Sexual Orientation Is Not a Choice

As part of a college research assignment, I decided to explore the topic of human sexuality and it’s biological explanation. The research that I did was not exhaustive by any means, but it was enough to get some answers. I chose to look into the following three questions:

1. What biological differences are there between individuals with varying sexual orientations?
2. To what extent do “nature” and “nurture” play a role in human sexual orientation?
3. What role do hormones play in sexual orientation?

Summary of my research:

My research topic was the psychobiology of sexual orientation, and my questions asked about: (1) the biological differences between individuals of varying sexual orientations, (2) the extent to which “nature” and “nurture” play a role in sexual orientation, and (3) what hormones play a role in sexual orientation and what that role is. The most consistent bottom line from the research I found was that, in determining one’s sexual orientation, there is a biological factor at play, which refers to the “nature” part of my second question. While the exact ratio of “nature” and “nurture” in determining one’s sexual orientation remains undetermined, one study did claim that over half is determined by genetics (Garcia-Falgueras & Swaab, 2010). Multiple twin studies have consistently shown that monozygotic twins have a higher sexual orientation concordance rate than dizygotic twins, which supports the role of genetics (Diamond & Whitam, 1993; Gilman et al., 2000). However, the lack of a one-hundred percent concordance rate does indicate that environment, or “nurture”, does play a role to some degree (Gilman et al., 2000). As far as non-genetic biological differences, multiple brain structure differences have been shown in non-heterosexual men, such as a smaller suprachiasmatic nucleus, larger anterior commissure, and larger frontal part of the hypothalamus (Balthazart, 2011; Garcia-Falgueras & Swaab, 2010). As far as my third question, Garcia-Falgueras & Swaab (2010) indicated a role of testosterone, while Balthazart (2011) indicated a role of androgens, in non-heterosexual women. In my opinion, the most interesting and surprising part of my research was the idea that a non-heterosexual orientation could be the result of varying degrees of masculinization of the brain and genitals because the brain sexually differentiates months after the genitals do (Garcia-Falgueras & Swaab, 2010). Overall, I was able to find a lot more information about my questions than I thought I would. Unfortunately, most of my answers were arguably simplistic, as a lot of the research surrounding sexual orientation is correlational, and I am left with questions about the causal mechanisms of those correlating factors. For example, I am curious how the brain structure differences in non-heterosexual men affect sexual orientation, or if they are effects themselves.

Below is information about the sources that I chose to include in my summary.

MiniReview: Hormones and Human Sexual Orientation

This article is a review of the merits and shortcomings of the research that was compiled regarding a biological explanation for human sexual orientation. There are many who hope to find a biological explanation to prove that their sexual orientation is not a choice, but there are some who hope for a biological explanation because it would raise the possibility of a “fix” or “cure”. Animal studies have provided insight into human processes, but findings can’t always be transferred to humans. A study with male rats showed that lesioning the preoptic area led to a preference for other males instead of females. Another study found that the ovine sexually dimorphic nuclei in male-oriented rams were significantly smaller, had fewer neurons, and expressed aromatase at reduced levels compared to those in female-oriented rams. Both a study that looked at oto-acoustic emission frequencies, and a study that looked at 2D:4D finger ratios, had findings that indicate that lesbians were exposed to higher concentrations of androgens in their lives. However, the correlating results were not found in gay men. Other studies have found that, in gay men, the suprachiasmatic nucleus and the anterior commissure were larger, and the interstitial nucleus of the anterior hypothalamus number 3 was smaller. However, these studies did not look to see if there were similar findings in women of varying sexual orientations. The article concluded that their evidence strongly supported the idea that human sexual orientation is not a choice, but rather is a result of biological factors, but also that more research needs to be done on the subject.

This article was particularly useful for answering my first question. It did a good job at providing multiple possible explanations, but all of the studies on humans were correlational studies. These correlations offer insight into potential biological explanations, but they do not show if the correlating factors are actually the causes of human sexual orientation differences. The animal studies did have stronger results that showed causation as opposed to just correlation. However, while animal research can provide insight into human processes, the results can’t simply be transferred to humans, as humans are very different from animals. There was also information that helps with my third question, in particular the information about androgen in lesbian women. This article did a good job at offering me possible answers to my questions, but I now have the question of if these findings are simply correlations, or if there is actually some degree of causation.

