Editing Sexual orientation (section)

== Causes ==

The exact causes for the development of a particular sexual orientation have yet to be established. To date, much research has been conducted to determine the influence of genetics, hormonal action, development dynamics, social and cultural influences—which has led many to think that biology and environment factors play a complex role in forming it.<ref name="pediatrics2004" /><ref name="Kersey-Matusiak" /><ref name="Lamanna" />

=== Biology ===

{{Main|Biology and sexual orientation}}

Research has identified several biological factors which may be related to the development of sexual orientation, including [[gene]]s, [[Prenatal hormones and sexual orientation|prenatal hormones]], and [[Human brain|brain]] structure. No single controlling cause has been identified, and research is continuing in this area.<ref name="twin-adol">{{cite journal|last2=Bruckner|first2=H.|year=2001|title=Opposite-sex twins and adolescent same-sex attraction|url=http://iserp.columbia.edu/sites/default/files/working_papers/2001_04.pdf|journal=American Journal of Sociology|volume=107|issue=5|pages=1179–205|doi=10.1086/341906|last1=Bearman|first1=P.S.|archive-url=https://web.archive.org/web/20121220235217/http://iserp.columbia.edu/sites/default/files/working_papers/2001_04.pdf|archive-date=2012-12-20|url-status=dead|citeseerx=10.1.1.483.4722|s2cid=13094910}}</ref>

Although researchers generally believe that sexual orientation is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences,<ref name="pediatrics2004" /><ref name="Lamanna" /><ref name="Stuart" /> with biological factors involving a complex interplay of genetic factors and the early uterine environment,<ref name="Lamanna" /><ref name="rcp2007" /> they favor biological models for the cause.<ref name="pediatrics2004" /> There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.<ref name="Bailey" /> Scientists do not believe that sexual orientation is a choice,<ref name="pediatrics2004" /><ref name="Kersey-Matusiak" /><ref name="Lamanna" /> and some of them believe that it is established at conception.<ref name="Vare">Vare, Jonatha W., and Terry L. Norton. "Understanding Gay and Lesbian Youth: Sticks, Stones and Silence." Cleaning House 71.6 (1998): 327–31: Education Full Text (H.W. Wilson). Web. 19 April 2012.</ref> Current scientific investigation usually seeks to find biological explanations for the adoption of a particular sexual orientation.<ref name="pediatrics2004" /> Scientific studies have found a number of statistical [[Biology and sexual orientation#Biological differences in gay men and lesbian women|biological differences between gay people and heterosexuals]], which may result from the same underlying cause as sexual orientation itself.<ref>{{cite web |last1=Rahman |first1=Qazi |title='Gay genes': science is on the right track, we're born this way. Let's deal with it. |url=https://www.theguardian.com/science/blog/2015/jul/24/gay-genes-science-is-on-the-right-track-were-born-this-way-lets-deal-with-it |work=The Guardian |date=24 July 2015 |access-date=3 January 2020 |archive-date=16 August 2021 |archive-url=https://web.archive.org/web/20210816181957/https://www.theguardian.com/science/blog/2015/jul/24/gay-genes-science-is-on-the-right-track-were-born-this-way-lets-deal-with-it |url-status=live }}</ref>

==== Genetic factors ====

[[Gene]]s may be related to the development of sexual orientation. A twin study from 2001 appears to exclude genes as a major factor,<ref name="twin-adol" /> while a twin study from 2010 found that homosexuality was explained by both genes and environmental factors.<ref>{{Cite journal|last1=Långström|first1=Niklas|last2=Rahman|first2=Qazi|last3=Carlström|first3=Eva|last4=Lichtenstein|first4=Paul|date=2010-02-01|title=Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden|journal=Archives of Sexual Behavior|language=en|volume=39|issue=1|pages=75–80|doi=10.1007/s10508-008-9386-1|issn=0004-0002|pmid=18536986|s2cid=11870487}}</ref>  However, [[experimental design]] of the available twin studies has made their interpretation difficult.

