Homosexuality

Genome Wide Association Analysis (GWAS)

 * Ganna et al. (2019) a recent large scale GWAS analysis, looked at 477,522 individuals in the United Kingdom and United States, did a replication analysis with 15,142 people from the U.S. and Sweden, and a follow-up using different sexual preferences. Not only did they find that homosexuality was a polygenic trait, meaning that it's influenced by many genes, they also found that the loci they examined explained 8%-25% of the variation in same-sex attraction. This means that half or less than half of the reason someone is a homosexual is because of their genes.
 * Limitations include the fact that GWAS analysis can not detect all the genes in the human genome, this is because GWAS analysis often detect noise from gene variants not associated with the trait being looked at. To combat this, researchers set a threshold that a genetic marker needs to pass in order to be detected as a true signal rather than just noise; this in turn often leaves out markers that are associated with the trait being looked at but they have small effects, thus being counted in as noise. Due to this problem, it's highly possible that the results found by Ganna et al. are underestimates. The actual variation in SSA explained by genes could be higher than 8%-25%.
 * Sanders et al (2017) did a GWAS analysis on male homosexuality

Twin Studies
In genetics, one of the most widely used methods of estimating heritability is the use of twin studies.


 * Bailey et al. (2000) looked at twins from the Australian National Health and Medical Research Council Twin Registry, with this sample being representative and this allowing us to make inferences because the sampling technique used was a probability sample. Their heritability estimates differed by sex, with their findings being largely significant for men and not women. The heritability estimate of SSA for men was 0.45 and for women being 0.08, meaning that 45% of the variance in SSA for men was genetic but for women it was less that 1%. In conjunction with their heritability estimates, Bailey et al. also reported that their confidence intervals (CI), which tell us the range of values our true value lies in, was 95% (meaning that you can be 95% certain the true value lies in your ranges produced) and had ranges from 0.00-0.71 for men and 0.00-0.67 for women. This CIs are very wide and this means that the wide CIs equal less accuracy. Due to their wide CIs, it's hard to make any judgment.
 * Kendler et al. (2000) attempted to estimate the role of genetic and environmental factors in the determination of sexual orientation in a more representative sample by using a self-report questionnaire in a U.S. national sample of twin and non-twin sibling pairs in household samples. Sexual orientation was classified as heterosexual or non-heterosexual (bisexual or homosexual). All analyses demonstrated familial resemblance for sexual orientation. Resemblance was greater in the monozygotic twins than in the dizygotic twins. The heritability estimate of SSA for men was 0.62%
 * Langstrom et al. (2010) examined 3,826 identical twins and 1,506 fraternal twins to measure the heritability of SSA, with homosexuality being defined in terms of lifetime same-sex partners. Their heritability estimate produced was 0.39 for having one lifetime same-sex partner and 0.34 (95% CI=0.00-0.59) for total number of same-sex partners (95% CI=0.00-0.53). Non-shared environmental variables experienced by one twin but not the other explained 61% and 66% of the variance, but shared environment explained none of the variation. For women, the heritability of having one lifetime same-sex partner was 0.19 (95% CI=0.00-0.49) and 0.18 for for lifetime same-sex partners (95% CI=0.11-0.45). Non-shared environments explained 64% and 66% of the variance, but shared environment explained 17% and 16%. These results show that while the heritability of SSA is not large, it is also not insignificant. The environment still plays a large role.
 * Bearman and Bruckner (2002) took advantage of Add Health data and looked at children in grades 7-12. In their sample of 18,841 adolescents, reported SSA, 3.1% reported a same-sex romantic relationship, and 1.5% reported same-sex sexual behavior. In their analysis, they did not find a significant genetic influence on SSA. The heritability of same-sex attraction was 7.7% for male identical twins and 5.3% for females. It's possible that as someone ages, the heritability of SSA increases with age. In other words, SSA might be more environmental at a younger age and moderately genetic at an older age. Further research would be needed to support this hypothesis.

In conclusion, the heritability of SSA is most likely less than 0.50. This means that the environment could play a larger role than genes. Although it's popular in contemporary society to say that homosexuals are "born this way", this does not seem to be the case. Based off of the available evidence, SSA is not set in stone and already innate at a young age, but genetics do play a significant role in SSA.

Fraternal birth order
The more older brothers a man has from the same mother, the greater the probability is that he will have a homosexual orientation. The most consistent bio-demographic correlate of sexual orientation in men is the number of older brothers (fraternal birth order).


 * Slater & Cantab 1962 shows that homosexual men tend to have a greater number of older siblings than comparable heterosexual men
 * This phenomenon is found in homosexual males across different races, cultures, and widely separated geographic regions.
 * Blanchard 1997, Blanchard et al 1998, Ellis & Blanchard 2001, Blanchard 2001, Puts et al 2006, shows that each older brother increased the odds of homosexuality in a later-born brother by 28-44%
 * Biological brothers increase the odds of homosexuality in later-born males even if they were reared in different households, whereas non-biological siblings, such as step-brothers or adopted brothers, have no effect on male sexual orientation.

