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In today’s day and age, different sexualities and gender identities are quickly becoming more accepted in mainstream society as we know it. Despite the change, there’re many individuals who believe that having a different sexual orientation or perhaps gender identity is a choice that is frowned upon in our society. In order to dismantle this belief, research and biology of the brain are undoubtedly necessary. Researching the brain on basis of sexuality is a fairly new topic of discussion, because it is somewhat difficult and confusing or not understood.
In this paper we will explore the different identities of gender, sex ,and sexual orientation and the main biological reasons behind these.
In order to discuss the biology of gender identity and sexual orientation, it is important to first examine the differences between multiple definitions that are often mistakenly interchanged. Sex, gender, sexual orientation, and gender identity. Sexual orientation is defined by LeVay (2011) as “the trait that predisposes us to experience sexual attraction to people of the same sex as ourselves, to persons of the other sex, or to both sexes” (p.
1). The typical categories of sexual orientation are homosexual, heterosexual and bisexual. Vrangalova and Savin-Williams (2012) found that most people identify as heterosexual, but there are also groups of people that identify as mostly heterosexual and mostly gay within the three traditional categories (p. 89). This is to say that there are not three concrete groups, but sexual orientation is a continuum and one can even fluctuate on it over time.
LeVay (2011) also defines gender as “the set of mental and behavioral traits that differ…between males and females” (p. 37). More times than not, gender or sex are used interchangeably when they undoubtedly should not be. Gender isn’t based on physical attributes. Sex is the word that should be used when speaking about biological characteristics.
Some people get to the point of being diagnosed with gender dysphoria when they significantly question their gender identity (Callahan, 2009, p. 72). In the fifth version of the Diagnostic and Statistical Manual of Mental Disorders (DSM), gender dysphoria (previously gender identity disorder) was added for individuals who feel that their sex at birth does not match the gender they identify with, continuing for at least six months (American Psychiatric Association, 2015). There are also disorders that are not listed in the DSM that are considered sex chromosome disorders. Sex chromosome disorders generally do not affect the sexual orientation, but they are important to discuss because they show why it is necessary to distinguish between sex and gender. The most common sex chromosome disorders that affect individuals are Turner syndrome and Klinefelter’s Syndrome. Turner syndrome is characterized by a missing or incomplete second X chromosome in girls and women.
Because of this, the symptoms often experienced are having short stature, deformation of the sex organs, and many other physical consequences like obesity and congenital heart disease (Turner Syndrome Society, 2011). Some women with Turner syndrome sometimes have problems with feeling completely feminine because of their altered physical appearance and occasional inability to have children. Even though most women with Turner syndrome identify as heterosexual, many have problems with being labeled homosexual because of the misconception that they are considered “intersex” and not fully feminine (Callahan, 2009, 63-68). The most common sex chromosome disorder for men is Klinefelter’s syndrome. Men with Klinefelter’s have an additional one or more X chromosomes due to a random error in genetic formation. The symptoms vary, but normally they are significantly taller, have longer arms, larger breasts and smaller testicles and penis (Mayo Clinic Staff, 2015). As stated before, it is necessary to distinguish between sex and gender because in the case of Turner syndrome, a less masculine looking man does not signify a more feminine gender identity.
A huge debate happening all around the world is the presence or not of a natural cause of differing gender identities and sexual orientation. Many consider homosexuality and opposite gender identity a choice, but there is increasingly more and more biological research on the topic to deny that. It is highly controversial and has shown to be very difficult to argue. So far, there is no research that has been done that has proven 100% that different sexualities and gender identities are biological, but there are many different studies that have found certain parts of the brain that seem to correlate with it. In 2013, Zhang, Liu, and Rao published an article on the relation of serotonin to sexual preference.
The authors found in male mice that 5HT (serotonin) is required to produce a sexual preference. In male rats that did not produce 5HT, they did not have a sexual preference (as cited in Zhang, Lui, & Rao, 2013). In the current study done by Zhang et al, they found that female mice lacking 5HT, when presented to both male and female mice, generally preferred female mice. This was shown by less sniffing of the head and genitals in the male rats. Female rats lacking 5HT preferred the odor of a female rat’s genitals to a male’s as well. The results are slightly uncertain because the mice that were tested lacking 5HT had been that way since they were embryos. Therefore, the authors were unsure if the sexual preference change was caused by the lower levels of serotonin or the indirect developmental effects. To test this, the present researchers used the drug PCPA to reduce some of the 5HT in normal rats. As far as mating, the female rats that received PCPA showed a significant decreased in their preference of males over females.