In 2012, a large, comprehensive&nbsp;[[Genome-wide association study|genome-wide linkage study]]&nbsp;of male sexual orientation was conducted by several independent groups of researchers.<ref>{{Cite journal|last1=Sanders|first1=A.R.|last2=Martin|first2=E.R.|last3=Beecham|first3=G.W.|last4=Guo|first4=S.|last5=Dawood|first5=K.|last6=Rieger|first6=G.|last7=Badner|first7=J.A.|last8=Gershon|first8=E.S.|last9=Krishnappa|first9=R.S.|date=May 2015|title=Genome-wide scan demonstrates significant linkage for male sexual orientation|journal=Psychological Medicine|volume=45|issue=7|pages=1379–88|doi=10.1017/S0033291714002451|issn=0033-2917|pmid=25399360|s2cid=4027333|url=https://semanticscholar.org/paper/b1264b91af61e2c27a4b2f7eb33dc1bba8d0bf06|access-date=2019-12-01|archive-date=2021-08-24|archive-url=https://web.archive.org/web/20210824071648/https://www.semanticscholar.org/paper/Genome-wide-scan-demonstrates-significant-linkage-Sanders-Martin/b1264b91af61e2c27a4b2f7eb33dc1bba8d0bf06|url-status=live}}</ref> Significant linkage to homosexuality was found with genes on chromosome [[Xq28]] and chromosome 8 in the pericentromeric region. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation." It was the largest study of the genetic basis of homosexuality to date and was published online in November 2014.<ref>{{cite journal|last1=Sanders|first1=A.R.|last2=Martin|first2=E.R.|last3=Beecham|first3=G.W.|last4=Guo|first4=S.|last5=Dawood|first5=K.|last6=Rieger|first6=G.|last7=Badner|first7=J.A.|last8=Gershon|first8=E.S.|last9=Krishnappa|first9=R.S.|last10=Kolundzija|first10=A.B.|last11=Duan|first11=J.|last12=Gejman|first12=P.V.|last13=Bailey|first13=J. M.|title=Genome-wide scan demonstrates significant linkage for male sexual orientation|journal=Psychological Medicine|date=17 November 2014|volume=45|issue=7|pages=1379–1388|doi=10.1017/S0033291714002451|pmid=25399360|s2cid=4027333|url=https://semanticscholar.org/paper/b1264b91af61e2c27a4b2f7eb33dc1bba8d0bf06|access-date=1 December 2019|archive-date=24 August 2021|archive-url=https://web.archive.org/web/20210824071648/https://www.semanticscholar.org/paper/Genome-wide-scan-demonstrates-significant-linkage-Sanders-Martin/b1264b91af61e2c27a4b2f7eb33dc1bba8d0bf06|url-status=live}}</ref>

However, in August 2019, a [[genome-wide association study]] of 493,001 individuals concluded that hundreds or thousands of genetic variants underlie homosexual behavior in both sexes, with 5 variants in particular being significantly associated. They stated that in contrast to linkage studies that found substantial association of sexual orientation with variants on the X-chromosome, they found no excess of signal (and no individual genome-wide significant variants) on Xq28 or the rest of the X chromosome.<ref>
*{{cite journal |last1=Zietsch |first1=Brendan P. |last2=Neale |first2=Benjamin M. |last3=Perry |first3=John R. B. |last4=Sanders |first4=Alan R. |last5=Martin |first5=Eden R. |last6=Beecham |first6=Gary W. |last7=Harris |first7=Kathleen Mullan |last8=Auton |first8=Adam |last9=Långström |first9=Niklas |last10=Lundström |first10=Sebastian |last11=Lichtenstein |first11=Paul |last12=Team16 |first12=Paul |last13=Sathirapongsasuti |first13=J. Fah |last14=Guo |first14=Shengru |last15=Abdellaoui |first15=Abdel |last16=Busch |first16=Alexander S. |last17=Wedow |first17=Robbee |last18=Maier |first18=Robert |last19=Nivard |first19=Michel G. |last20=Verweij |first20=Karin J. H. |last21=Ganna |first21=Andrea |title=Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior |journal=Science |date=30 August 2019 |volume=365 |issue=6456 |pages=eaat7693 |doi=10.1126/science.aat7693 |pmid=31467194 |language=en |issn=0036-8075|pmc=7082777 }}
*{{cite web |title=Genetics of Sexual Behavior |url=https://geneticsexbehavior.info/what-we-found/ |website=Genetics of Sexual Behavior |publisher=geneticsexbehavior.info |access-date=30 August 2019 |date=28 February 2018}}
*{{cite journal |last1=Lambert |first1=Jonathan |title=No 'gay gene': Massive study homes in on genetic basis of human sexuality |journal=Nature |volume=573 |issue=7772 |pages=14–15 |date=29 August 2019 |doi=10.1038/d41586-019-02585-6 |pmid=31481774 |language=en|bibcode=2019Natur.573...14L |doi-access=free }}</ref>