Childhood trauma

 * The American Psychiatric Association states: "...no specific psycho-social or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse. Sexual abuse does not appear to be more prevalent in children who grow up to identify as gay, lesbian, or bisexual, than in children who identify as heterosexual".
 * Holmes et al (1998) in the Journal of the American Medical Association reported that "Abused adolescents, particularly those victimized by males, were up to 7 times more likely to self-identify as gay or bisexual than peers who had not been abused".
 * Limitations may include that the study went on to state that victims may self-identify as gay or bisexual before the abuse occurs, implying that their non-heterosexual orientation and identity is not caused by the abuse, and reported that no longitudinal study had determined a causal relationship between sexual abuse and sexual orientation.


 * Doll et al. (1991) analyzed 1,001 adult homosexual and bisexual men who attended STD clinics from May 1989–1990 and found that 37% of participants reported that they have been encouraged or forced to have sexual contact with an older or more powerful partner before the age of 19; 94% of these cases occurred with men. The median age of the participants first age of contact was at just 10 years old. Of the cases looked at, 51% involved the use of force and 33% involved anal sex.
 * Runtz and Briere (1986) utilized a questionnaire on 278 undergraduate female students to assess the effects of child abuse. Not only were those who were abused as children more likely to to express more negative behaviors, but they were also more likely to have “homosexual contact.” Referencing, 2 previous studies — with one finding a positive link and the other finding no link, the authors suggest that more research is needed on this issue.
 * Wilson and Widom (2009), a 30-year longitudinal study published in the Archives of Sexual Behavior, although the authors found that men with histories of childhood sexual abuse were more likely to report ever having had same-sex sexual partners, they did not find any "significant relationships between childhood physical abuse or neglect and same-sex sexual orientation in adulthood"; neither men nor women with histories of childhood physical abuse, sexual abuse, or neglect reported more same-sex sexual partners in the previous year or same-sex romantic cohabitation compared to men and women without such histories. Authors of the study speculated that "sexual abuse may result in uncertainty regarding sexual orientation and greater experimentation with both same- and opposite-sex relationships", but may not affect ultimate sexual orientation.
 * Roberts, Glymour, and Koenen (2012), they found that history of sexual abuse increased the prevalence of same-sex attraction by 2.0% points and same-sex identity by 0.7% points. The effects were more significant for men and the association between child abuse and SSA could differ by type of abuse and sex.
 * Bailey and Bailey (2013) and Rind (2013) responded
 * Roberts, Glymour, and Koenen (2013) responded again
 * Bailey, Ellingson, and Bailey (2014) responded for a third time
 * Roberts, Glymour, and Koenen (2014) responded for a fourth time

Childhood gender nonconformity

 * Bailey and Zucker (1995)is a meta-analysis of 48 studies that quantitatively reviews childhood sex-typed behavior in homosexual and heterosexual men and women. Effect sizes were large for both men and women, with men's significantly larger.
 * Bem (2008) argues that there is a casual link between childhood gender expression and sexual orientation. His theory "Exotic becomes erotic" or EBE offers an explanation for both opposite sex and same-sex desire. He does not intend his model as an absolute prescription for all individuals, but rather as a modal or average explanation. He suggests that biological variables affect early temperament, which in turn influence the development of preferences for sex typical or atypical play. Such preferences for sex typical or atypical play. Such preferences lead in turn to feeling different from opposite or same-sex peers. He further postulates that individuals develop attractions to those from whom they feel different during childhood. Thus, early gender interests lead to later physiological or sexual attractions.
 * Riegger (2008) Retrospective studies have also shown large differences in childhood gender nonconformity, but these studies have been criticized for possible memory biases. The authors studied an indicator of childhood gender nonconformity not subject to such biases: childhood home videos. They recruited homosexual and heterosexual men and women (targets) with videos from their childhood and subsequently asked heterosexual and homosexual raters to judge the gender nonconformity of the targets from both the childhood videos and adult videos made for the study. Pre-homosexual children were judged more gender nonconforming, on average, than pre-heterosexual children, and this pattern obtained for both men and women. This difference emerged early, carried into adulthood, and was consistent with self-report. In addition, targets who were more gender nonconforming tended to recall more childhood rejection. The convergence of results from these three research approaches increases our confidence in the validity of their shared conclusion that degree of gender nonconformity versus gender conformity is a predictor of future sexual orientation and that gender-nonconforming children tend to become homosexual adults
 * One should take methodological limitations into account when considering the results of this study. However, these limitations are different from the biases that may affect retrospective studies of heterosexual and homosexual adults using only self-reports or prospective studies of very atypical children.

Familial relationships

 * Gay men report having had less loving and more rejecting fathers, and closer relationships with their mothers, than non-gay men.
 * Limitations of this finding may include the fact CGN could effect familial relations


 * For men, same-sex marriage was associated with having older mothers, divorced parents, absent fathers, and being the youngest child.
 * Homosexual men have significantly more older male siblings than the homosexual women, who, in turn, have significantly more siblings than heterosexual men.



Martos et al 2014



Glick et al 2012

Milestone