Science doesn't stay still. The purpose of the scientific method is to question what we already know, and to see whether there are better explanations to describe what we see and experience in the world around us. Two classic examples from physics: Albert Einstein creating a new framework for understanding space and time with his theory of relativity; and the double-slit experiment demonstrating that a photon can, and must, be thought of as both a particle and a wave. (Arguably, the latter just demonstrates that our mental models struggle with reality.)
Ditto in biology. We understand more about how our bodies work than we did 20, 40, 60 years ago. We're able to tailor vaccines to tackle novel diseases in record times. The frontier moves, all the time.
Hence you'll find people online who will solemnly tell you that while it used to be thought that there were just two sexes, male and female, there's now "a scientific consensus" that "sex is a spectrum". You will usually then get into a back and forth about what they mean by "sex" (sometimes it's actually gender, which isn't the same thing: sex is your body, gender is in your mind) and what they mean by "spectrum" (usually they mean continuum, i.e. an unbroken set). And when you ask for some evidence of this "consensus", you'll often be presented with a link to one of the following articles. They're all misleading, or wrongly interpreted.
Binary, not bimodal
Before we tackle them, let's be clear. Sex — biological sex — is emphatically not a spectrum. It's not bimodal either. Human height is bimodal: there are two peaks, around average female height and average male height, and a spread of others below, between and above. But with sex, you're either male or female: the fertilised egg proceeds down one of two pathways towards the destination where it will produce either small gametes (sperm, in humans) or big ones (ova, in humans). The union of those two different gametes is how humans reproduce. There's no third sex, no ova-ova union or sperm-sperm union.
At which point people always say "what about intersex people? They're a third sex/not part of the binary." This is a misunderstanding, brought about by the word itself. People with intersex conditions are a subset of the larger group who have what's called DSDs, differences of sexual development. (Previously "disorders" was used rather than "differences", but has since been thought too ungenerous. Nobody wants to be told they've got a disorder.) A DSD is
a generic definition encompassing any problem noted at birth where the genitalia are atypical in relation to the chromosomes or gonads.
(Source: "Disorders of sex development: a new definition and classification", Feb 2008, Best Practice and Research Clinical Endocrinology & Metabolism. The full text isn't available for free, but useful parts of the paper are.)
The word "intersex" was used because at birth these people have ambiguous (external) genitalia which make it difficult to determine their sex. But you can, with careful diagnosis, determine their sex, and it's always male or female: like everyone else, their bodies are on one of only two pathways, to produce big gametes, or small ones. DSDs thus include what was called "intersex", and also those where other genetic events have occurred during embryo development.
(Update: if you're tempted to use the word "intersex", please instead read this heartfelt post by a woman with a DSD — Complete Androgen Insensitivity Syndrome (CAIS) — who points out that it's misleading, and often misused:
As well as intersex having no clear definition, the term is also frequently misunderstood to refer to gender identities. This was highlighted in the recent paper by Peter Hegarty that found the term intersex was frequently understood as referring to social or gender identities. Emi Koyama also noted that it was more common for people without a DSD to identify as intersex, than those with a DSD, who mainly see themselves as men and women with a medical condition.
An essay by Traci O'Keefe even encouraged people to identify as intersex as a way to 'mess with the system' — and even stated that 'intersex' people who objected to this were transphobic.
Please don't do this; as the author of that post points out at length, it's colossally disrespectful to people who have specific medical conditions. I really commend her post if you want an idea of how DSDs are misdescribed, and how hard it is to get proper recognition.)
What, though, if the person has a DSD and doesn't produce any gametes? Well, think of the embryonic development as a train network with just two destinations. You get on the train with your ticket, and it heads for the destination. Sometimes, though, the train gets held up, or even stops. But if someone were to ask you "where are you going?" you wouldn't say "oh, just to this place in the middle of nowhere". You'd say the destination. Male or female. There isn't an in between.
Furthermore, you can't say that someone who doesn't produce gametes doesn't have a sex; otherwise you're arguing that pre-pubescent children don't have a sex, and that post-menopausal women don't. Both are plainly absurd claims. The strength of the gamete-based sex definition is that it predicts so much: at birth you can forecast what role someone can play in reproduction, which is how the species continues. You can also make a fair guess about how their body will look in adulthood, especially if you know what their parents look like: they'll look quite like their father if male, their mother if female.
