Sex on a spectrum: biological perspectives of intersexuality and transexuality

//Sex on a spectrum: biological perspectives of intersexuality and transexuality

Sex on a spectrum: biological perspectives of intersexuality and transexuality

2023-04-25T00:57:57-07:00 April 25th, 2023|News|

By Vishwanath Prathikanti, Anthropology ’23

Author’s note: This past quarter I took ANT158, Evolution of Sex: A Biological Perspective. I had falsely believed prior that most of our understanding of sex and sexuality was from a psychological perspective resulting from differences in hormonal cascades that occurred before birth. It was enlightening to learn about evolutionary theories behind sexuality, the relatively high frequency of intersexed individuals, and how different cultures are shaped because of it. For this paper, I wanted to focus on two groups I was previously unaware had so many biological basises; intersexed individuals and trans individuals. I hope to help someone correct misconceptions such as the fallacy that according to biology, there are only two genders.


Many understand the difference in sexes as a difference in gonads, or reproductive parts. Males have testes and females have ovaries, with the growth of each determined by hormonal cascades. However, many do not understand that even sex exists on a spectrum; if a gene is not expressed or a hormone is not released, there may be a mismatch between the genetic code and the expression of that code for the individual. Such individuals fit the intersexed definition, though an exact definition of what an intersexed individual is has been a subject of controversy in the scientific community.

Perhaps one of the most important contributors to the acceptance of intersex individuals as something more than fringe cases was Dr. Anne Fausto-Sterling, who published a number of books and papers on intersex individuals. In a literature review summarizing research from the 1950’s to 2000, Fausto-Sterling and colleagues first presented the notion that the percentage of intersex individuals in the population may be as high as 2% [1]. In this paper, they defined intersex as any individual that deviates from the idea that there are only two sexes via a wide variety of biomarkers. These deviations can present themselves in chromosomal, gonadal, or hormonal levels in individuals. In other words, the key difference between transsexuals and intersexed individuals is that intersexed individuals always have some kind of observable biological element, and there are a wide array of markers. Transsexuals simply have to identify as something besides their gender assigned at birth, and some may have biological markers associated with the opposite sex and others may not. A famous example of a biological marker in transsexuals, the BSTc region of the brain, is discussed further.

In her 2012 book, Sex/Gender: Biology in a Social World, Fausto-Sterling expands more towards brain-sex and the potential mismatch between physical characteristics and their gender identity. A misunderstanding of what brain-sex is may be a contributing factor to the perpetuation of gender being the only thing on a spectrum. Brain-sex refers to the complicated ways in which hormones, gene expression and genetic imprinting by the father and/or mother affects the child and the way their brain works. It is not limited to how a person perceives their sex, and this myth may contribute to the idea that gender and sex are completely different and one (sex) refers to biology and the other (gender) to the brain’s perception of identity. In reality, both are linked to biology [2].

Despite these efforts, sex existing on a spectrum is still challenged. In 2002 for example, Leonard Sax posited that the rate of intersexed individuals was much lower—around 0.018% [3]. However, Sax came to this number through a strangely strict definition of intersex; Sax posits that if an individual had an XXY chromosome and had some cells with XX and others with XY configuration, this person would not be intersexed, as their cells technically match their chromosomes. To be intersexed according to Sax, someone must have a mismatch between phenotypic sex and genotypic sex. For example, a person under Sax’s definition would be intersex if they had Complete Androgen Insensitivity Syndrome, if they had XY chromosomes but they never developed male genitalia due to a defect in androgen receptors [3]. 

Such a definition, however, is comparatively much less valuable than what Fausto-Sterling tells us. Her definition of intersex is simply being somewhere in between a man and a woman, and her definition seeks to dismantle the myth that there are only two sexes. While detractors claim that they seek a more clinically rigorous definition, like Sax, it is possible that it furthers the myth that intersex individuals are simply outliers in society, and sex exists in a binary system.

Similarly, if we understand that we can exist on a sexual spectrum, it becomes understandable why transgender individuals, or people that want to switch the gender imposed on them due to their genitalia, exist in society. In addition to the research conducted by Fausto-Sterling indicating a disconnect between external genitalia and internal genes and hormones, there is other clear evidence showing a biological basis for transgendered individuals. Specifically, a study conducted by Zhou and colleagues examined the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a white matter band that acts as a relay site during a stress response. The BSTc is essential for sexual behavior due to multiple reasons, but perhaps most importantly, it is the major center of the aromatization process essential in converting testosterone to estrogen, the two most common male and female growth hormones [4]. It also forms unique connections with the amygdala and hypothalamus, making it highly influential in growth and development. Uniquely, the BSTc region is much larger in males than females, and is directly related to the testosterone and/or estrogen it helps create and regulate. Zhou and colleagues found that the BSTc region among transgendered women and non-transgendered women, where neither groups were on any kind of hormonal therapy that would affect the size of their BSTc, were similar in size. This female brain structure in a genetically male individual supports the notion that gender identity develops as a result of the developing brain [4].

While the research base for transgendered and intersexed individuals is very strong, cultural pushback is rooted in either misinformation or a sense of feeling threatened transgendered individuals. One prominent example of this can be seen in their participation in sports. Over the years, the regulation of trans peoples’ participation in sports has led to absurd levels of regulation, notably for people that do not identify as trans. Most famously are the numerous cases against Caster Semenya, multi-gold winning olympic track athlete, on the basis of testosterone, a shaky metric. The International Association of Athletics Federations (IAAF) currently states that to compete in the olympics, a woman should have testosterone levels below 5 nmol/L. Otherwise, they must compete as a male or receive testosterone blockers to compete as a female. However, in a review of nearly 700 elite athletes, Healy and colleagues found that 16.5% of men had testosterone levels below the 5 nmol/L limit and 13.7% women had testosterone levels above the limit [5]. The IAAF conducted their own study that upheld their regulations, but importantly, they opted to exclude outliers that they deemed having “differences of sexual development,” something they have been criticized for but have not rectified as of the publishing of this paper [6]. These discriminatory practices perhaps further fuel ignorance on the subject of intersexed individuals, and do not properly tackle biology in sexuality. 

The reality is that human beings are more complicated than we’d like to admit. “Bodies are not bounded,” Fausto-Sterling emphasizes in the conclusion to her book. “We will learn a lot about the science of sex and gender in the years to come. But to the extent that our social settings and thus experiences change, at least some of the subtleties of sex and gender will remain a moving target” [2].


  1. Blackless, M., Charuvastra, A., Derryck, A., Fausto-Sterling, A., Lauzanne, K., & Lee, E. (2000). How sexually dimorphic are we? Review and synthesis. American Journal of Human Biology, 12(2), 151–166.<151::AID-AJHB1>3.0.CO;2-F
  2. Fausto-Sterling, A. (2012). Sex/Gender: Biology in a Social World. Routledge.
  3. Sax, L. (2002). How common is intersex? A response to Anne Fausto-Sterling. Journal of Sex Research, 39(3), 174–178.
  4. Zhou, J.-N., Hofman, M. A., Gooren, L. J. G., & Swaab, D. F. (1995). A sex difference in the human brain and its relation to transsexuality. Nature, 378(6552), Article 6552.
  5. Healy, M. L., Gibney, J., Pentecost, C., Wheeler, M. J., & Sonksen, P. H. (2014). Endocrine profiles in 693 elite athletes in the postcompetition setting. Clinical Endocrinology, 81(2), 294–305.
  6. Pielke Sr, R., Tucker, R., & Boye, E. (2019). Scientific integrity and the IAAF testosterone regulations. The International Sports Law Journal, 19.