==== Hormones ====
{{Main|Prenatal hormones and sexual orientation}}

The hormonal theory of sexuality holds that just as exposure to certain hormones plays a role in fetal [[sex differentiation]], hormonal exposure also influences the sexual orientation that emerges later in the adult. Fetal hormones may be seen as either the primary influence upon adult sexual orientation or as a co-factor interacting with genes or environmental and social conditions.<ref name="BornGay-passim">Wilson, G., & Q. Rahman, ''Born Gay: The Psychobiology of Human Sex Orientation'', ''op. cit.''</ref>

For humans, the norm is that females possess two X sex chromosomes, while males have one X and one Y. The default developmental pathway for a human fetus being female, the Y chromosome is what induces the changes necessary to shift to the male developmental pathway. This differentiation process is driven by [[androgen]] hormones, mainly [[testosterone]] and [[dihydrotestosterone]] (DHT). The newly formed testicles in the fetus are responsible for the secretion of androgens, which will cooperate in driving the sexual differentiation of the developing fetus, including its brain. This results in sexual differences between males and females.<ref>{{cite journal|last=Siiteri|first=PK|author2=Wilson, JD |title=Testosterone formation and metabolism during male sexual differentiation in the human embryo.|journal=The Journal of Clinical Endocrinology and Metabolism|date=Jan 1974|volume=38|issue=1|pages=113–25|pmid=4809636|doi=10.1210/jcem-38-1-113}}</ref> This fact has led some scientists to test in various ways the result of modifying androgen exposure levels in mammals during fetus and early life.<ref name=Levy>{{cite book|last1=LeVay|first1=Simon|title=Gay, Straight, and the reason why. The science of sexual orientation.|date=2011|publisher=Oxford University Press|isbn=978-0-19-993158-3|pages=45–71, 129–56}}</ref>

==== Birth order ====

{{Main|Fraternal birth order and sexual orientation}}

A significant volume of research has demonstrated that the probability of a male growing up to be gay increases with each older brother he has from the same mother. Known as the ''fraternal birth order'' (FBO) effect, scientists attribute this to a [[Prenatal development|prenatal]] biological mechanism – specifically a maternal immune response to male fetuses – since the effect is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. This process, known as the ''maternal immunization hypothesis'' (MIH), would begin when cells from a male fetus enter the mother's circulation during pregnancy. These cells carry Y-proteins, which are thought to play a role in brain masculinisation (sex-differentiation) during fetal development. The mothers immune system builds antibodies to these Y-proteins. These antibodies are later released on future male fetuses and interfere with the masculinization role of Y-proteins, leaving regions of the brain responsible for sexual orientation in the 'default' female-typical arrangement, causing the exposed son to be more attracted to men over women. Biochemical evidence for this hypothesis was identified in 2017, finding that mothers with a gay son, especially those with older brothers, had significantly higher levels of anti-bodies to the NLGN4Y Y-protein than mothers with heterosexual sons.<ref name=":22">{{Cite journal|last=Balthazart|first=Jacques|date=2018-01-09|title=Fraternal birth order effect on sexual orientation explained|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=115|issue=2|pages=234–236|doi=10.1073/pnas.1719534115|issn=0027-8424|pmc=5777082|pmid=29259109|doi-access=free}}</ref><ref name=":02">{{Cite journal|last1=Bogaert|first1=Anthony F.|last2=Skorska|first2=Malvina N.|last3=Wang|first3=Chao|last4=Gabrie|first4=José|last5=MacNeil|first5=Adam J.|last6=Hoffarth|first6=Mark R.|last7=VanderLaan|first7=Doug P.|last8=Zucker|first8=Kenneth J.|last9=Blanchard|first9=Ray|date=2018-01-09|title=Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=115|issue=2|pages=302–306|doi=10.1073/pnas.1705895114|issn=0027-8424|pmc=5777026|pmid=29229842|doi-access=free}}</ref>