Update: among the responses to this piece was this question from Redbladeking, which I've chosen to answer here because it's so good.
redbladeking: I guess my question would be: can you elaborate on why you believe no gametes isn't proof of a third sex (that being the lack of a sex)? I'm aware of your train metaphor, but I'm afraid that I just don't understand it and find it to be arbitrary. You say having no gametes is like being on a train that stops just short of its destination. While I believe I understand what you're trying to say (there was an intended destination, that destination being either male or female but the "train" stopped just short), it seems more like an explanation or excuse to defend the idea of a binary rather than an objective observation of the world. For example, the metaphor I like to use is computer binary code where, the "computer" is biological sex, so instead of just the 0 and 1 binary for computing, the sex binary has 0, 1, and 2. 0 being no gametes, 1 being male (or female) and 2 being female (or male, which number is which sex doesn't really make or break this metaphor).
This is an excellent question. The answer requires focussing on why and how living things reproduce. Consider yourself: you're here because your biological parents had a child. Why did they do that? It's hard to explain in any logical way; people just seem to have a drive to reproduce. All living things show it. Richard Dawkins offered a revolutionary way of thinking about why living things reproduce: it's to pass our genes one. He posits that genes are "selfish" — they want to be immortal, in effect, by being passed down and passed down through the generations. Our bodies may die, but if we've reproduced then our genes have a chance to be passed on and inch ever closer to immortality.
Now, if a member of a species that normally reproduces by sexual reproduction (such as humans) cannot produce any gametes, how does it reproduce? Short answer: it can't. The genes never get passed on. In terms of evolution, an organism that cannot reproduce is a dead end. In terms of gene reproduction, it's a full stop.
This, then, is why we don't think of "no gametes" (in the mature person) as a third sex: there's no future in it — they can't reproduce with the male or the female.
Two points to be made. First, if even you can't ever produce gametes, that doesn't mean you're not a valuable member of the human race. It just means you can't reproduce by any standard means. (Cloning might be available, but isn't currently legal.)
Second: in the train/destination metaphor, once you're heading down on of the lines, even if you don't reach the station (gamete production), you're still male or female.
OK: back to the main article.
The Helmuth mandate
We'll get on to the articles in just a moment, but a quick diversion first. Scientific American features a few times below. Since a new editor, Laura Helmuth, took over in April 2020 it seems that hard biological science is out of favour. This has seriously angered some longstanding former contributors. Instead there are articles about how maybe legendary biologist EO Wilson was racist and how modern mathematics is confronting its white, patriarchal past. To which the reaction of a lot of people who used to be loyal readers has been:
However, I think I've figured out what's going on. Helmuth, who has a solid background as a science writer, will be reporting to the VP of Magazines at Springer Nature, who said that her qualities "make her a clear choice to lead Scientific American and grow its reach to new audiences as it enters its 175th year." Emphasis added, because that's what this is about: trying to keep it alive. The traditional SA reader will, like many print magazine readers, be dying off. So my instinct — based on quite a few years in magazines — is that Helmuth has been tasked by her Veep of Magazines with producing something that will appeal to younger, university-educated buyers. They don't even have to read it — just putting it on the coffee table will be enough to burnish their progressive credentials if it has an image like this (from the spring/summer 2022 edition. A Special Collector's Edition, no less):
There will also have been internet readership targets, of course. The risk is that in aiming for this younger audience, knowing from media data what their views are on various topics, you fall prey to audience capture. The linked article is subtitled "How influencers become brainwashed by their audiences", and it's excellent. It does describe the dilemma that Scientific American faces: it can't tell its audience they're wrong about something that they deeply believe in, because that will torpedo online reputation when the Twitter mob descends, and hence sales, and hence Helmuth's job. So she's annoyed some of the older contributors, and readers, in order to find a new, younger audience. I can't find figures for SA's circulation in the past few years. I wonder when we'll find out if Helmuth's fulfilled her task?
