From an evolutionary perspective, homosexuality is a trait that should not persist, especially considering the Darwinian model of natural selection. This theory suggests that factors that reduce fitness and fecundity or fruitfulness and fertility should progressively disappear over time following the course of evolution and natural selection. However, there are several studies that might explain the evolutionary function of and genetic basis for the appearance and maintenance of homosexuality in human population.
Italian geneticist Andrea Camperio Ciani has carried out several studies that investigated the relationship between homosexuality and fertility of women. So far, his studies revealed that female relatives of homosexual men tend to have more children than female relatives of heterosexual men. This suggests that male homosexuality is a manifestation of an evolutionary advantage.
In the 2009 study of Ciani and Francesca Iemmola that replicated but expanded a similar study conducted in 2004, results confirm that male homosexuality is a strong genetic trait. To be specific, the study recruited 98 heterosexual probands and 152 homosexual probands—a total of 250 probands. Note that a proband is a term used most often in medical genetics and other medical fields to denote a particular subject being studied or reported on. In the study, the 250 probands accomplished a self-administered questionnaire that collected biographical data, Kinsey Scale results, and fecundity data based on live-born offspring.
Ciani and other researchers conducted another similar study in 2012 that investigated 161 female European proband subjects and scrutinized possible influences, including physiological, behavioural, and personality factors—61 female probands were either mothers or maternal aunts of homosexual men and the remaining 100 female probands who were were either mothers or maternal aunts of heterosexual men were used as a control.
Results of the studies revealed that homosexuality in men is a manifestation of an evolutionary benefit acquired and enjoyed by their families. Mothers and maternal aunts of gay men tend to have significantly more offspring than the maternal relatives of straight men, thus suggesting that there are sexually antagonistic genetic factors that influence both homosexual orientation and fecundity of women. These genetic factors provide mothers and maternal aunts of homosexual men more advantage than the mothers and maternal aunts of heterosexual men—particularly by promoting female reproductive ability and making women more attractive to men.
Sexually antagonistic selection is a component of the Darwinian evolution model that describes how certain genetic factors spread in the population by giving a reproductive advantage to one sex while disadvantaging the other.
Ciani also found out that the mothers and maternal aunts of homosexual men display fewer gynaecological disorders or complications during pregnancy. They are also more extroverted, as well as funnier, happier and more relaxed. Furthermore, they have fewer family problems and social anxieties.
Nonetheless, the results suggest that genetic factors that are partly linked to the X-chromosome and that influence homosexual orientation in males are not selected against because they increase fecundity in female carriers, thus offering a solution to the Darwinian paradox and an explanation of why natural selection does not progressively eliminate homosexuals.
Another study involving twins has also investigated the link between genetic factors, homosexuality, and heterosexual mating advantage. Researchers Brendan P. Zietsch et al examined a large data set in which a large community-based twin sample numbering to 4,904 anonymously completed a detailed questionnaire examining sexual behaviours and attitudes.
Initial analysis of the data revealed that psychologically masculine females and masculine males exhibited sexual behaviours and attitudes that make them more likely to be nonheterosexual while complete heterosexual females and males had more opposite-sex sexual partners. Through a further statistical modelling of the data, the researchers uncovered an interesting trend in which heterosexuals a nonheterosexual twin tend to have more opposite-sex partners than heterosexual twin pairs. When taken together, these results suggest that genetic factors that result in predisposition toward homosexuality may confer a mating advantage in heterosexuals. Thus, the presence of these genetic factors also explains the evolution and maintenance of homosexuality in the population.
The Kin Selection Hypothesis might also explain the evolutionary role of homosexuality. Accordingly, the theory suggests that homosexual men could help perpetuate the family genes by acting altruistically toward nieces and nephews. Evolutionary psychologists Paul L. Vasey, David S. Pocock, and Doug P. VanderLaan tested this hypothesis on the Pacific Island of Samoa where men who prefer same-sex partners are widely accepted and recognised as a distinct gender category called fa’afafine.
Previous research revealed that the fa’afafine are more altruistically inclined toward their nieces and nephews than either Samoan women or heterosexual men. They are willing to babysit, tutor their nieces in art and music, and provide financial assistance for medicine or education, among others.
The earlier 2006 study of Vasey, Pocock, and VanderLaan tested the basic prediction of the Kin Selection Hypothesis by comparing the altruistic tendencies between the fa’afafine and heterosexual men. Results revealed that both groups do not differ in terms of overall generosity and financial allocation. However, the fa’afafine reported greater avuncular tendencies—or uncle-like behaviour—than heterosexual men. Vasey and VanderLaan further tested the Kin Selection Hypothesis in a 2010 using a larger and independent sample. Results of their study revealed that the fa’afafine exhibit significantly higher avuncular tendencies even when compared to a more closely matched control group that also lacks direct parent care responsibilities—or heterosexual men with no children.
To further test the Kin Selection Hypothesis, Vasey and VanderLaan made a follow-up study in 2010 to investigated the psychology of the fa’afafine and determine if their altruism is directed specifically toward their kin rather than children in general. Results revealed that not only the fa’afafine are more altruistic than Samoan women or heterosexual men, they are also more altruistic and avuncular behaviour toward their kin while displaying weaker tendencies toward other children. From an evolutionary perspective, the researchers noted that the emergence and maintenance of same-sex attraction in certain population are critical to perpetuate genes shared by family members or relatives.
