Here’s a quick update on my critique of a letter issued by three organismal-biology-society Presidents claiming that sex isn’t binary—not in humans and, indeed, not in any species. The signers were the Presidents of the Society for the Study of Evolution (SSE), the American Society of Naturalists (ASN), and the Society of Systematic Biologists (SSB), and the letter is archived here.
That letter implicitly claimed to represent the opinion of all 3500 members of these societies, even though they were never polled about their opinions. I know many members, and I also also know that a lot of them do NOT agree with the letter and its misguided contentions. While I am no longer a member of the SSE, I of course disagree with the letter: that’s why I wrote a long critique.
The three-society letter gives the email addresses of two Presidents, and I expect that disaffected members may make their dissent known, for I’ve been contacted by several of them. But, as always, I urge readers to make known their feelings whatever they are.
Just two comments.
First, three past Presidents of the SSE have already publicly disagreed with the letter and its claims. One of them was me, but here are comments on my post made by two others: 
I would think this would give the three officers who signed that letter some pause, as Presidents are elected by a poll of all SSE members. And I would suggest that the three societies backtrack and poll their members to say, anonymously, if they agree with the letter. It’s not right that they claim to represent the consensus view of their membership, much less a majority view.
Second, how do the Presidents regard sex in their own research? Before I give some information on that, I wanted to relate the issue to something I discussed in my first book, Speciation, co-written with Allen Orr. In the first chapter and Appendix, we describe the many competing definitions of species, and suggested that the best one for motivating research on the problem of speciation—why nature comes in discrete groups rather than existing as a continuum—is the Biological Species Concept (BSC), which regards species as groups of organisms that have reproductive barriers preventing or impeding gene exchange with other groups.
The interesting thing to me was that although people have argued fiercely in the literature about what a “species” is, when it comes to speciation, the process whereby species are formed in nature, virtually every paper equates speciation with “the origin of reproductive barriers.” That is an implicit admission that yes, species are groups separated by genetically-based reproductive barriers (these barriers need not be absolutely complete). To me, this validates the BSC, for when people actually do research on the origin of species, they research the origin of reproductive isolation. Your research is where the rubber meets the road, and says a lot about how you regard definitions and concepts.
That prompted me and one other person to look up whether the authors of the “sex isn’t binary” letter regard sex as binary in their own research. The answer is “yes.”
I didn’t have to look hard before I found this feature on the website of the ASN President, whose lab sells stickleback fish to other researchers. They sell two types: males and females. What about the other sexes? After all, there should be more, right?
But if you go back through President Bolnick’s own research papers, you will see clearly that he mentions just two sexes, males and females. Here’s one of his research papers (from Nature Communications), but I can’t be arsed to look at them all. Click on the title below to go the paper, which mentions “male” or “female” 171 times.
The abstract and a table: the sexes were studied in two species of fish as well as in mice and HUMANS. And—you guessed it—in all these species only two sexes are mentioned in humans! Curious, no? Red lines in the articles below are mine:
From a table in the paper. Men and women in humans, fish and mice? Is that all they used? Why did they divide up the species that way?
And Emma Hilton at the University of Manchester saved me the trouble of having to investigate the work of the SSE President.
Also Boggs 🙈 pic.twitter.com/XiL0Q4XWJl
— Emma Hilton (@FondOfBeetles) February 9, 2025
Finally, again you don’t have to look hard to see that the SSB President also divides species into males and females in her research. Here’s one paper that mentions males or females 40 times. I again put an extract below (click on title to go to text):
You can amuse yourselves, if you wish, by doing similar searches, but you will find the same thing: when the three Presidents are considering sex in their own research, there are always two, males and females. Are you going to tell me that doesn’t say anything about a sex binary?
They really need to retract the part implying that they represent 3500 members,* and resend the corrected letter.
*The authors of the letter don’t explicitly state that the letter represents the collective opinion of the 3500 members, only that their three societies represent 3500 members: “Our three scientific societies represent over 3500 scientists, many of whom are experts on the variability that is found in sexual expression throughout the plant and animal kingdoms.” This actually makes their letter worse, since it very much looks like the authors are attempting to pad the weight of their letter by including their entire memberships without actually consulting them. It’s not a good look.