Bunking up
And now, on to the articles. Note that the first three Scientific American ones predate Helmuth! First:
Scientific American: Beyond XX and XY (September 2017).
Who wrote the article? The actual text on the graphic is written by Amanda Hobbs, with "expert review by Amy Wisniewski at the University of Oklahoma Health Sciences Center". Wiskiewski is a professor of urology whose specialisms include DSDs.
What's it really about? How the interplay of chromosomes and genes can lead to DSDs, because you get much more complex sex chromosomal configurations than just XX and XY: there's XXY or X- or XYY, and they produce a range of physical configurations.
Does it say "sex is a spectrum" or something like it? Not on the graphic. In the article online introducing it, credited to the graphic artist (but likely written with Hobbs), it says "the gender with which a person identifies does not always align with the sex they* are assigned at birth, and they may not be wholly male or female. The more we learn about sex and gender, the more these attributes appear to exist on a spectrum." Appear to exist: certainly, DSDs lead to people whose physical sexual attributes don't fit the obvious template we're used to, and there's a range of them. In that sense, there's a spectrum of phenotypes (the physical manifestation of the person's genes, ie genotype). But appearances are deceptive: we can, and do, determine the sex of people with DSDs. It's just trickier than with 99.8% of the population.
On the graphic, there's a coloured spectrum across the page atop a diagram of how different DSDs develop (for example, you start with 46, XY sex chromosomes and then get 5-ARD and end up with "male hormones; male characteristics"). But if you study the boxes carefully, you'll find each one at the "birth" layer mentions either testis or ovaries. Not both; not neither. The lines look intriguingly like a rail network, as it happens, with two broad destinations on the left and right of the page, and a deep divide between.
Does it demonstrate that sex is a spectrum? No. Lots of the people who cite it haven't read it closely enough. But they want sex to be a spectrum (we'll discuss why later), and there's a spectrum-y graphic, so for them that'll do. But it's in fact about how much more complicated sexual development can be than we are taught in school.
Scientific American: Stop using phony science to justify transphobia (June 2019)
Who wrote it? "Simón(e) D Sun is a doctoral candidate in the Tsien Lab at New York University's Neuroscience Institute". (Now at Cold Spring Harbour Laboratory.)
What's it really about? Why transgender people should be respected. However the word "gender", and where it might reside in a body, is never defined.
Does it say "sex is a spectrum" or something like it? No. There's a good explanation of how the embryo becomes sexed — the delicate dance of genes such as SRY — but nowhere does that suggest sex is anything but male or female. Then it talks about brains. The brain is not a reproductive sex organ. Then it talks about sex hormones, which vary across age, pregnancy status and sex. "The binary sex model not only insufficiently predicts the presence of hormones but is useless in describing factors that influence them," the author scolds us. "Environmental, social and behavioural factors also influence hormones in both males and females, complicating the idea that hormones determine sex."
Except: nobody said that hormones determine sex. This is a strawman entirely pulled out of thin air. Your sex determines your hormones. Being male, I'm not going to have a surge of progesterone each month. The second sentence in that quote above completely defuses the first. Environmental, social and behavioural factors influence our hormones. The binary sex model in fact predicts the presence of hormones really, really well. Levels vary because, well, levels of hormones vary: ask any woman having her period. (On second thoughts perhaps don't.)
Does it demonstrate that sex is a spectrum? No. It demonstrates that your sex doesn't predict everything about your brain or hormonal levels. This is hardly news. And yet bizarrely the final paragraph begins "While this is a small overview, the science is clear and conclusive: sex is not binary, transgender people are real." Sorry, but it absolutely doesn't show that sex is not binary. The article itself only talks about males and females, and says that how we behave — due to our brains and hormones — isn't binary, a point nobody would argue with. But those aren't our reproductive systems. The point about transgender people is also strange. What's the relevance? Gender isn't the same as sex.
Nature/Scientific American: Sex redefined: the idea of 2 sexes is overly simplistic (October 2018)
Who wrote it? Claire Ainsworth, a freelance science journalist. Ainsworth is well-respected and has a qualification in developmental genetics. She writes about biology and biomedicine. This piece originally appeared in Nature, and was reprinted by SA, because they're part of the same publishing group. Nature is a highly respected science journal. Ainsworth's article was a "news" article, so not itself a peer-reviewed piece of science. But she's qualified in the field, and there is a rigorous editorial process at Nature. (I know, I've written for it.)
