
      What makes a person gay or straight? This simple question has been the subject of countless debates over the past few decades. Even now, with the Supreme Court having legalized same-sex marriage on a national scale, there are still those who butt heads on the topic of sexual orientation. There are many people who argue that sexual orientation is a choice made by an individual, as if it’s a conscious decision that can be changed at any time like a favorite color. However, in recent years scientific evidence has been introduced that argues that sexual orientation may, in fact, be a biological aspect that can develop as early as the gestation of the fetus (Diamond). Some may turn up their nose at this concept, not just detractors of gay and lesbian rights, but supporters as well. The idea of an all-in-one ‘gay gene’ that determines a person’s sexual orientation has been used against LGBT rights in the past by construing homosexuality as a mental illness. But despite evolving social views and the overall progressive mindset that is growing in modern society, there is still evidence that points towards a biological origin for sexual orientation. While there isn’t a specific gene or part of the brain someone can point to and say: ‘this is what causes a person’s sexual orientation to be gay or straight’ there are multiple factors that can play into how it develops in a person that are genetic in nature.

      One such piece of evidence of a biological origin is the genetic relationship between families and twins. A study performed by Franz J. Kallman during the 1950s claimed that if one twin was homosexually oriented, then it was likely that the other twin was as well. Of course, this claim was immediately shot down due to the era it was conducted in. A multitude of studies were performed to counter Kallman’s original research, arguing that homosexuality was an entirely social construct with no biological connections. This notion of sexual orientation being tied to upbringing or social constructions was upheld until the 1980s when new evidence was produced by Richard Pillard and his colleagues. Pillard examined families where at least one member was homosexual and found that if at least one male family member was homosexual, there was an 18 to 25 percent chance that another male member was also homosexual (Pillard qtd. in Diamond). A similar study conducted by Michael Bailey found that in women, 12 to 20 percent of female family members had female homosexual siblings (Bailey qtd. in Diamond). This research indicates that genetics are a factor in developing sexual orientation. This is demonstrated by the likelihood of the offspring of gay individuals also being gay. From these twin studies, there have been attempts to research into isolating specific genes that might influence sexual orientation, the hypothetical ‘gay genes’ mentioned earlier.

      Research into the chromosomes and pedigree of homosexual men and women has determined that homosexuality is in fact likely to run in families, as found in a study performed by Dean Hamer and colleagues in the early 1990s. This research showed that was little to no rate of homosexuality in paternal lines and that homosexual orientation was more likely to manifest through the X chromosome which can only be passed down through mothers. Following this discovery, Hamer and colleagues went on to search for physical evidence of this trait that might occur through some chromosomal pattern. Their research found a remarkable pattern of genes in the X chromosome of male homosexual brothers. Of forty selected pairs of gay brothers, thirty-three shared this pattern (Hamer qtd. in Diamond).

      Another significant conclusion came in 1995 in a study by William Turner, who observed more than two hundred families with homosexual male and female members. Turner compared the male-female parental sibling ratio in each family and found a very large unbalanced secondary sex ratio in the maternal generation of male, but not female, homosexuals, a highly significant ratio of twice (or more) as many aunts as uncles. The maternal uncle to aunt ratio for female homosexuals was about one to one. Turner then took these statistics and compared them to other conditions that were known to be transmitted genetically such as color blindness and hemophilia A (Turner qtd. in Diamond). Through his research, Tuner found that homosexuality is a semidominant genetic phenomenon, supporting the claim of a genetic factor in sexual orientation.

      A more recent study by the Academy of Science of South Africa theorized that epigenetics were a factor in determining sexual orientation. Epigenetics are the heritable phenotype passed on in addition to the traditional genetic basis of inheritance. An embryo developing in the womb has a specific sensitivity to hormones which influence the development of genitalia and other characteristics. The team behind the study hypothesized that epigenetics could regulate the genetic code responsible for the embryo’s reaction to androgen. Normally the epi-marks which tell the genes how to interact disappear before the embryo develops. However sometimes these epi-marks remain behind and can be passed on from parent to offspring. This means that epi-marks that made an embryo resistant to androgens could be theoretically passed on to a male embryo (Academy…). These changes in responsiveness to androgens could lead to changes in sexual identity and sexual origin. 

      According to research conducted by the Department of Bioengineering and Therapeutic Sciences as The University of San Francisco (also known as the Savic laboratory), there are multiple structural and functional brain differences that determine a person’s sexual orientation. In a study the Savic laboratory conducted, they found a key difference between heterosexual men and women and their homosexual counterparts in a part of the brain known as the hypothalamus. The hypothalamus is a part of the brain involved in many functions, including reproduction. Savic reported that homosexual males exhibited more female patterns than heterosexual males, and homosexual women showed more male-like features than heterosexual women. Whether the observed sex-atypical characteristics occur during gestation period, as is the case with gender identity and sexual orientation, has yet to be confirmed (Savic).

     Research from the Netherlands was some of the first to point to structural differences in the brain between homosexual and heterosexual individuals. Research performed by F. Swaab and M. A. Hofman found a region of the brain that appears to be larger in homosexual males than in heterosexual males. Similarly, this region of the brain was found to differ in size between men and women and that in male-to-female transsexuals it was closer in size to females than males. The researchers suggested that while this region began development in the prenatal stages of childbirth, it was found to be at its highest level of differentiation between the ages of two and four (Swaab & Hofman qtd. in Diamond). This suggests that human sexual orientation develops early on. While the comparison between the brains of heterosexual and homosexual brains is relatively new, the study of the difference between sexes in the brain was conducted in research dating back to 1948. This study did not garner much attention until much later when Simon LeVay discovered a region of the brain tied to sexuality and arousal that appears larger in heterosexual males compared to heterosexual women and homosexual males.