Additionally, while it may be true that there is “[variability] found in sexual expression throughout the plant and animal kingdoms,” this does not mean that the experts reject the *definition” of the sex binary. How something is defined and how that something is expressed are not the same thing.
The British Medical Association attempted the very same tactic to discredit the UK’s Cass Review of ‘gender affirming’ therapies, and that too badly backfired when their members objected en mass and forced them to change their position. The parallel is striking! https://www.bbc.co.uk/news/articles/c3vxlnkv3x0o
Just read the BBC article you cite. Thank you for pointing me to it. Yep. Same or similar modus operandi. Padding the numbers to give your argument extra weight it doesn’t deserve. Seems like a form of administrative malpractice to me.
I attempted to steelman this seeming-hypocrisy as an intellectual exercise. (Also, I’m a programmer, not a biologist, so my terminology might be imprecise.) The best I can come up with is: “sex is not binary, but it is bimodal. The majority of non-asexual organisms are either male or female, so it makes sense to do research treating those as distinct categories. However, we must remember that these categories are not completely universal, and that intersex conditions can be found in both human and animal populations.”
Then I realized that this essentially becomes the tepid-but-defensible motte of a motte and bailey argument, with a radical-but-absurd “lol sex isn’t real” bailey awaiting re-occupation once the critics leave the room.
Yes, Jerry points out frequently that intersex and other conditions make the situation more bimodal than mathematically binary. But nowhere else is the exaggeration done to declare us people on a continuum. Most humans have 4 limbs, 5 sets of fingers and toes, and grow hair at some point in our lives. All of these are also technically bimodal (there being humans with more or fewer limbs and digits, and some lack all body hair). But we are defined as quadrupedal pentamerous mammals.
There is no bimodality in sex. Sex is based on male and female reproductive cells. You can plot the bimodality of their sizes if you like, and I’d encourage you to try because you’ll find it insightful. But they’re not the same thing – they are functionally very different. They’re as ‘bimodal’ as the sizes of coconuts vs elephants.
The bigger problem with the claim of bimodality is that this concept requires a scale that can be quantified. So how is this scale in the case of sex quantified? How do you calculate that someone is 95.623% male and 4.377% female? Unless this can be objectively clarified you are not talking about a real spectrum, but merely using it as a metaphor, and that is linguistic trickery, not science. It’s OK to just say it. There is no bimodality in sex.
Yes, the nuance of referring to taxonomic terms like “quadrupedal” or indeed “pentadactyl” is that these are characteristics of the species, not prerequisites for individuals to be correctly classified as members. A six fingered human is still a human. Likewise a male whose primary sex characteristics did not develop normally is still male. If you consult the medical dictionaries you will find that DSDs are almost all sex-specific. The only genuinely ambiguous conditions are vanishingly rare – nothing like as prevalent as red hair – and what they certainly are not is a ‘sex’ as in constituting a distinct reproductive role of our species.
Yes the motte-and-bailey nature of these arguments is exactly right. Spectrum in the streets, but binary in the sheets.
The Society for Integrative and Comparative Biology also prominently features gender ideology and has a history of taking political positions. https://sicb.org/deij/
I learned of this recently when I was contemplating joining as a member.
Just a wild thought here :
Might be useful to get a clear statement on gender performativity (Judith Butler) – a social construct.
I think it’d simply be a sort of non overlapping avoidance of the matter – like science and post structural critical theory are addressing entirely different domains. We have heard this idea before.
I think the time has come, though, for a strong rebuke of gender performativity – or I just have only read Colin Wright on the conflict, and not the whole literature.
So are they going to issue retractions of those papers?
The situation reminds me of Francis Collins, who is a very competent geneticist by all accounts. Of course he was director of the NIH and of the Human Genome Project. When he writes or speaks about biology, he is dead-on accurate. Nuanced. He says all the right words. But he is also captured by evangelical religion so when a subject turns that way its as if his head spins around and a different Francis Collins is talking because he is saying errant nonsense.
I wonder if these two people even realize what each is saying and thinking while sitting on the same shoulders.
