Theories: Link Between Evolution and Homosexuality

Theories: Link Between Evolution and Homosexuality

From an evolutionary perspective, homosexuality should not exist and persist in human societies, especially considering the Darwinian model of natural selection, which argues that factors that reduce fitness and fecundity or fruitfulness and fertility should progressively disappear over time following the course of evolution and natural selection.

Scholars from different disciplines have explored the evolutionary function and possible genetic basis for the appearance and maintenance of homosexuality in the human population. While the entire topic is still in its early stage, the relevant studies collectively represent an emerging theory that draws the link between evolution and homosexuality.

Understanding the Link Between Evolution and Homosexuality

Sexual Antagonism and Mating Advantage

Italian geneticist Andrea Camperio Ciani has carried out several studies that investigated the relationship between homosexuality and the fertility of women. 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.

A 2009 study by Ciani and Francesca Iemmola replicated and expanded a similar study conducted in 2004. It involved 98 heterosexual probands and 152 homosexual probands who accomplished a self-administered questionnaire that collected biographical data, Kinsey Scale results, and fecundity data based on live-born offspring.

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. Results of the survey suggest that homosexuality is a strong genetic trait.

Ciani and other researchers conducted another similar study in 2012 that investigated 161 female European probands and scrutinized possible influences, including physiological, behavioral, and personality factors. 61 female probands were either mothers or maternal aunts of homosexual men and the remaining 100 female probands who were either mothers or maternal aunts of heterosexual men were used as a control.

An interpretation of the results explained that homosexuality in men might be 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 gynecological 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.

The collective results also 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, thereby 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 4,904 anonymously completed a detailed questionnaire examining sexual behaviors and attitudes.

Initial analysis of the data revealed that psychologically masculine females and masculine males exhibited sexual behaviors and attitudes that make them more likely to be non-heterosexual while complete heterosexual females and males had more opposite-sex sexual partners.

Through further statistical modeling of the data, the researchers uncovered an interesting trend in which heterosexuals a non-heterosexual twin tend to have more opposite-sex partners than heterosexual twin pairs. When taken together, these results suggest that genetic factors that result in a predisposition toward homosexuality may confer a mating advantage in heterosexuals. Thus, the presence of these genetic factors also explains the maintenance of homosexuality in the human population, thereby providing a link between evolution and homosexuality.

Homosexuality and Kin Selection Hypothesis

The Kin Selection Hypothesis might also explain the evolutionary role of homosexuality. This 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 were men who prefer same-sex partners are widely accepted and recognized 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 heterosexual Samoan women or men. They are willing to babysit or provide care, tutor their nieces in arts and music, and provide financial assistance for education or medicine, among others. Note that an 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 showed 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 behavior—than heterosexual men. Vasey and VanderLaan further tested the Kin Selection Hypothesis in 2010 using a larger and independent sample. Results showed that the fa’afafine exhibited significantly higher avuncular tendencies even when compared to a more closely matched control group that also lacks direct parental 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 investigate 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 behavior 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 a certain population are critical to perpetuating genes shared by family members or relatives.

Genetics and Male Sexual Orientation

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, for 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. These SNPs represent differences of a single letter in the genetic code. They measured the extent to which each of the SNPs was shared by men in the study.

The only trait unambiguously shared by all participants was homosexuality. All other traits, including the color of hair, height, and intelligence, among others, varied by several degrees between each brother in a pair and between all sets of brothers. Whatever SNPs consistently identified in the same genetic locations across the group would most likely 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 chromosome 8. Xq28 was first identified in 1993 as having a possible link to homosexuality and sexual behaviors in animals. On the other hand, the link between 8q12 and homosexuality was first suggested in 2005. Both Xq28 and 8q12 are two regions in the human genome that might have ties with the manifestation of 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 combination of factors. The better conclusion is that the presence of these two regions in the genome of gay men had predisposed them toward homosexuality, thereby suggesting a genetic basis on the emergence of this sexual orientation in the human population.

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 band of nerve fibers 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. The same feature has been observed in left-handed individuals. According to the researchers, the size of the corpus callosum is largely inherited, thereby suggesting a genetic factor in sexual orientation.

It is 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, and there is not established genetic basis linking evolution and homosexuality.

Role of Genetics and Environmental Factors

N. Långström, Q. Rahman, E. Carlström, and P. Lichtenstein investigated how genetics and environment play an important role in the development of homosexual orientation. They surveyed 3,826 same-gender identical and fraternal twin pairs or about 7,652 individuals to determine the sexual orientation and sexual behavior 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 behavior and other individual-specific environmental factors—not to be confused with parenting or social environment—accounted for around 64 percent. What this means is that men become homosexual because of different development pathways instead of a single pathway. For women, genetics accounted for roughly 18 percent of the variation in homosexual behavior, 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 remain modest while non-shared environmental factors, which may include hormonal exposure in the womb or fetal development, dominate. Moreover, heredity had approximately the same influence as shared environmental factors in women, whereas the latter had no impact on sexual behavior in men.