The effect becomes stronger with each successive male pregnancy, meaning the odds of the next son being gay increase by 38–48%. This does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the first born son, to 4% for the second, 6% for the third and so on.<ref name=":22" /><ref name="Blanchard199722">{{cite journal|author=Blanchard R|year=1997|title=Birth order and sibling sex ratio in homosexual versus heterosexual males and females|journal=Annual Review of Sex Research|volume=8|pages=27–67|pmid=10051890}}</ref> Scientists have estimated between 15% and 29% of gay men may owe their sexual orientation to this effect, but the number may be higher, as prior miscarriages and terminations of male pregnancies may have exposed their mothers to Y-linked antigens. The fraternal birth order effect would not likely apply to first born gay sons; instead, scientists say they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence brain development.<ref name=":02" /> This effect is nullified if the man is left-handed.<ref>{{Cite journal|last1=Blanchard|first1=R.|last2=Cantor|first2=J.M.|last3=Bogaert|first3=A.F.|last4=Breedlove|first4=S.M.|last5=Ellis|first5=L.|year=2006|title=Interaction of fraternal birth order and handedness in the development of male homosexuality|url=http://msu.edu/~breedsm/pdf/Blanchard2006IntFratHandHBFinal.pdf|journal=Hormones and Behavior|volume=49|issue=3|pages=405–14|doi=10.1016/j.yhbeh.2005.09.002|pmid=16246335|s2cid=16151756|access-date=2007-05-18|archive-date=2020-11-29|archive-url=https://web.archive.org/web/20201129162835/https://msu.edu/~breedsm/pdf/Blanchard2006IntFratHandHBFinal.pdf|url-status=live}}</ref> [[Ray Blanchard]] and [[Anthony Bogaert]] are credited with discovering the effect in the 1990s.<ref>{{Cite journal|last1=Bogaert|first1=Anthony F.|last2=Skorska|first2=Malvina|date=2011-04-01|title=Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review|url=http://www.sciencedirect.com/science/article/pii/S0091302211000227|journal=Frontiers in Neuroendocrinology|series=Sexual Differentiation of Sexual Behavior and Its Orientation|language=en|volume=32|issue=2|pages=247–254|doi=10.1016/j.yfrne.2011.02.004|pmid=21315103|s2cid=45446175|issn=0091-3022|access-date=2020-07-09|archive-date=2020-07-09|archive-url=https://web.archive.org/web/20200709203424/https://www.sciencedirect.com/science/article/pii/S0091302211000227|url-status=live}}</ref> [[J. Michael Bailey]] and [[Jacques Balthazart]] say the FBO effect demonstrates that sexual orientation is heavily influenced by prenatal biological mechanisms rather than unidentified factors in socialization.<ref name=":1">{{Cite journal|last=Bailey|first=J. Michael|date=2018-01-01|title=The Fraternal Birth Order Effect Is Robust and Important|journal=Archives of Sexual Behavior|language=en|volume=47|issue=1|pages=18|doi=10.1007/s10508-017-1115-1|pmid=29159754|s2cid=35597467|issn=1573-2800}}</ref><ref name=":22"/>

=== Environmental factors ===

{{Main|Environment and sexual orientation}}

In the field of genetics, any factor which is non-genetic is considered an ''environmental influence''. However, environmental influence does not automatically imply that the social environment influences or contributes to the development of sexual orientation. There is a vast non-social environment that is non-genetic yet still biological, such as [[prenatal development]], that likely helps shape sexual orientation.<ref name="Bailey">{{cite journal|vauthors=Bailey JM, Vasey PL, Diamond LM, Breedlove SM, Vilain E, Epprecht M|date=2016|title=Sexual Orientation, Controversy, and Science|journal=[[Psychological Science in the Public Interest]]|volume=17|issue=21|pages=45–101|doi=10.1177/1529100616637616|pmid=27113562|doi-access=free}}</ref>{{rp|76}}

