That quote is from a good article in Nature on how sex is non-binary — my only quibble would be with that “now”. You’d have to define “now” as a window of time that encompasses the entirety of my training and work in developmental biology, and I’m getting to be kind of an old guy. Differences in sex development (DSDs) are common knowledge, and rather routine — and coincidentally, I’m giving an exam on sex chromosome anomalies today.

The article works through a lot of basic concepts: chimeric sex, genetic vs. cellular vs. organismal sex, and the development of sexual characters. I was so happy that they did not trigger one of my pet peeves, the claim that we all start out as female — we don’t, we start out sexually indifferent.

That the two sexes are physically different is obvious, but at the start of life, it is not. Five weeks into development, a human embryo has the potential to form both male and female anatomy. Next to the developing kidneys, two bulges known as the gonadal ridges emerge alongside two pairs of ducts, one of which can form the uterus and Fallopian tubes, and the other the male internal genital plumbing: the epididymes, vas deferentia and seminal vesicles. At six weeks, the gonad switches on the developmental pathway to become an ovary or a testis. If a testis develops, it secretes testosterone, which supports the development of the male ducts. It also makes other hormones that force the presumptive uterus and Fallopian tubes to shrink away. If the gonad becomes an ovary, it makes oestrogen, and the lack of testosterone causes the male plumbing to wither. The sex hormones also dictate the development of the external genitalia, and they come into play once more at puberty, triggering the development of secondary sexual characteristics such as breasts or facial hair.

That’s exactly right.

The major point of the article is something a lot of people deny: that sex is complicated, there’s more than two states of human existence, and most importantly, that biology verifies the existence of a continuum of sexual differentiation. Drag this article out next time someone tries to argue that biology supports their simplistic version of a discrete sexual dichotomy.

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. Many transgender and intersex activists dream of a world where a person’s sex or gender is irrelevant. Although some governments are moving in this direction, Greenberg is pessimistic about the prospects of realizing this dream — in the United States, at least. “I think to get rid of gender markers altogether or to allow a third, indeterminate marker, is going to be difficult.”

So if the law requires that a person is male or female, should that sex be assigned by anatomy, hormones, cells or chromosomes, and what should be done if they clash? “My feeling is that since there is not one biological parameter that takes over every other parameter, at the end of the day, gender identity seems to be the most reasonable parameter,” says Vilain. In other words, if you want to know whether someone is male or female, it may be best just to ask.

I’ll also add that it’s not just biology that supports the idea that sex is a spectrum. It’s also the case of psychology and sociology — any science that has to address sex differences.


  1. #1 Sapiensiate
    February 18, 2015

    Not only do I completely agree with this, but also I think it should be extended to many other aspects of biology. For example even in the classification of species we like a strict “what box are you in” type taxonomy, whereas in reality it exists more as a spectrum.

    This should not be surprising given that complex biological systems are… well complex. And complexity often means that the results follow a distribution. Given a large enough sample size you will see evidence of this distribution; but where to draw the line? Better to admit that the line doesn’t exist.

  2. […] La idea de solo dos sexos es simplista. Los biólogos creen que hay un espectro más amplio [eng] […]

  3. #3 lyle
    February 18, 2015

    Here I think the parrot fish is instructive, it changes sex during its life from male to female to male. So what is “natural” depends on where you look. As another example all be it not sex related directly consider the suprise when life was found around hot springs in the deep oceans (tube worms etc) that live of sulphur not sunlight. Biology is more varied than our minds are willing to admint. Let alone what might happen if the remnants of life were to be found on Mars.

  4. #4 Craig Thomas
    February 19, 2015

    Did you just call psychology ans sociology, “sciences”?

  5. #5 Craig Thomas
    February 19, 2015

    Lyle, you seem to be having no problem describing the Parrot Fish’s sex as it stands at any one moment in time, despite the fact it changes. Why do you think that is?