This is all so very confused.
Why can’t people understand that it is possible for the definition of sex we use to have absolutely no moral weight? And that it’s possibly distinct from whatever legal definition we use?
If some politician had decided to write an E.O. clarifying that the legal definition of sex was to be based on someone’s self-assertion, and included any number of gender categories, that would not change the biological definition of sex. It would still sit, underneath, useful for making scientific predictions.
Similarly if Trump had instead written an E.O. that stated that the LEGAL definition of sex was to be based on external genitalia at birth, it too would not impact the biological definition of sex, even though external genitalia in humans is merely a very strong correlate with bio-sex.
Importantly, neither of these E.O.’s would have been “unscientific” as the 3 societies try (weakly) claim. They would simply create and define separate legal constructs that each overlap only partly with the biological (gametic) definition of sex. Would using these definitions have different implications for the rights of women and for trans and intersex people? Yes. That’s what legal definitions are for! But it’s very much not what scientific definitions are for.
I would also be interested to find out if they think that any of the creatures they identify as “male” and “female” produce gametes which do not correspond to those labels. After all, they were very clear that the Trump administration’s statement that “sex … is based on the size of the gamete that the resulting individual will produce” is “contradicted by extensive scientific evidence”.
So obviously they must have been expecting to find sticklebacks, mice, butterflies and birds whose sexes have nothing to do with their gametes. Really surprising that they don’t seem to have discussed that possibility in these papers …
Do any biologists believe that because 1 out of 700 humans is born with a third chromosome #21, our species is NOT “diploid”? Of course not. But such a claim would be easier to argue than sex not being binary. Sex(ual) reproduction is a strategy that requires exactly two sex types in anisogamous species, defined by their gametes. The presence of individuals with non-normative secondary sex characteristics changes nothing about the biological/evolutionary definition of sex. It also has no ethical bearing on how we as a society should treat others, as Jerry correctly points out. The fact that the presidents of these three societies can’t bring themselves to make that basic fact clear, despite relying on this very definition for their biological research, is absurd and an embarrassment.
It’s Lysenkoism, on a milder scale. I’m sure there were many Soviet geneticists who knew that Lysenko was full of bunk, but knew what would happen to them if they pointed that out.
Here, we have careerist scientists worried about losing influence or their position if they don’t toe the line this “sex is a spectrum” bunkum.
Someone needs to tell them that they can stop pandering to the fringe trans movement and get back to just doing science.
The rigorous male/female binary published within these Society presidents’ own work is easily explained: they neglected to ask the animals how they identify.
A major oversight and ethically questionable! Retractions are in order.
And of course resignations of the editorial staff of those papers that are full of harmful stereotypes. How could they allow the oppression of non-binary sticklebacks?
I am sure death followed for those maligned fish!
I think the research examples make it pretty clear that evolutionary biologists are fully aware that there are two distinct sexes in plants and animals. But the pressure to so great right now to conform inaccurate definitions of sex that they feel compelled to do some virtue signaling, probably just to avoid the conflict. After all, most scientists are most comfortable talking about their research, not the never ending socio-political conflicts in American culture.
But they don’t have to say anything! There’s no need for virtue signaling like this – they could simply ignore the issue and avoid all conflict altogether. I’m not in this world- is there really still that much pressure to put out these kinds of ludicrous statements?
I encounter the ‘sex is a spectrum’ nonsense with some frequency. My (unoriginal) reply is to ask ‘where is you Nobel prize for finding a sex between male and female?’. Sadly, we live in an era where reality denial is commonplace. The PoMo worldview dominates the ‘progressive‘ left. According to them, ‘2 + 2 = 4’ reeks of the white supremacist patriarchy. The PoMo ideology was invented (in part) by Michel Foucault. He died from HIV/AIDS. Reality caught up with him.
Good argument!
A similar response to the spectrum believers is: “There’s a Nobel Prize waiting for the finder of the 3rd gamete!”
Excellent post! And it really amused me. We in Europe do need something to cheer us up now :-(.
Seems like a lot of commenters use the term sex and don’t differentiate it from gender. Isn’t that a point of confusion?