What is biological sex?

 min read
April 27, 2023

How do you define one’s biological sex?

Sex, as opposed to gender identity, usually refers to one’s anatomy, physiology, and related biology. Most people reduce "biological sex" to chromosomes and genitalia, or even just to what gender someone assigned at birth, but the reality is that biological sex is a spectrum and not a binary. Yes, most people fall into two prototypical categories of male and female, but not everyone does. About 2% of the population does not. This 2% is “intersex,” meaning, “between sex.” (Read more about what intersex means from the Intersex Society of North America.) Biological diversity exists everywhere – including within biological sex. So, biological sex is, too, a spectrum.

How is “biological sex” a spectrum?

There are five main factors of biological sex:

  1. Sex Chromosomes: most assume this means XX or XY, but variations such as XXY, XXX, XYY, and X also occur. These are often called sex chromosomal abnormalities, but that doesn’t necessarily mean there is anything wrong with them. Sex chromosomal abnormalities are far more common than most people understand. Some individuals are also born with “Mosaic Genetics” wherein some cells are XY and some are XX. See more here: https://www.ncbi.nlm.nih.gov/books/NBK115545/.
  2. Hormones: usually testosterone and estrogen, including their derivatives like estradiol and progesterone, dihydrotestosterone and other androgens. (Androgens are the class of hormones that include testosterone, and testosterone is probably the main hormone in this group, and certainly the most well known, but it is only one of several androgens that are usually considered the main “male” sex hormone.) Keep in mind that hormones are important for so many more things other than sexual maturation, though! [Hormones & Behavior, Lecture, Professor Hooven, 2018]
  3. Expression of hormones: hormones are ineffective without functioning hormone receptors. Folks with Complete Androgen Insensitivity Syndrome (CAIS) have XY chromosomes and testes, but do not have funcitoning androgen receptors, resulting in no effects of testosterone. Find more details below.
  4. Internal genitalia: testes, vas deferens, fallopian tubes, uterus, ovaries, etc.
  5. External genitalia: include penis, vulva, clitoris, scrotum (not necessarily with descended testes. Testes remain internal until puberty when, most natal males, they descend. However, sometimes they do not and they remain internal.) More about this here: http://www.embryology.ch/anglais/ugenital/diffmorpho04.html.)

Okay, so what does this mean? What does it mean that there is a spectrum?

Most folks' anatomical and physiological characteristics fall into two main prototypical categories that most people call "male" and "female." However, these two categories are not comprehensive. Not all bodies fall into these neat categories because biological sex is much mroe nuanced. Many people's bodies exhibit variations from this binary, making biological sex a spectrum. While many might try to sequester intersex folks as "abnormality" or "dysfunctional," in reality, intersex characteristics are part of human biological diversity. It's key to remember that chromosomes don’t always indicate everything about sex, neither do internal/external genitalia.

Let's Explore an Example: CAI

A note on terminology: complete androgen insensitivity is commonly referred to as CAIS, as referred to here previously. However, many dislike the final word, “syndrome” because pathologizes their bodies in a way that does not feel accurate. As a result, many refer to the set of characteristics simply as Complete Androgen Insensitivity or CAI. For this reason, this article will also use "CAI" primarily.

People with CAI have an XY chromosomal make-up, which usually means they also have testes. (The Y chromosome usually means that testes will develop; the formation of testes is based solely on the presence of the Y chromosome, not on endogenous testosterone.) Functioning testes then produce testosterone. In folks with CAI, the testes most often function, and therefore produce testosterone. But, because these individuals have androgen (testosterone) receptors that are either not present or not functioning, their bodies exhibit NO effects of that testosterone. Because reception of testosterone is necessary for the formation of external genitalia, penis and scrotum do not form. Folks with CAI are almost always sexed as female at birth because they have a vagina and clitoris and their external genitalia "appears female." Interestingly, these individuals also almost always identify as women. CAI is not usually diagnosed until late puberty, and only because menstruation never occurs. Additionally, because testosterone is converted to estrogen (through a process called aromatization), during puberty, these folks go through a very typical estrogen-driven puberty. Folks with CAI also appear hyper feminine due to the complete lack of testosterone - they have no body hair, very smooth and clear skin, lower muscle mass, and so on. Watch a fantastic video about CAI here: INTERSEX EXPLAINED! | Complete Androgen Insensitivity.

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References & Further Reading





https://en.wikipedia.org/wiki/Sexual_differentiation — check out the citations at the bottom of this article for a deep dive into the biology if you’re interested