What's it really about? The difficulty of ascribing sex in edge cases — such as mosaicism, or some DSDs — and the inadvisability of forcing people with DSDs, especially with ambiguous genitalia, into one or the other group (through surgery, typically) without their consent.
Does it say "sex is a spectrum" or something like it? The quote "biologically, it's a spectrum" appears, from Eric Vilain, a clinician and the director of the Center for Gender-Based Biology at UCLA. But he's not referring to sex. Here's the paragraph:
Since the 1990s, researchers have identified more than 25 genes involved in DSDs, and next-generation DNA sequencing in the past few years has uncovered a wide range of variations in these genes that have mild effects on individuals, rather than causing DSDs. "Biologically, it's a spectrum," says Vilain.
So Vilain is referring to the spectrum of variations in genes that are involved in causing DSDs. Not, emphatically, sex.
Towards the end, the phrase "if biologists continue to show sex is a spectrum" appears:
…says Vilain: "It might be difficult for children to be raised in a gender that just does not exist out there." In most countries, it is legally impossible to be anything but male or female.
Yet if biologists continue to show that sex is a spectrum, then society and state will have to grapple with the consequences, and work out where and how to draw the line.
The strange thing about this phrase's appearance is that neither the biologists nor the article has shown that sex is a spectrum. It's shown that the ways that sex is exhibited, through DSDs, is a spectrum. You have someone who has fathered children who turns out to have a uterus. (They're male. Their uterus is non-functional and they never produced ova.) Someone appears to be female, yet has internal testes. But that doesn't mean sex is a spectrum. It's worth asking what "sex is a spectrum" would even mean, in practice. Would you produce sperm on 20 days of the month, perhaps ova the other 10? You can certainly say gender is on a spectrum. Who's going to argue with that? Some men present in a way that society sees as feminine ("camp" is an oft-used phrase), some women present in a way that society sees as masculine ("butch" was often popular). Some cultures have genders for feminine men, and for masculine women. This doesn't mean the men produce sperm that are a bit ova-like, or ova that have tails. Gender is separate of sex. It's in your mind.
Ainsworth got so many questions about this phrase "if biologists continue to show that sex is a spectrum" that she responded in a tweet. Here's the thread, pictured:
"Two sexes, with a continuum of variation in anatomy/physiology". Absolutely. This conforms with what we see, and doesn't argue at all with the gametic theory. If you won't hear it from me, will you hear it from someone with a DPhil in developmental genetics?
Does it demonstrate that sex is a spectrum? No. Ainsworth, the author, is quite clear that it doesn't.
Cade Hildreth: Do gametes (eggs/sperm) create a sex binary? Science says no (March 2023)
Who wrote it? Cade Hildreth, whose self-description is "an LGBTQ+ entrepreneur, real estate investor, former USA Rugby Player, and fitness fanatic". The categories on Hildreth's homepage/blog are Biohacking, Confidence, Cryptocurrency, Entrepreneurship, Finance, Fitness, Gold and Silver, Increasing Income, Investing, LGBTQ+, Nutrition, Podcast and Real Estate. Elsewhere on the site it says "Cade is the Founder of BioInformant.com, the first and only market research firm to specialize in the stem cell industry. With 15 years of experience, Cade is a media expert on stem cell market trends, recently interviewed by the Wall Street Journal, Los Angeles Business Journal, Xconomy, and Vogue Magazine."
Hildreth takes extraordinary care not to have any sex or gender attached (read this strangulated 2012 writeup on winning a rugby player of the year award) but clearly played for the USA Women's Rugby team. (A later selfie on Instagram shows Hildreth showing off double mastectomy scars, aka "top surgery".) Hildreth studied biology and chemistry at Dartmouth College and Smith College, and biotechnology and molecular biology at Georgetown University.
What's it really about? Well. It immediately jumps into DSDs, without mentioning that they're DSDs, and uses those (such as the existence of people who have "streak gonads" which don't produce gametes) to suggest that sexing such people is hard, so we should use other attributes to assign sex. And then, because other (physical) attributes are not binary, therefore sex isn't binary.
