How Homosexual Behavior Has Persisted And Its Advantages In Human Society
Sexual behavior is a vital function in nearly every living organism on Earth. Sexual reproduction and thus the passing on of genes to offspring is an instinctual behavior exhibited in most organisms. Throughout human history, sexual behavior is a prominent part of life that has enabled our species to continue to prosper and reproduce. However, sexual behavior that does not directly lead to reproduction may be assumed to be disadvantageous. Homosexual behavior is often believed to be a “Darwinian Paradox”. This is because concentration on reproductive sex is lowered when homosexual behavior is exhibited, which would typically be considered a loss in evolutionary fitness. Yet, homosexual behavior has persisted in the human population. This essay will explain theories of why homosexual behavior has carried on and the advantages this exhibited behavior has in the human population. Due to the higher availability of research on male homosexuality, the focus will be on male homosexual behavior unless otherwise indicated. Despite what may be considered a disadvantageous behavior, for homosexual genetic factors and behavior to be maintained in the human population there must be underlying advantages that may not be immediately noticed without in depth analysis and research.
Delving into the genetic and heritability factors that establish homosexuality will help to understand why these tendencies have lasted and its role in human evolution. These factors include, but are not limited to: antagonistic sexual selection, social or group bonding, adaptive feminizing effects on the male homosexual brains, mixed homosexual and heterosexual behavior, environmental factors, and maternal effects. Understanding the evolution of homosexual occurrence in other animal species, especially in those of closely related species to humans, can be helpful to understand why this behavior occurs in the human population. Homosexual behavior has been observed in many species of mammals and even birds. Studies have shown that homosexual behavior has been used to form same-sex alliances in primates. For example, Old-World monkeys are more closely related to humans and they display homosexual behavior more frequently than New-World monkeys. In some nonhuman primates, homosexual behavior can even be as common as heterosexual behavior. Same-sex sexual partners are sometimes defended and guarded, and this type of behavior is displayed in the silverback gorillas of Virunga. These same sex alliances can be formed for a variety of reasons, whether it be for safety or even to obtain food when food is associated with sex. Early hominids relied heavily on alliances and cooperation to survive and obtain resources. There is no reason to suppose that these alliances formed likely through sex, excluded sexual relations between members of the same gender.
An individual that participates in same-sex behavior is not excluded from also participating in heterosexual behavior. An individual prone to homosexual behavior may not avoid heterosexual behavior. Heterosexuals may also on occasion participate in homosexual acts or have the desire to but avoid acting on them. Homosexuality and heterosexuality are not distinct categories and that there is a broad range of sexual behavior. There can be many reasons that a homosexual man or woman could pursue heterosexual relations instead. Some of these reasons include social, cultural, religious, or personal reasons that keep them from committing their homosexual desires. During many time periods it was dangerous for one to be identified as a homosexual so often these people may have avoided this type of behavior and instead married or reproduced with someone of the opposite gender. This is one way that someone with genetic factors that make them predisposed to homosexual behavior may choose to avoid it and instead reproduce and pass on these genes through heterosexual behavior. Heterosexuals may also on occasion participate in homosexual acts or have the desire to but avoid acting on them. It often needs to be reminded that homosexuality and heterosexuality are not distinct categories and that there is a broad range of sexual behavior. The opposite side of the spectrum is where a person that would typically only display heterosexual behavior participates in what Dewar (2003) refers to as opportunistic homosexuality.
Opportunistic homosexuality is where an individual participates in homosexual behavior for the benefit of its social function. Opportunities like this include forming alliances as mentioned previously and for its use in rituals and social bonding. Kimmel (2006) provides many examples of cultures where homoerotic sexual rituals are done. The Tarfs subculture of the Nacirema has quite a few rituals described where the men participate in sexual acts together. These opportunistic moments of homosexual behavior were not the only times that sexual behavior was likely expressed. Many of the participants in these rituals or opportunistic homosexuality likely also had heterosexual sex in other instances. Rituals and social bonding are two examples where the social structure of a group has an impact on the prominence of homosexual behavior. The social, economic, and cultural structure can influence whether a society expresses homosexual behavior.