    I think the function of sex is pretty clear to most of us who don’t live in the post-modern world.

  6. #6 Zoe Brain
    February 19, 2015

    While you’re up to speed on the issue PZ, many in the medical profession are not, trust me on that.

    Especially psychiatrists, whose knowledge of endocrinology and genetics is often scanty, or worse, just plain wrong. God help the Intersex person who falls into the hands of “Christian Counselors”, whose intentions may be good, but whose knowledge about the biology is literally from the Dark Ages or before.

    The general public still believes that anomalies are “one in a billion”, that they can safely be ignored. Men are XY, Women XX, that’s all there is to it.

    While this article is yet more ammunition to use, frankly, if there isn’t enough already, one more round won’t help, not when you’re fighting tens of thousands of pastors and their flocks in the tens of millions. They know what they know, God said it, they believe it. And vote for legislation accordingly.

    Progress is being made in education, and even if it wasn’t, we’d still have to try. But there’s now something of a backlash, with conservative legislators mandating that such dangerous knowledge no longer be taught, as it “goes against family values”.

  7. #7 See Noevo
    February 19, 2015

    How did we evolve two sexes to begin with?

  8. #8 Lyle
    February 19, 2015

    A link to the Wikipedia article on Parrotfish:

    It describes them as sequential hermaphrodites.
    What I learned about them was in the diving speciality Carribean Reef Ecology. (And the various books on the animals and plants of the reef)
    Yet another example of the complexity look at this wikipedia article on the various ways sex is determined in different classes of animals and plants x,y w,x and u,v being three common systems of chromosomes.
    Interestingly the article points out that the platypus has 10 sex chromosomes but does not use the sry gene of higher mammals.
    I think the article makes the point that nature has evolved several systems for determining the sex of an organism.

  9. #9 Lyle
    February 19, 2015

    Re #6
    It has long been known that xxy xyy and variations with more sex chromosomes exist in humans (generally not being advantageous to having descendents). Mental retardation is common in the less usual sex chromosomal states for example.
    Re #7
    It turns out some single celled animals will fuse and exchange genetic material, to get the benefit of different genes, which is likley where sex as part of reproduction came from originally.
    Let alone talk about reproduction in plants which I don’t fully understand but flowering plants do have 2 different stages in their life cycle (as do lower plant)

  10. #10 See Noevo
    February 19, 2015

    To Lyle #9:

    “It turns out some single celled animals will fuse and exchange genetic material, to get the benefit of different genes, which is likley where sex as part of reproduction came from originally.”

    If the “goal” was genetic exchange and it was being accomplished absent gender and sex, then why (and how) did gender and sex evolve, and stick around for billions of years?

  11. #11 Lucid
    February 19, 2015

    I think this whole thread is politically correct bullshit: it’s like saying that there is no real distinction between fish and bird because there are some flying fish. Nobody would get that idea because even with some flying fish the concept is still very useful, natural and has high predictive power, most likely because it captures differences in the underlying biology that matter.

    Regarding Sex:

    1. Sex is extremely binary in its genetic base. XY is clearly different from XX, DNA being a discrete (almost binary) thing. Thus even most inter-sex persons are biologically clearly male or female (according to their chromosomes). And this matters, because it determines with whom they can (potentially) procreate. There is not much more important in our lives.

    2. Chimera and chromosome disorders with intersex condition are very rare exceptions. Most of these people -btw- have much graver problems then sliding sex scales, e.g. dying before even sexual-maturity. These exceptions certainly do not reduce the viability of the concept of sex.

    3. Of course there is a point were the typical development between sexes departs, sometimes more sometimes less. So what? You still can’t conceive children with your own (chromosomal) sex no matter how.

    4. It’s true that live most likely started out uni-sexual and many bacteria etc still are. Binary sexual differentiation developed because (so to speak) it allows the species to extract more information from the evolutionary environment, thus being more adoptable than without (because of genetic recombination). Furthermore sexual dimorphism enable specialization, thus better total work efficiency. So intersex people are most likely biologically less attractive than normal people, nothing to do with society.