While the particular genes involved in homosexuality are not yet identified, another 2014 study draws the links between genetics and male sexual orientation. Over the past five years, researchers Alan Sanders et al had collected blood and saliva samples from 409 pairs of gay brothers, including sets of twins or a total of 818 participants. They subjected these samples under a genetic analysis to look at the locations of genetic markers called single nucleotide polymorphisms or SNPs—differences of a single letter in the genetic code. Subsequently, they measured the extent to which each of the SNPs was shared by men in the study.
The only trait unambiguously shared by all 818 participants was homosexuality. All other traits, including colour of hair, height, and intelligence, among others varied by several degrees between each brothers in a pair and between all sets of brothers. Thus, whatever SNPs consistently identified in the same genetic locations across the group would most likely to be associated with homosexuality.
Results revealed five SNPs but two stood out because they are shared more commonly among the participants. These are the Xq28 in the X chromosome and 8q12 in the chromosome 8. Xq28 was first identified in 1993 as having a possible link to homosexuality and sexual behaviours in animals. On the other hand, the link between 8q12 and homosexuality was first suggested in 2005. Both Xq28 and 8q12 are thereby two regions in the human genome that have clear ties with homosexuality.
Sanders et al reminded that the results do not suggest that they have found specific genes for homosexuality. Instead, the results suggest that both Xq28 and 8q12 could be part of a combinations of a factor. The better conclusion is that the presence of these two regions in the genome of gay men had predisposed them toward homosexuality.
The study of Witelson et al also establishes the genetic basis for homosexuality by exploring a possible association with handedness and brain structure. Handedness has been associated with differences in the structure and function of the brains between right-handed and left-handed people. Particularly, left-handed individuals have a larger region of the posterior corpus callosum, a thick ban of nerve fibres connecting the two brain hemispheres. Interestingly, an earlier research has also revealed that there are a higher proportion of left-handers in the homosexual population.
Results of the study of Witelson et al revealed that homosexual men have larger posterior corpus callosum than heterosexual men—as mentioned, feature that has been observed in left-handed individuals. According to the researchers, the size of the corpus callosum is largely inherited suggesting a genetic factor in sexual orientation.
It is, however, very important to remember that genes or genetic and evolutionary factors alone are not responsible for the emergence and maintenance of homosexuality. There remains no consensus in the scientific community about why a person develops a particular sexual orientation. Niklas Långström, Qazi Rahman, Eva Carlström, Paul Lichtenstein investigated how genetics and environment play an important role in the development of homosexual orientation. In doing so, they surveyed 3,826 same-gender identical and fraternal twin pairs or about 7,652 individual to determine the sexual orientation and sexual behaviour of each individual within a twin pair.
Studies of identical twins and fraternal twins are often used to unravel the genetic and environmental factors responsible for a trait. While identical twins share all of their genes and their entire environment, fraternal twins share only half of their genes and their entire environment. Therefore, greater similarity in a trait between identical twins compared to fraternal twins shows that genetic factors are partly responsible for the trait.
Results of the study revealed that that genetics accounted for around 35 percent of the differences between men in homosexual behaviour and other individual-specific environmental factors—not to be confused with parenting or social environment—accounted for around 64 percent. This means that men become homosexual because of different development pathways instead of a single pathway. For women, genetics accounted for roughly 18 percent of variation in homosexual behaviour, non-shared environment for roughly 64 percent, and 16 percent for the family environment.
The key takeaway from the study of Långström et al is that genetic influences are important but remains modest while non-shared environmental factors, which may include hormonal exposure in the womb or foetal development, dominate. Moreover, heredity had approximately the same influence as shared environmental factors in women, whereas the latter had no impact on sexual behaviour in men.
Details of the study of Iemmola and Ciani are in the article “New evidence of genetic factors influencing sexual orientation in men: Female fecundity increase in the maternal line” published in 2009 the journal Archives of Sexual Behaviour. More details of the 2012 follow-up study of Ciani et al are in the article “Factors associated with higher fecundity in female maternal relatives of homosexual men” published in The Journal of Sexual Medicine. Details of another 2012 follow-up study from Ciani and Pellizzari are in the article “Fecundity of paternal and maternal non-parental female relatives of homosexual and heterosexual men” published in the journal PLoS One.
More details of the study of Zietsch et al are in the article “Genetic factors predisposing to homosexuality may increase mating success in heterosexuals” published in 2009 in the journal Evolution and Human Behavior.
Details of the study of Vasey, Pocock, and VanderLaan are in the article “Kin selection and male androphilia in Samoan fa’afafine” published in 2007 in the journal Evolution of Human Behaviour. Details of the follow-up study of Vasey and VanderLaan are in the article “Avuncular male tendencies and the evolution of male androphilia in Samoan fa’afafine” published in 2008 in the journal Archives of Sexual Behavior. Details of the follow-up study of Vasey and VanderLaan are in the article “An adaptive cognitive dissonance between willingness to help kin and nonkin in Samoan fa’afafine” published in 2010 in the journal Psychological Science.
Further details of the study of Sanders et al are in the article “Genome-wide scan demonstrates significant linkage for male sexual orientation” published in 2015 in the journal Psychological Medicine. More details of the study of Witelson et al are in the article “Corpus callosum anatomy in right-handed homosexual and heterosexual men” published in 2008 in the journal Archives of Sexual Behaviour.
More details of the study of Långström are in the article “Genetic and environmental effects on same-sex sexual behavior: A population study of twins in Sweden” published in 2010 in the journal Archives of Sexual Behavior.