Balthazart, J. (2011). Minireview: Hormones and Human Sexual Orientation. Endocrinology, 152(8), 2937–2947. https://dx.doi.org/10.1210%2Fen.2011-0277

Fraternal Birth Order and Birth Weight in Probably Prehomosexual Feminine Boys

Talks about a phenomenon called the fraternal birth order effect, which refers to a male’s odds of being homosexual increasing proportionately with his number of older brothers. This particular study was looking to confirm those findings once again, but this time by looking at birth weight. Their subjects made up three groups: a prehomosexual group comprised of boys who had been admitted to a clinic because of their feminine behavior, a control group of preheterosexual boys, and a group of girls. They found that prehomosexual boys who had two or more older brothers weighed significantly less at birth than preheterosexual boys who had two or more brothers. They also found no significant difference in birth weight between prehomosexual and preheterosexual boys with less than two brothers, or between preheterosexual boys and girls. They concluded that their findings indicate that the biological mechanism that increases boy’s sexual orientation based on their number of older brothers operates before they are born. While this research does show a biological factor at play when it comes to sexual orientation, that biological factor is not genetic. It does say that not all homosexual males can attribute their sexual orientation to this phenomenon, but the majority of homosexual males with three or more brothers can. The rest can likely attribute it to other factors, whether biological, environmental, or other.

This study provided me with information about both my first and second questions. It demonstrated a biological difference between homosexual males and heterosexual males, though that difference is dependent on other factors as well (number of older brothers). As far as my second question, it indicates that, at least for some homosexual males, sexual orientation is determined by a non-environmental factor. The study does show that there is a biological factor at play, but it also shows that it is not a genetic factor. That leaves me with the question of what that biological factor is. The study also showed that the biological factor was limited to those with more than two older brothers, so there is still the question of what factors determine homosexuality in boys with fewer than two brothers.

Blanchard, R., et al. (2002). Fraternal birth order and birth weight in probably prehomosexual feminine boys. Hormones and Behavior, 41(3), 321–327. https://doi.org/10.1006/hbeh.2002.1765

Homosexual Orientation in Twins: A Report on 61 Pairs and Three Triplet Sets

This study took inspiration from previous studies on homosexuality with monozygotic and dizygotic twins. They used the Kinsey scale to measure the sexual orientations of 61 sets of twins, 12 of which were given the questionnaire in person, 20 of which had one twin do it in person, and the other mail it in, and 29 of which had both twins mail it in. Based on the results, the twins were categorized into one of three categories. They were classified as “concordant” if they had either the same number on the Kinsey scale or numbers next to each other, “partially concordant” if their numbers were within two to three of each other, and “disconcordant” if they were four or more away from each other. With monozygotic twins, they found the concordance rate to be 65.8% percent and the disconcordance rate to be 28.9%. In addition, they found that monozygotic twins who were concordant also tended to have remarkably similar sexual interests and histories. With dizygotic twins, they found the concordance rate to be 30.4% and the disconcordance rate to be 60.8%. The study concluded that biological factors do play a role in determining sexual orientation, but how those factors do so and to what extent they do so is yet to be understood.

This study was helpful in answering my second question, which had to do with “nature” and “nurture”. I was hoping to find a twin study to look at because they are very useful when looking at the roles of genetics and environment. This study in particular was good because it was inspired by similar previous studies but aimed to be more credible. They made strong efforts to cut out any sets of twins that they deemed untrustworthy, and even talked to third party sources who were close to a particular participant to verify their answers. This means the results of this study are hopefully more reliable than the studies that inspired it. It did a good job at answering my second question to the extent that it provided evidence for there being a genetic factor in sexual orientation. However, it didn’t provide much insight into any environmental factors, as only two sets of twins were raised apart. This means that their environments were the same growing up, so they weren’t able to be compared to twins who had different environments growing up. Therefore, my question remains as to what extent environmental factors play a role. Also, this study does provide evidence that at least part of the biological basis for human sexuality has to do with genetics. However, it doesn’t confirm whether or not genetics is the only biological factor at play or explain the processes that those factors facilitate in order to have an effect on sexual orientation.