====Social factors====
There is no substantive evidence to support the suggestion that early childhood experiences, parenting, sexual abuse, or other adverse life events influence sexual orientation. Hypotheses for the impact of the post-natal social environment on sexual orientation are weak, especially for males.<ref name="Bailey" /> Parental attitudes may affect whether or not children openly identify with their sexual orientation.<ref name="AmPsycholAssn-whatis" /><ref name="pediatrics2004" /><ref name="rcp2007" /><ref>"Different aspects of sexual orientation may be influenced to a greater or lesser degree <nowiki>[</nowiki>p. 303:<nowiki>]</nowiki> by experiential factors such that sexual experimentation with same-gender partners may be more dependent on a conducive family environment than the development of a gay or lesbian identity." Susan E. Golombok & Fiona L. Tasker, ''Do Parents Influence the Sexual Orientation of Their Children?'', in J. Kenneth Davidson, Sr., & Nelwyn B. Moore, ''Speaking of Sexuality: Interdisciplinary Readings'' (Los Angeles, Calif.: Roxbury Publishing, 2001) ({{ISBN|1-891487-33-7}}), pp. 302–03 (adapted from same authors, ''Do Parents Influence the Sexual Orientation of Their Children? Findings From a Longitudinal Study of Lesbian Families'', in ''Developmental Psychology'' (American Psychological Association), vol. 32, 1996, 3–11) (author Susan Golombok prof. psychology, City Univ., London, ''id.'', p. xx, & author Fiona Tasker sr. lecturer, Birkbeck Coll., Univ. of London, ''id.'', p. xxiii).</ref><ref>"Whereas there is no evidence from the present investigation to suggest that parents have a determining influence on the sexual orientation of their children, the findings do indicate that by creating a climate of acceptance or rejection of homosexuality within the family, parents may have some impact on their children's sexual experimentation as heterosexual, lesbian, or gay." ''Do Parents Influence the Sexual Orientation of Their Children?'', ''ibid.'', in ''Speaking of Sexuality'', ''id.'', p. 303 (adapted per ''id.'', p. 303).</ref> Though it has since been found to be based on prejudice and misinformation, it was once thought that homosexuality was the result of faulty psychological development, resulting from childhood experiences and troubled relationships, including childhood sexual abuse.<ref name=AmPsycholAssn-whatis /><ref name="AmPsychiAssn-Sexual orientation" />  Such hypotheses "have been associated with highly charged political, moral and theological grounds for wanting to believe that it can".<ref>{{cite journal |last1=Cook |first1=Christopher C. H. |title=The causes of human sexual orientation |journal=Theology & Sexuality |date=2021 |volume=27 |issue=1 |pages=1–19 |doi=10.1080/13558358.2020.1818541|s2cid=225189040 }}</ref>

=== Influences: professional organizations' statements ===

The [[American Academy of Pediatrics]] in 2004 stated:<ref name="pediatrics2004" />
{{blockquote|The mechanisms for the development of a particular sexual orientation remain unclear, but the current literature and most scholars in the field state that one's sexual orientation is not a choice; that is, individuals do not choose to be homosexual or heterosexual. A variety of theories about the influences on sexual orientation have been proposed. Sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences. In recent decades, biologically based theories have been favored by experts. Although there continues to be controversy and uncertainty as to the genesis of the variety of human sexual orientations, there is no scientific evidence that abnormal parenting, sexual abuse, or other adverse life events influence sexual orientation. Current knowledge suggests that sexual orientation is usually established during early childhood.}}

The [[American Psychological Association]], the [[American Psychiatric Association]], and the [[National Association of Social Workers]] in 2006 stated:<ref name="Calif-amici" />
{{blockquote|Currently, there is no scientific consensus about the specific factors that cause an individual to become heterosexual, homosexual, or bisexual – including possible biological, psychological, or social effects of the parents' sexual orientation. However, the available evidence indicates that the vast majority of lesbian and gay adults were raised by heterosexual parents and the vast majority of children raised by lesbian and gay parents eventually grow up to be heterosexual.}}

The [[Royal College of Psychiatrists]] in 2007 stated:<ref name="rcp2007" />
{{blockquote|Despite almost a century of psychoanalytic and psychological speculation, there is no substantive evidence to support the suggestion that the nature of parenting or early childhood experiences play any role in the formation of a person's fundamental heterosexual or homosexual orientation. It would appear that sexual orientation is biological in nature, determined by a complex interplay of genetic factors and the early uterine environment. Sexual orientation is therefore not a choice, though sexual behaviour clearly is.}}

The American Psychiatric Association stated in 2011:<ref name="AmPsychiAssn-Sexual orientation" />
{{blockquote|No one knows what causes heterosexuality, homosexuality, or bisexuality. Homosexuality was once thought to be the result of troubled family dynamics or faulty psychological development. Those assumptions are now understood to have been based on misinformation and prejudice.}}

A legal brief dated September 26, 2007, and presented on behalf of the American Psychological Association, California Psychological Association, American Psychiatric Association, National Association of Social Workers, and National Association of Social Workers, California Chapter, stated:<ref name="Calif-amici" />

{{blockquote|Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation – heterosexuality, homosexuality, or bisexuality – is determined by any particular factor or factors. The evaluation of ''amici'' is that, although some of this research may be promising in facilitating greater understanding of the development of sexual orientation, it does not permit a conclusion based in sound science at the present time as to the cause or causes of sexual orientation, whether homosexual, bisexual, or heterosexual.}}