A boomer feminist named Anne Fausto-Sterling wrote a popular book 26 years ago arguing that there are more than two sexes. I wrote a post showing that her reasoning is very bad and, of course, the conclusion is wrong https://open.substack.com/pub/hereticalinsights/p/a-response-to-fausto-sterlings-sexing.
TL;DR: her entire argument is that intersexual people exist, but 1) she greatly overestimates how common they are and 2) intersexuality is a misnomer. These people have…
Good article. There’s some more abstract ideas about the distribution of sex karyotypes I’d like to mention as well:
\1. Almost binary: a purely binary model has >98% fit to the actual distribution
\2. Asymptotically binary: we can converge the actual distribution onto binary exactly for almost no cost. Intersex people are extremely rare, economically unneeded, and barely reproduce.
\3. Evolutionarily binary: evolution selects for binaryness and intersex genotypes are the result of mutational processes, not selective ones.
The empirical bimodal distribution objectively has all of these properties. These properties, to me, are sufficient enough to allow one to say “there are two sexes.” Basically, intersex genotypes exist, but don’t matter, and no fact about them existing can produce the logical implication that they matter.
Interestingly, the 1.7% of the world being intersex, which is where your >98% thing comes from, originated with the Sexing the Body book. Most of that 1.7% still have normal chromosomes, i.e., XX or XY. Only three of the ten conditions that she listed have abnormal chromosomes (Turner syndrome, Klinefelter syndrome, and other “non-XX or non-XY”), which in total make up 0.00193% of the population. So, in reality, 99.99807% of the population either has XX or XY chromosomes. From my understanding, if you use the looser classification of Y chromosome=male, no Y chromosome=female, you sex the person correctly about 99.9998% of the time (the only exception would be for a small subset of “true hermaphrodites,” where they have a Y chromosome but clearly have a much more female-oriented reproduction system).
And, yes, from the perspective of evolutionary biology, it makes no sense for there to be more than two sexes. There are only two types of gametes in the population, so a group that is either sterile or produces one of the two gametes cannot be a third sex. Colin Wright, if you’ve heard of him, wrote a paper explaining this recently https://link.springer.com/article/10.1007/s10508-025-03348-3
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