Summary: Working Theories Linking Evolution and Homosexuality

Based on the discussions above, especially the overview of the existing body of literature about the possible link between evolution and homosexuality, several working hypotheses could be surmised and summarized.

The series of studies by Ciani et al. and Brendan P. Zietsch et al. suggest homosexuality remains pervasive in a population because it confers heterosexual individuals with some evolutionary or more specifically, reproductive advantage. These studies show that the presence of homosexual men increases the reproductive probability of and mating success in both heterosexual men and women, thus serving as a natural and evolutionary mechanism for reproduction.

It is also interesting to look at the link between evolution and homosexuality on both familial and communal levels. The series of studies involving Samoan fa’afafine came into a working theory that the emergence and maintenance of homosexual men in a given family provide a survival advantage. Because these individuals do not reproduce, they help in ensuring the continuity of their clans by channeling their resources, including their avuncular tendencies, toward the younger members of their families.

The appearance of homosexuality also has a genetic basis. Homosexual men have some genetic variations compared to heterosexual men. However, more studies need to be done to determine all genes that predispose an individual toward a homosexual orientation. Nonetheless, the persistence of these genes can also be a starting point to investigate further the link between evolution and homosexuality, particularly on a genetic level.

Note that the studies mentioned above have several limitations. For starters, the conclusions and suggestions are simply working hypotheses. The entire topic remains considerably at its early stage. More studies need to be done to replicate, confirm, or reject these assumptions. Furthermore, it is obvious that these studies also focused on male homosexuality. Additional studies should be done on homosexual women, as well as other sexual orientations.


  • Camperio Ciani, A. S., Fontanesi, L., Iemmola, F., Giannella, E., Ferron, C., and Lombardi, L. 2012. “Factors Associated with Higher Fecundity in Female Maternal Relatives of Homosexual Men.” The Journal of Sexual Medicine. 9(11): 2878-2887. DOI: 1111/j.1743-6109.2012.02785.x
  • Camperio Ciani, A. and Pellizzari, E. 2012. “Fecundity of Paternal and Maternal Non-Parental Female Relatives of Homosexual and Heterosexual Men.” PLoS ONE. 7(12): e51088. DOI: 1371/journal.pone.0051088.
  • Iemmola, F. and Camperio Ciani, A. 2008. “New Evidence of Genetic Factors Influencing Sexual Orientation in Men: Female Fecundity Increase in the Maternal Line.” Archives of Sexual Behavior. 38(3): 393-399. DOI: 1007/s10508-008-9381-6
  • Långström, N., Rahman, Q., Carlström, E., and Lichtenstein, P. 2008. “Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden.” Archives of Sexual Behavior. 39(1): 75-80. DOI: 1007/s10508-008-9386-1
  • Sanders, A. R., Martin, E. R., Beecham, G. W., Guo, S., Dawood, K., Rieger, G., Badner, J. A., Gershon, E. S., Krishnappa, R. S., Kolundzija, A. B., Duan, J., Gejman, P. V., and Bailey, J. M. 2014. “Genome-Wide Scan Demonstrates Significant Linkage for Male Sexual Orientation.” Psychological Medicine. 45(7): 1379-1388. DOI: 1017/s0033291714002451
  • Vasey, P. L., Pocock, D. S., and VanderLaan, D. P. 2007. “Kin Selection and Male Androphilia in Samoan Fa’afafine.” Evolution and Human Behavior. 28(3): 159-167. DOI: 1016/j.evolhumbehav.2006.08.004
  • Vasey, P. L., and VanderLaan, D. P. 2010. “An Adaptive Cognitive Dissociation Between Willingness to Help Kin and Nonkin in Samoan Fa’afafine.” Psychological 21(2): 292-297. DOI: 10.1177/0956797609359623
  • Vasey, P. L. and VanderLaan, D. P. 2008. “Avuncular Tendencies and the Evolution of Male Androphilia in Samoan Fa’afafine.” Archives of Sexual Behavior. 39(4): 821-830. DOI: 1007/s10508-008-9404-3
  • Witelson, S. F., Kigar, D. L., Scamvougeras, A., Kideckel, D. M., Buck, B., Stanchev, P. L., Bronskill, M., and Black, S. 2007. “Corpus Callosum Anatomy in Right-Handed Homosexual and Heterosexual Men.” Archives of Sexual Behavior. 37(6): 857-863. DOI: 1007/s10508-007-9276-y
  • Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J. H., Keller, M. C., Macgregor, S., Wright, M. J., Bailey, J. M., and Martin, N. G. 2008. Genetic Factors Predisposing to Homosexuality May Increase Mating Success in Heterosexuals. Evolution and Human Behavior. 29(6): 424-433. DOI: 1016/j.evolhumbehav.2008.07.002