Changes in the social structure of a group may even lead to evolutionary changes that increases the rate of genetic factors influencing homosexuality. Dewar (2003) argues that the introduction of the agricultural economic structure led to additional selective pressures that enabled adaptive homosexual genetic changes. Studies have shown that homosexual men have superior linguistic function compared to the average man. Cooperation is also a more common behavior shown by homosexual men. In the agriculture structure, linguistic abilities and the behavior to co-operate may have been far more adaptive and beneficial than behavior like aggression would be in these types of communities. Therefore, homosexual men were likely at a reproductive advantage with these traits and is likely linked to genetic factors that influence homosexuality.
It is thought that at the start of the agriculture era, bisexual behavior was exhibited by humans similarly to other social animals. The agriculture system and need for improved communication allowed homosexual behavior to remain adaptive and allowed further evolution that then led to the development of exclusive homosexual behavior. Compared to hunter-gatherer communities, homosexuality was more frequent in agriculture communities. Specifically, a cereal farming community allows larger populations of people to reside in the same area which may have played a role in the frequency of homosexuality. Having the capability of higher concentrated populations of people in these cereal farming communities can be known to cause psychosocial stressors due to the crowding effects of concentrated populations. It was proposed by Barber (1998) that psychosocial stressors may lead to maternal effects that increase the frequency in homosexuals.
Maternal effects have been theorized to play a role in the occurrence of homosexuality. These effects can be due to stresses that impact the mother that then in turn impact the development of the fetus. In the same study conducted by Barber (1998), the psychosocial stressors of living in a crowded environment were observed to see if population size played a role in the frequency of homosexuality. The larger communities with more crowding were found to be associated with a higher frequency of homosexuality. The higher frequency of homosexuality is likely due to the stressors that impact the development of a child in the womb via maternal effects. Modern urban and agriculture communities can be assumed to have higher rates of homosexual behavior than our hunter-gatherer ancestral communities would have observed. Numerous other sources of maternal effects have been studied to see how they influence the occurrence of homosexuality.
Maternal immunological effects caused by the fraternal birth order effect is another form of maternal effects that could play an influence on sexual orientation. The fraternal birth order was a study that found that the occurrence of male homosexuality was found to be more common and frequent as the number of older brothers increased. As more sons are born, the likelihood of the immune effect brought on by the mother’s immune system increases. The proposed mechanism of the maternal immune hypothesis is that the male fetus’s brain development can be affected by the mother’s immune system. The male fetus has male specific substances such as Y chromosome proteins that the mother’s immune system recognizes as foreign. The mother then develops antibodies that cross into the blood brain barrier of the male fetus which causes changes in the brain development. Since these antibodies would specifically interact with sex-dimorphic brain development, the sexual orientation of the son(s) could be influenced and thus lead them to become attracted to the same sex. The birth order and number of prior males born could cause increasing levels of immunization response from the mother and thus increase the sexual development effect in a male fetus. Complications that arise in testing the maternal immune hypothesis are mentioned by Bogaert and Skorska (2011) like the unknowns of how abnormal pregnancies could influence the fetus, the specific antigen involved in fraternal birth order and sexual orientation, and the reproductive and medical history of the mother. Once these barriers are resolved, it will be interesting to see if the immune response mechanism is one method that contributes to homosexual development in men.
Same-sex behavior has been proposed to arise from heritable factors as well as environmental factors: nature versus nurture. Studies investigating twins have been particularly enlightening in helping to distinguish the levels of genetic and environmental influences involved in homosexual behavior. A population study of twins using the Swedish Twin Registry was conducted by Långström, et al. (2010) to reach these conclusions and avoid self-selected volunteers that were done in previous twin studies. This study was able to come to the conclusion that same-sex behavior can be connected with both heritable factors and from environmental factors. Another twin study was conducted in Australia that was also trying to determine proof of heritability in same-sex behavior. It was found that there were significant heritability values found for overall sexual orientation in both males and females. These twin studies show proof that sexual orientation and thus the occurrence of homosexual behavior have genetic factors that are passed down to offspring.
