | journal = [[Archives of Sexual Behavior]] | issn = 0004-0002  | pages = 551–555 | doi = 10.1007/s10508-019-01573-1 | pmid = 31691074 | arxiv = 1711.09692 | hdl = 1721.1/131866.2 | s2cid = 37620479 | url = https://dspace.mit.edu/bitstream/1721.1/131866.2/6/10508_2019_1573_ReferencePDF.pdf }}</ref> but the reviewers noted that Khovanova's methodology restricted samples to one-son and two-son families which resulted in lost data and possibly [[Bias (statistics)|biased results]], and also made the [[Statistical assumption|assumption]] that female offspring could be ignored for simplicity and clarity while the female fecundity effect is specified in terms of total offspring rather than male offspring exclusively.<ref>{{cite journal|last1=Blanchard|first1=Ray|last2=Krupp|first2=Jurian|last3=VanderLaan|first3=Doug P.|last4=Vasey|first4=Paul L.|last5=Zucker|first5=Kenneth J.|year=2020|title=A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality|journal=Proceedings of the Royal Society B|publisher=Royal Society|volume=287|issue=1923|doi=10.1098/rspb.2019.2907|pmid=32183625 |pmc=7126035}}</ref>

In 2022, the ''Journal of Sex Research'' published a study of 26,542 men and 33,534 women that had entered [[Same-sex marriage in the Netherlands|same-sex unions in the Netherlands]] as compared with 4,607,785 men and 4,405,635 women that found robust evidence of a fraternal birth order effect on both male and female homosexuality and no support for a female fecundity effect, but the researchers qualified that their findings could be a result of sampling through a same-sex unions registry and only considered maternal siblings rather than other female relatives (e.g. sisters, maternal aunts, maternal grandmothers), and as such, that the existence of a female fecundity effect could not be discounted.<ref>{{cite journal|last1=Ablaza|first1=Christine|last2=Kabátek|first2=Jan|last3=Perales|first3=Francisco|year=2022|title=Are Sibship Characteristics Predictive of Same Sex Marriage? An Examination of Fraternal Birth Order and Female Fecundity Effects in Population-level Administrative Data from the Netherlands|journal=Journal of Sex Research|publisher=Routledge|volume=59|issue=6|pages=671–683|doi=10.1080/00224499.2021.1974330|pmid=35040387 }}</ref> In 2024, ''Archives of Sexual Behavior'' published a study comparing the fertility of the parents and grandparents of 459 homosexual men and 79 homosexual women to 7,312 heterosexual men and 3,352 heterosexual women in the [[Czech Republic]] that found higher fertility for only the paternal grandmothers of homosexual men and with a small effect size, leading the researchers to conclude that their findings did not support an antagonistic pleiotropy explanation of homosexuality.<ref>{{cite journal|last1=Fořt|first1=Jakub|last2=Flegr|first2=Jaroslav|last3=Kuba|first3=Radim|last4=Kaňková|first4=Šárka|year=2024|title=Fertility of Czech Gay and Straight Men, Women, and Their Relatives: Testing the Sexually Antagonistic Gene Hypothesis|journal=Archives of Sexual Behavior|volume=53 |issue=5 |pages=1747–1761 |publisher=Springer Science+Business Media|doi=10.1007/s10508-024-02827-3|doi-access=free|pmid=38472605 |pmc=11106150}}</ref>

In 2021, ''[[Nature Human Behaviour]]'' published a study that used the results from a pair of genome-wide association studies (GWAS) that found that opposite-sex behavior individuals that were carriers of genetic variants associated with same-sex behavior had a greater number of opposite-sex sexual partners on average, and that [[Genetic algorithm|computer simulations]] suggested that such a mating advantage could explain the maintenance of such genetic variants in human populations.<ref>{{cite journal|last1=Zietsch|first1=Brendan P.|last2=Sidari|first2=Morgan J.|last3=Abdellaoui|first3=Abdel|last4=Maier|first4=Robert|last5=Långström|first5=Niklas|last6=Guo|first6=Shengru|last7=Beecham|first7=Gary W.|last8=Martin|first8=Eden R.|last9=Sanders|first9=Alan R.|last10=Verweij|first10=Karin J. H.|year=2021|title=Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans|journal=Nature Human Behaviour|publisher=Nature Portfolio|volume=5|issue=9|pages=1251–1258|doi=10.1038/s41562-021-01168-8|pmid=34426668}}</ref> In 2024, ''[[Science Advances]]'' published a GWAS study that found that only the genetic variants associated with male bisexual behavior to be associated with greater offspring while the genetic variants associated with exclusive same-sex behavior were negatively associated with the number of offspring.<ref>{{cite journal|last1=Song|first1=Siliang|last2=Zhang|first2=Jianzhi|year=2024|title=Genetic variants underlying human bisexual behavior are reproductively advantageous|journal=Science Advances|publisher=American Association for the Advancement of Science|volume=10|issue=1|doi=10.1126/sciadv.adj6958|pmc=10796114}}</ref> In 2025, ''[[Trends (journals)|Trends in Genetics]]'' published a review analyzing the two studies that concluded that the evolutionary explanation of the persistence of genetic variants associated with same-sex behavior despite apparent fitness costs remained uncertain, but that antagonistic pleiotropy is a plausible persistence mechanism.<ref>{{cite journal|last1=Felesina|first1=Thomas|last2=Zietsch|first2=Brendan P.|year=2025|title=Emerging insights into the genetics and evolution of human same-sex sexual behavior|journal=Trends in Genetics|publisher=[[Cell Press]]|doi=10.1016/j.tig.2024.12.005|pmid=39880707}}</ref>