      Another study found that homosexual men compare more to women when performing tasks involving spatial awareness and dexterity. These differences were recorded by Sanders and Ross-Field, who found that on performing tasks such as The Water Level Task, which assesses spatial relation, and targeted throwing and other performance related tasks, heterosexual men differed significantly from women and homosexual men. Other researchers have found similar differences in sexual orientation regarding verbal fluency and speaking skills. Much like the physical and mental exercises in the previously mentioned research, male homosexuals performed similarly to heterosexual females compared to heterosexual males. Overall, heterosexual men performed better at tasks related to spatial awareness compared to homosexual men and heterosexual women.

      It is likely that if hormones influence sexual orientation, it most likely occurs during prenatal gestation. Observations have indicated that this is the most likely stage for a ‘critical period’ as Simon LeVay puts it in his book Gay, Straight, and the Reason Why. LeVay’s argument provides evidence as to why this period of development is so important to the origin of sexual orientation.

      During the gestation period the maturation of the reproductive organs and release of their hormones affects structural changes in the genitalia and nervous system. This development period influences the orientation of the fetus towards male or female behaviors. Recently it has been proposed that this prenatal development period influences sexual orientation and sexual identity. The strongest evidence for this argument comes from females subjected to androgens prior to birth. This occurs dues to issues with the fetus or problems with the mother’s metabolism. These individuals, depending on the number of androgens they had been exposed to and the period of development during which it occurred vary in results of the exposure but overall show signs of masculine behavior and lesbianism. Similarly, males who are feminized during prenatal development are more likely to adapt to female patterns and mannerisms.

       A study performed by a group led by neuroscientist Yi Rao of Peking University and the National Institute of Biological Sciences in Beijing, has shown that a chemical found in the brain known as serotonin can affect a male mouse’s decision on which gender of mice it chooses to try and mate with. Serotonin is known to affect the sex drive of both mice and humans, and using this knowledge, the genetically developed a group of male mice that lacked the ability to produce their own serotonin. Rao found that almost half of the male mice lacking serotonin ignored the female mice and attempted to mate with the male mice instead. Although there are certainly implications behind what this research means for sexuality in humans as well as mice, this research should not be taken at face value and requires further developments before a conclusion can be drawn.

      Some would argue that there are environmental factors that influence sexual identity. But there are studies where this produces conflicting evidence. In 1983 Imperato-McGinley and her colleagues studied a group of male individuals indigenous to the Dominican Republic who, due to a genetic flaw, were born without genitalia. Their parents assumed they were girls and raised them as such. Once puberty occurred, their genitalia began to develop and despite being raised as females from birth, every individual in the study switched to a heterosexual male lifestyle. Regardless of their upbringing and social development, the group easily moved on to sex-appropriate orientation. This adds evidence to the argument that sexual orientation is more biological in nature rather than a social force. It is important to consider population figures when observing the prevalence of homosexual activity. It seems reasonable to assume that if homosexuality was influenced by culture, such behavior would be more prevalent in societies that tolerate it mos. In societies where homosexual behavior is more widely excepted, such as Denmark, it was found that they had some of the lowest homosexual populations.

      A recent study found positive association between childhood abuse and homosexual orientation in adulthood. This study attempted to find a causal link between maltreatment and sexuality. Effects of sexual abuse on men’s sexual orientation were substantially larger than on women’s. Effects of non-sexual maltreatment were significant only for men and women’s sexual identity and women’s same-sex partners. While evidence claims much of the association between maltreatment and sexual orientation may be because of maltreatment on sexual orientation, the results of the study suggest that causal relationships driving the association between sexual orientation and childhood abuse are complex and varied in nature.

    The implications of research into the origin of sexual orientation oftentimes lead to repercussions in politics and society at large. The results of a study are likely to be scrutinized by individuals on both ends of the argument in an attempt to be used for their own gain. As undergrad journalist Jihan Ryu notes, “Ever since modern science gained significant social prestige and the power of authority, the public has primarily reacted with trust to any article published in a prestigious scientific journal.” The goal of research into sexual orientation is noble in origin, to seek an answer to an aspect of human nature that has eluded us for years, it might even help to persuade those who are against sexual orientation towards a more neutral stance. But often the research produced with this goal in mind ends up antagonizing the original intent of the research and makes it difficult to study sexuality without being accused of a bias towards one side or the other. So this brings us back to the original question: What makes a person gay or straight? Even now scientists from all fields are working on an answer to that simple question. But for now, there is strong evidence arguing towards a biological factor over a social force, as well as the notion that sexual orientation is a ‘choice’ an individual makes. At the same time, scientists must be aware of this social bias and ensure that the integrity of their research is maintained to mitigate the misuse of their information. Instead of arguing for the significance of a ‘gay gene’ we should instead treat all sexual orientations and identities with the same weight and interest as we do research into homosexuality. Treating all sexual identities as topics worth of in-depth discussion, we can negate any presumed bias and instead promote a neutral, unbiased approach to research and discussion. The idea of a biological aspect that can define a person’s sexual orientation may seem controversial to many, but there’s overwhelming evidence claiming just that. 