They also sniffed female rats’ genitals more often and for longer periods of time. Because of the use of PCPA, it can indicate that serotonin does indeed control the sexual preference of adult female rats. There are 14 5HT receptors in the rat brain, and it is yet to be determined which receptors have to do with sexual preference (Zhang, Liu & Rao, 2013, 1-6). It was also found that a hormone called the leuteinizing hormone (found in both males and females—essential for reproduction) plays a role in sex differences. Gladue, Green, and Hellman discovered that the leutinizing hormone may be a marker of homosexuality in other animals (as cited in Jenkins, 2010, p. 282). It is unclear yet if the leutinizing hormone has any role specifically in determining sexuality prenatally in humans.
Also in the realm of biological explanations for sexual orientation, but found in 1992, was the research of Allen and Gorski on the anterior commissure. The anterior commissure is a sexually dimorphic structure of the brain that is a small connection between the two cerebral hemispheres and plays a role in pain and the sensation of it. The anterior commissure was studied by Allen and Gorski to find a relation between its size and the sexual orientation of the person before death. Brains used in the study were previously identified as belonging to homosexual men, heterosexual men, and heterosexual women. After being cut, measured and analyzed with an analysis of variance (ANOVA), it was found that the area of the anterior commissures in homosexual men was significantly larger than those of heterosexual men and women. Even after dividing the size of the anterior commissure by the full brain weight, that of the homosexual men were still 36% larger than heterosexual men. An additional variable was used that compared the anterior commissures of heterosexual and homosexual men that all died of AIDS. Homosexual men’s anterior commissures were still larger compared to heterosexual men who died from AIDS. Because of this, AIDS is not thought to increase the anterior commissure. The researchers are not certain of when the sexual orientation differences in the anterior commissure develop, but for many lab animals, sexual dimorphisms occur right around birth (Herbert, 2008, 3115).
A more recent topic being discussed is sexual differentiation in the brain being related to both gender identity and sexual orientation. It is important to discuss that gender identity and sexual orientation are often, but not always, closely related. Swaab and Garcia-Falgueras make it very clear that the brain develops before birth with the chance of being male or female and then taking one direction or the other within the first two months of pregnancy (2009). However, sexual differentiation of the brain occurs much later in the second half of pregnancy, giving independence and the chance to develop an opposite gender identity from biological sex (transsexuality) can occur. There are also other events during development that can affect sexual differentiation. Fetal rat brain cells can undergo differentiation in a tissue culture without having sex hormones. Hormones alone do not cause sexual differentiation. It is thought that the differences in brain structures from hormones and brain cells interacting are what is the basis for gender roles, gender identity, sexual orientation, and sex differences.
All of these things can influence later behaviors in life and they are usually permanent. In terms of transsexuals, there have been female-like structures in a male brain and male-like structures in a female brain. These differences were found in the central nucleus of the stria terminalis in the thalamus (BSTc) and in the third interstitial nucleus in the hypothalamus (INAH3). The INAH3 is generally smaller in gay men than straight men, and as seems to be a trend, the size in gay men and women is strangely similar (LeVay, 2011, p. 198). These two structures have been shown to be involved in different aspects of sexual behavior in rats. Swaab and Garcia-Falgueras found that in transsexuals that went from male to female, the two brain structures mentioned above were completely female (2009, p. 21). These structures were ruled out as being female due to hormone injections (Swaab & Garcia-Falgueras, 17-20). They “match the gender that transsexuals feel they belong to, and not the sex of their sexual organs…” (Swaab et al, p. 22). Male to female transsexuals also show female-like processing when viewing sexual stimuli of the preferred sex.
Unfortunately, some of these things do not become apparent in the brain until well into adulthood, so transsexualism cannot be diagnosed before the individual feels completely sure of it (Swaab et al, 22). While thus far, this paper has only discussed sexuality and gender identity as a natural occurrence, there are other things that can influence gender identity and sexual orientation. For example, there is a phenomenon called the “fraternal birth order effect” that says that the chance a boy or man will be homosexual increases with the older brothers he has. The interesting thing about this phenomenon is that the influence is because of the mother, not the individual’s older brothers. This is explained by an immune response in the mother where the chance of a Y chromosome increases with every pregnancy.