This argument is fallacious because there are only two alternatives for which gamete you produce ("none" isn't a gamete); only two destinations on the railway network, even if your train halts before the final stop. Yes, it can be difficult to determine the sex of someone with a DSD (it may require karyotyping/gene sequencing and tests of hormone levels), but it isn't impossible. (It may be necessary, because some DSDs can lead to problems in later life.)
Does it say "sex is a spectrum" or something like it? Um. The opening paragraph reads:
Gametes cannot be used to create a sex binary in humans, because no gametes can be present along with ambiguous or mixed gonadal tissue. When this happens, other sex markers have to be considered, none of which are binary.
As above, "no gametes" doesn't mean "no sex". A post-menopausal woman doesn't stop being female. "Ambiguous" just means "hard to figure out". Mixed tissues, sure, that happens too — mosaicism is known — but the existence of DSDs shows that embryology is complicated, not that there's a third or multiple sexes.
Hildreth won't give up, though, and repeats the same point a little further down:
This third possibility of ambiguous gonads coupled with no gamete production clearly defies the ability to use gametes to create a sex binary.
Again: no. Refer to the Scientific American diagram from "Beyond XX and XY" if you like: the "ambiguous gonads" example is "mixed gonadal dysgenesis" (45X/46,XY), and such a person may be male or female, depending on the specifics of their situation. They're still, like all the rest of us, part of the sex binary. And emphatically human.
Hildreth goes on in this vein for a while, citing various DSDs while never calling them such, and suggesting that because such people's physical bodies present in confusing ways, this proooooves! that sex isn't binary. Again, no. (Still only male or female. Ask Prof Wisniewski.) And then there's this:
An example of a person with this blend of traits is Castor Semenya, the renowned 800m runner. The Olympic Committee has not found a way to classify her sex because it varies across the biological sex markers, which is why they have defaulted to measuring her fluctuating androgen levels on a event by event basis.
The Olympic Committee might not classify Semenya's sex, but I don't look to the IOC for medical diagnoses. Semenya is known to be 46, XY DSD. That's a male configuration in which paternity is possible. The IOC very much can classify Semenya's sex, though possibly don't want to publicise it, because it's personally and medically sensitive. For Hildreth not to know this suggests a lack of attention to what's been a huge topic of discussion literally for years.
Does it demonstrate that sex is a spectrum? No. It throws up a lot of chaff about DSDs, without explaining how they occur, and suggests multiple "markers" of sex: chromosomes, gonads, hormones, secondary sex characteristics, external genitalia, internal genitalia, gametes, gene expression, brain structure, and hormone receptor sensitivity.
The truth though is that for 99.8% of people you can discard all of those except gametes. And for most of the remaining 0.2%, chromosomes will get you most of the way. For those remaining, gene expression will probably suffice. (Brain structure is never going to indicate sex. It doesn't belong on the list at all.)
Who wrote it? Marianne J Legato, an emeritus (and distinguished) professor at Columbia University. Legato has written for years about how disease experience differs depending on gender. (Well, sex.)
What's it really about? That people vary a lot, and we shouldn't say that just because someone is male or female, or their sex is difficult to determine, that we know how they will behave. ("Phenotype" means "what your body looks like", as opposed to "genotype" which is what your DNA looks like.)
Does it say "sex is a spectrum" or something like it? No. It discusses how the embryo becomes sexed, and how DSDs can make determining sex confusing. It then talks about the brain (once more: not a reproductive sex organ) and whether sexuality and gender identity resides there. It then discusses whether there's a physical basis for homosexuality.
Yet at the end, in the summary, it says
The view that the world's population can be separated into a clearly defined dyadic unit of male and female is defunct; not only clinical observations, but molecular biology has established that sexual identity is on a continuum, with an enormous potential for variance.
Well, that depends a lot on what you mean by "sexual identity". Throughout the molecular biology part of the paper, Legato is absolutely clear that the embryo becomes sexed, either male or female. The section about DSDs points out that people may feel that their sex matches their genitals, rather than the more complex story their karyotype tells. They think they're male, or female.
