However, there are average differences in the brain within and between males and females that when aggregated together allow us to predict someone's sex with up to 97% accuracy.[8]

This is specifically done through analyzing the overall morphology of the cerebral cortex.

It is true that trans individuals may have specific structures in the brain that are similar to that typically seen in the opposite sex. As the author mentions, some specific regions in gay men are similar to straight women.

But these similarities do not change one's sex.

The existence of subtle brain similarities to the opposite sex does not mean that such individuals ARE the opposite sex. It just means that one might have specific structures that are atypical.

This does not change your sex anymore than other atypical physical differences.

Using brain similarities and differences to argue that someone IS the opposite sex is akin to using sex differences in height to argue that a short male who falls in the typical female range IS a female.

Variation does not make someone 'less male' or 'less female'

PART THREE--Endocrinology

The author uses hormone levels to 'disprove' the sex binary: "But like all things biology, hormones cannot be limited to the pubescent idea of “estrogen = female and testosterone = male."

This is true, as males and females are both exposed to T and E.

These hormone levels, like differences in the brain, exist as bimodal distributions, an average for males and an average for females.[9]

But variance in hormones, even extreme variance, does not disprove the sex binary.

For example, a male who has less T than the average male does not become 'less male' or 'more female.' He is still a male, as his body is organized around small gametes.

Other variations in physiology and anatomy do not disprove the binary, but rather show the variation within.

Thus, you may notice a pattern in all three aspects: (1) genetics, (2) neurobiology, and (3) endocrinology.

In all three categories, variation does not mean sex itself is a spectrum, but rather, that sex-related traits are bimodal--an avg for males and an avg for females.

Sex differences within and between males and females exist as bimodal distributions. There is variation of hormone production, brain structure, and external appearance, but this does not make a 'sex spectrum.'

Rather, it makes a spectrum of anatomy and physiology.[10]

The reason why we can place traits such as hormone levels on bimodal distributions is BECAUSE of the sex binary.

Because there are two gametes of differing size, there are two sexes, with variation within males and variation within females.[11]

The author does not go wrong in presenting basic scientific facts about the variation of sex-related traits.

Where they go wrong is arguing that because variation of genes, brain structure, and hormones, that male and female cannot be reliably defined.

"The science is clear and conclusive: sex is not binary, transgender people are real," they write.

The problem is that sex on the reproductive level IS binary. There are only two reproductive functions, and thus, two sexes.

The existence of trans people does not disprove this model, and it does not invalidate them. Rather, trans individuals represent the variation in biology, identity, and expression found within the sex binary.

We don't have to deny the fundamental reality of sex to accept them.

The author tells us to stop using phony science to justify transphobia.

But I have a different request: when will activists and authors of major publications stop using complex medical conditions such as DSDs in arguments about unrelated identities?

And finally, when will activists stop using the beautiful complexity and diversity of biology to justify the dismantling of male and female under the guise of 'science'?

Sources:
[1] Inoubli, A., et al. (2011). Karyotyping: Is It Worthwhile in Transsexualism? The Journal of Sexual Medicine, 8(2).

[2] Sax, L. (2002). How common is lntersex? A response to Anne Fausto‐Sterling. Journal of Sex Research.

[3] Gilbert, SF. (2000). Chromosomal sex determination in mammals. Developmental Biology, 6th edition. Sunderland (MA), Sinauer Associates.

[4] Kimball, J. (2020). Sex chromosomes. http://LibreText.org .

[5] Kashimada, K., Koopman, P. (2010). Sry, the master switch in mammalian sex determination. Development, 137.

[6] Matson, C., et al. (2011). DMRT1 prevents female reprogramming in the postnatal mammalian testis. Nature.

[7] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

[8] Luo, Z., et al. (2019). Gender identification of human corticol 3-D morphology using hierarchical sparsity. Frontiers in Neuro.

[9] Ellis, L. (2011). Identifying and explaining apparent universal sex differences in cognition and behavior. Personality and Individual Differences, 51.

[10] Schmitt, D. (2016). Sex and gender are dials, not switches. Psychology Today.

[11] Lehtonen, J., Parker, G. (2014).

[12] Hayward, A., Gillooly, J. (2011). The cost of sex, Quantifying energetic investment in gamete production by males and females. PLoS ONE, 6(1).

[13] Marinov, GK. (2020). In humans, sex is binary and immutable. Academic Quest.

[14] Epelman, M., et al. (2005). Anisogamy, expenditure of reproductive effort, and the optimality of having two sexes. Operations Research, 53(3).

[15] D Sun, S. (2019). Stop using phony science to justify transphobia. Scientific American.

Thanks for reading! Constructive feedback is welcome, and if you have other research you'd like me to know about, let me know

'Why Sex Is Binary'

And if you're interested in learning more about why bimodal sex differences are the result of the sex binary, check out this thread: https://twitter.com/zaelefty/status/1279802563517276166?s=20

And finally, here's my response to the other Scientific American article, 'The New Science of Sex and Gender.' https://twitter.com/zaelefty/status/1280324312914894848?s=20