Bogaert states that this typically only shows in men with higher than average feminine behaviors (as cited in Jenkins, 2010, p. 282). The older-brother effect, another name for the birth order phenomenon, is called a phenomenon for a reason. The validity of it is often questioned. Many researchers have done trials of experiments on the older-brother effect and lots have no trend or even the opposite of what the phenomenon states. Blanchard and Bogaert estimated that with each older brother, the likelihood a man will be gay increases at a fixed percentage—about 33%. Larger studies, though, have shown to be nonlinear. When put into perspective, a man with no older brother experiences a 2% chance of being gay. It would take a lot of older brothers for this percentage to be correct, and in families with 11 or more sons; almost none of them are ever gay. They also added that 29% of gay men are gay because of the older-brother effect (LeVay, p. 247-256).
As one can see, the strength of this phenomenon is not as strong as people think it is. It is somewhat inconsistent. For girls and women, the chance of being born lesbian increases when their mother exposes them to nicotine, amphetamines, and thyroid-gland hormones while still in the womb. Swaab and Garcia-Falgueras report that there is no sound data to support that homosexuality is a choice or because of a less than adequate upbringing (2009). The two authors sum up their findings in a good way: “the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain” (p. 24). This explains that a person’s sex can be independent of their sexual orientation and/or gender identity.
There is a very close connection between psychology and the development between one’s sexual orientation. One could say that homosexuality comes along with a certain set of mental traits, but it is easy to stereotype individuals in this category. Gay men are often considered very feminine and lesbians masculine. This stereotype itself is enough to make it difficult for someone feeling different from their sexuality to not want to “come out.” These, in turn, makes the individual question their identity and feel uncomfortable in themselves. In reality, all gay men and lesbian women are different. There are gay men that verge so closely on heterosexual that one would not know they were gay just by looking at them. The same can go for lesbians. As said previously, sexual orientation is a grand continuum. There is no one model of a homosexual. There are, however, traits that researchers have found that represent the “average” homosexual when compared to heterosexuals. It is a hard thing to study in children when children do not know themselves what sexuality they identify with.
Gay men reported participating in less “rough and tumble” play and sports when they were children. Pre-gay children also say they participated in more gender atypical activities when they were younger—boys did not always play with cars and Legos, but things that are normally played with by both girls and boys. This is an example of gender-nonconformity. Gender non-conformity is a large indicator of homosexuality in adult life (LeVay, 2011). Gerulf Rieger led a study at Northwestern University in which gay and straight individuals provided childhood videos to examine the attitudes of themselves as children. Judges watched clips assembled by the researchers and they did not provide the current sexual orientation of the adults in the clips. They were easily able to label the pre-gay children as more gender-nonconformist than the pre-straight ones. These differences start to become visual to others when the child is around three.
Some pre-heterosexual children even exhibit gender-nonconformity traits (LeVay, p. 73-88). In adults, they are generally aware of their sexual orientation. LeVay writes that there are cognitive differences in gay and straight adults. The first he discusses are differences in visuospatial abilities. The mental rotation test shows the largest sex differences—men outperform women. There are also differences in the results of gay and straight participants in similar tasks. Gay men performed worse at a targeting task than straight men and lesbians better than straight women. Perhaps this difference is due to the similar ideas stated previously that pre-gay children play fewer sports between ages five and seven than pre-straight children and the same for pre-straight girls and pre-lesbian girls. Gay men have tended to perform similarly to straight women on visuospatial tasks. Another interesting and very common finding is handedness in people with different sexual orientations.
A lot of different studies have found a lot of different results in relation to being right- or left-handed. Some say there are no differences in handedness and others disagree. The general consensus written by LeVay is that gay men and lesbians are more likely not to be right-handed than straight men and women. Another interesting related fact is that left-handed gay people score -differently on cognitive tasks compared to right-handed gay men (LeVay, p.107-113). This poses many questions about how the body and the mind are connected in homosexual individuals.
Many different topics have been discussed in this paper and there are many widespread studies and opinions presented. Sexual orientation and gender identity are topics that have a lot of varying opinions and support in research. One thing is for sure, though. This topic in psychology and physiology is one that needs much more support. No one can fairly say that sexuality and gender identity are 100% biologically based. This paper is just a small sample of many different topics on the subject and the most recent attitudes on them.
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