Does it demonstrate that sex is a spectrum? No. It never tries to, and only ever suggests that there are two sexes.
Scientific American: Here's why human sex is not binary (May 2023)
Who wrote it? Agustín Fuentes, a professor of anthropology at Princeton University. He is the author of Why We Believe: Evolution and the Human Way of Being and Race, Monogamy and Other Lies They Told You: Busting Myths about Human Nature.
What's it really about? Good question. It start out by insisting that sex isn't, in fact, binary, and then saying that gametes are, in fact, binary, but that sex, somehow, isn't. The real thrust of his piece may be found buried in a somewhat defensive Twitter thread he produced after the article appeared (and a zillion people pointed out the many strawmen and contradictions in it):
So what he seems to be saying is that gametic definition is fine, but doesn't tell us how someone with one or the other will necessarily fit into society. Is this a surprise to anyone? Rather as some meetings could be an email, this article could have been a sentence.
Does it say "sex is a spectrum" or something like it? No. There's a lot of focus, in fact, on how there are only two gametes. But Fuentes's point, being an anthropologist, is that if you define people's social roles by the gametes they produce, you don't allow for the variability in social flexibility that humans actually show. At no point does he refute the idea that human biological sex is binary. He doesn't mention intersex people or DSDs.
There are some truly misleading links. Towards the end, he writes that "For humans, sex is dynamic". The hyperlink goes to a 2019 article in American Psychologist for which the summary says, in part, "This review describes 5 sets of empirical findings, spanning multiple disciplines, that fundamentally undermine the gender binary." GENDER binary. (Also, who thinks gender is binary?) For Fuentes to wilfully confuse sex and gender in this way really raises questions about his writing, and the editing. (This article, by the way, can be laid at Helmuth's door.)
Does it demonstrate that sex is a spectrum? No.
Endnote
The thing that puzzles me is why people — and always people advocating for trans issues — are so desperate for sex to be on a spectrum. To clarify, again: sex is physical, gender is in your mind. Otherwise how can your gender disagree with your sex, the condition that defines "transgender"?
I've asked this question frequently, but never had an answer that makes any sense. One possibility — which I raise, and maybe people can rebut — is that there's a wish for sex to be malleable in this way, so that one really can not just identify as the other sex, but become the other sex. If sex is a spectrum, maybe you could slide along the spectrum, like someone tuning a radio, and move from here to there?
Perhaps that's why defining sex variously as your hormone levels, your psychology, your brain mapping, or (something I've seen) wilfully confusing gender and sex offers some sort of comfort against the harsh biological reality — that no amount of hormones or surgery will change your genotype, even while they make your phenotype conform to your desired appearance. That doesn't mean that transgender people don't exist, or that they don't deserve to be respected just like anyone else. It simply means that our underlying biological sex doesn't change.
To finish: you might ask, well, where's the scientific research demonstrating two sexes in humans? I'd offer Biological sex is binary, even though there is a rainbow of sex roles, which is available in full online. Published in December 2022 by a trio of German biologists, the abstract begins:
Biomedical and social scientists are increasingly calling the biological sex into question, arguing that sex is a graded spectrum rather than a binary trait. Leading science journals have been adopting this relativist view, thereby opposing fundamental biological facts. While we fully endorse efforts to create a more inclusive environment for gender-diverse people, this does not require denying biological sex. On the contrary, the rejection of biological sex seems to be based on a lack of knowledge about evolution and it champions species chauvinism, inasmuch as it imposes human identity notions on millions of other species.
Species chauvinism! Don't do that. And in the introduction, they point out
there is no need to deny the biological concept of sex to endorse the rights of gender-diverse people, because biological sex and gender are two entirely separate issues.
It's a good paper, and I recommend it as background reading. Thanks for reading this far (or skimming). If there are other articles in worthwhile publications which claim sex is a spectrum, drop them in the comments and I'll read them and try to include them here in an update.
Also: you're free to disagree with me, and make your case for how I'm wrong. Please do, but note that you have to deal with the facts. Personal attacks don't make your case; quite the opposite. Also, they'll either get deleted, or used as examples of people who can't argue.