Familial traits further indicate that there are genetic and heritable factors involved in that trait. Same-sex behavior has been studied to see if it can be considered familial. In the Australian twin study, sexual orientation was also concluded to be familial. If a family member is inclined to same-sex behavior, then it is more likely a family member closely related to them will also be prone to homosexual behavior. This finding adds additional proof that homosexual behavior is heritable since close relatives of a person that has homosexual tendencies will also be more likely to have homosexual tendencies.
Homosexual traits can be passed down from the relatives of homosexuals. Using twin sampling, Zietsch et al., (2008) found that a heterosexual twin that has a nonheterosexual twin sibling is more likely to have a higher number of opposite-sex partners than a set of twins comprised of heterosexuals. Analysis of these results could mean that genes that influence homosexuality may somehow give an advantage to heterosexuals to obtain more sexual partners. Since these twins share many common genes, genes that influence homosexuality can be maintained in a population by passing them down from the heterosexual relative that has a mating advantage.
The genes that influence homosexual behavior in males may be linked to beneficial traits in female relatives of these males and is considered as antagonistic sexual selection. Genes leading to homosexual behavior in males could be impactful to levels of fecundity found in the female relatives of these men. When there are genetic factors influencing male homosexuality, this leads to a positive increase in the total fecundity. This phenomenon can be considered antagonistic sexual selection. In this situation, genetic factors that lead to male homosexual behavior could be considered a lowered fitness trait due to the loss of reproductive opportunities. These genetic factors influence the genes in related females to have higher fecundity which is attributed as a higher fitness. Therefore, genetic factors that influence male homosexuality potentially causes a lowering of fitness in those males and also an increase in the fitness in the female relatives of these men. This shows that traits that influence male homosexual behavior is a trait that exhibits antagonistic sexual selection. Antagonistic sexual selection has been shown to maintain diversity in genes in a population despite the lowered fitness in one gender because of the increased fitness in the other gender. This is one of the first well supported proof of a sexually antagonistic trait in the human genome. Education and culture lower fecundity which then causes an increase in sexually antagonistic genetic factors associated with homosexuality which then increases fecundity to keep the scales balanced (Camperio Ciani 2014). Alterations to the expression of genes (epigenetics) could also be a source for homosexual behavior rather than just the genes themselves.
Epigenetics has recently become a common proposal for another causation of homosexuality. A model was proposed by Rice et al., (2012) where epi-marks evolved to convert sexual dimorphic development that influence gonad trait discrepancies in opposite-sex descendants of the parent. These gonad trait discrepancies were observed in twin studies that attempted to identify homosexual traits and the genetic markers associated. Savolainen and Hodgson (2016) provide an example of the epigenetic model where a mother could pass down epigenetic marks to her son that cause the son to generate what would be typical female sexual preference and thus become sexually attracted to other men. Epigenetics could fill in the gap that twin gene studies were not able to provide answers for pertaining homosexual genetic markers.
Homosexual behavior has long been a behavior expressed in the human population. While it seems counterintuitive that this behavior would continue to be passed down through lineages, it has retained a significant prominence throughout human history. Homosexual behavior plays a role in forming social bonds and alliances. A rapid progression and increase in the occurrence of homosexual behavior could be associated with dynamic changes in the social structure of humans, such as in the movement towards agriculture communities and increased population. Alternatively, maternal effects could have their own influence in the onset of homosexuality. The heritability of homosexuality helps to maintain it and pass this behavior and the genes associated with it to offspring. Epigenetic factors and the sexually antagonistic characteristic of homosexuality also contributes to ensuring that this behavior is not eliminated from the human genome. As long as humans are around it appears that homosexual behavior will continue to be established in our population due to the advantages it holds and the genetic mechanisms ensuring their continued passage through the generations.