Diamond, M., Martin, J., Whitam, F. L. (1993). Homosexual Orientation in Twins: A Report on 61 Pairs and Three Triplet Sets. Archives of Sexual Behavior, 22(3), 187–206. https://doi.org/10.1007/bf01541765

Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation

The purpose of this article was to compile research and information regarding sex hormones and their role in sexual identity and sexual orientation. The article implies that sexual orientation is purely biologically determined and has to do with both genetics and our interaction with sex hormones. It also says that the genetic component of sexual orientation development is more than 50%. There has not been an exact gene identified as the determinant of sexual orientation, but multiple studies have found results indicating sexual orientation is transmitted maternally/linked to the X-chromosome, with the Xq28 gene of particular importance. In fetal development, sexual differentiation of the genitals occurs months before sexual differentiation of the brain. Discrepancies in the sexual differentiation of the two could account for the opposite-sex-typical behavior, such as same-sex attraction, that homosexuals exhibit. An example of this is females with congenital adrenal hyperplasia (CAH), which means that they were exposed to high levels of testosterone while they were in the womb. They tend to show more male-typical behavior when they are young, and high percentages of non-heterosexual females have CAH. Structural differences in the brain were also noted, as homosexual men have been found to have smaller frontal parts of the hypothalamus and larger suprachiasmatic nuclei. The study concluded that because the sexual differentiation of the brain and genitals occur at different times during fetal development, the degree of masculinization of the two may not match, which then affects one’s sexual orientation.

This source was helpful in answering all three of my questions. As far as my first question, it provided multiple structural differences within the brain that vary between individuals of different sexual orientations. It also talks about hormones and genetics, which ties into my second and third questions. As far as my second question, it provided me with a much more specific number to help me answer the “to what extent” aspect of my question; it said that over 50% of sexual orientation is determined by genetics, which refers to the “nature” part of my question. The fact that genetics is not the only biological aspect at play indicates that environment, or “nurture”, plays a much smaller role. As far as my third question, it gave me a specific hormone, testosterone, that plays a role in sexual orientation determination. Unfortunately, much of the information provided in the article was correlational, so many of the mechanisms surrounding sexual orientation determination are questions that remain. I am still left with questions about the role of the brain structure differences and genetics. However, I don’t believe this is a shortcoming of the article, but rather due to a lack of research and available information. The information on testosterone affecting genital and brain development and sexual differentiation has been the closest information on causality that I have been able to find.

Garcia-Falgueras, A. and Swaab, B. (2010). Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation. Endocrine Development, 17, 22–35. http://dx.doi.org/10.1159/000262525

Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs

The researchers in this study were looking to conduct a twin study about sexual orientation, as many others have done, but with a more representative and unbiased sample than has been previously used. They reached out to twins all over the U.S. and ended up with a large sample of twin and nontwin sibling pairs (1588 twins, 1380 nontwin siblings). They conducted their research by calling participants, getting their consent, and then sending them a questionnaire to be returned by mail. One question asked about the participant’s sexual orientation, which would determine their classification as either heterosexual or non-heterosexual. They found, like previous studies, that there was a higher concordance rate for monozygotic twins than dizygotic twins and nontwin siblings. They also found that the higher concordance rate for monozygotic twins was not due to being raised in more similar environments that the dizygotic twins and nontwin siblings. However, they did conclude that there are likely environmental factors that influence sexual orientation, as there was not a 100% concordance rate for monozygotic twins.

This study gave me information to help answer my second question because it talked about a genetic factor at play when it comes to sexual orientation. While I did look at another twin study, the sample size of this study was much larger than the other which increases its reliability. It was also significant that a goal of their study was to get a more representative and diverse sample, which they did by getting participants from all over the U.S., and by reaching out to potential participants as opposed to waiting for volunteers to approach them. Another interesting aspect of this study was that they did acknowledge a likely environmental factor at play with sexual orientation. This also helps with my second question because, while it doesn’t provide an exact ratio of their effect, it does acknowledge that both “nature” and “nurture” play a role in determining sexual orientation.

Gilman, S. E., Kendler, K. S., Kessler, R. C., & Thornton, L. M. (2000). Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs. American Journal of Psychiatry, 157(11), 1843–1846. https://doi.org/10.1176/appi.ajp.157.11.1843

There are countless studies out there supporting the idea that human sexuality has a strong biological basis. If you support conversion therapy, you’re not only a shitty person, you’re also stupid. #science