    5. There is either one sex or two sexes, not more. Why? Because the two sexes represent the two possible types of mating strategies: quantitative (male) vs qualitative (female), or spreading vs choosing/nurturing. There is simply is no third alternative.


  12. #12 John
    February 19, 2015


    Point five is reactionary biotruth nonsense, adds nothing and degrades the entire discussion. I can’t imagine why you included it.

  13. #13 Daulnay
    February 19, 2015

    Why do sexes exist?

    Probabilities and mutation. Sexes allow organisms to flush fatal mutations out of the gene pool. Sexual reproduction allows organisms to faithfully pass on much larger amounts of genetic information to the next generation. Those organisms can grow with more complex genetic instructions than non-sexual ones.

    Here’s how it works (simplified):
    Over time, each bit of genetic information (the nucleobases GATC) may get mutated into something fatal , call it a mutation rate m. The more genetic information (n) there is, the more likely a fatal mutation occurs: p = 1 – (1- m)^n.

    Suppose two sexual organisms reproduce, where each has developed a single fatal mutation. Because they are sexual, they will pass on the broken gene to 1/2 of their offspring.

    So, 1/4 of their offspring will have no fatal mutations, 1/2 will have 1, and 1/4 will have 2 fatal mutations.

    Sexual reproduction allows more elaborate genetically coded organisms to reproduce without dying out to mutations. That’s why it exists.

  14. #14 Lucid
    February 19, 2015


    I agree that point 5 is the least defensible one and probably more in the domain of philosophy.

    But imho it’s a very interesting theory for explaining why there are only two sexes and the only one really convincing.

    All sexual species show that quantitative/qualitative distinction in their mating strategies and it’s also hardly avoidable because the sex which gets pregnant has vastly higher costs and risks and thus needs to be much more careful. In humans pregnancy can easily be deadly without a supporting male and without advanced medicine (and effective safe medicine is a very recent invention in evolutionary terms so it couldn’t shape our behavior much yet).

    If you agree that this difference exists, what other third mating strategy could there be besides quality and quantity?
    It can’t be a mixture for game theoretic / competition-related reasons.

    There are dozens other thinkable reasons why no third sex develops, e.g. because it would be even more trouble to get 3 persons in bed together than two. Conception difficulty probably rises exponentially with the number of sexes ( O(n!) ), but it’s still only a ‘gradualistic’ argument, the cost might be worth the gain in some evolutionary situations. Still.. there is no known species with 3 sexes.

  15. #15 Lucid
    February 19, 2015

    Oh, and the social thing:

    1. our modern world has changed almost nothing with regards to basic mating strategies. Before industrial cloning there is not a chance this changes because the biological incentives are just to strong.

    2. Also, changing it forcefully is like putting wild animals in small cages. Its not natural and thus incompatible with the human condition. In human history societies who tried it killed themselves.
    Actually since the dawn of feminism movement psychologist measure progressively lower happiness in women who live in nontraditional partnerships. In our christian past women used to be the happier sex, now it’s men.
    And the differences got even bigger when females entered the workforce en masse. The trend continues. And europe – even more socialist and ‘modern’ than the US – will cease to exists due to unborn children in the near future.

  16. #16 See Noevo
    February 19, 2015

    To Lucid #11:

    “Binary sexual differentiation developed because (so to speak) it allows the species to extract more information from the evolutionary environment…”

    In other words, sex and gender accidentally/randomly appeared, along with the attraction between the two, were found advantageous, and so carried on.

    But HOW did the sex and gender evolve?

    “There is either one sex or two sexes, not more. Why? Because the two sexes represent the two possible types of mating strategies: quantitative (male) vs qualitative (female), or spreading vs choosing/nurturing. There is simply is no third alternative.”

    Why not three or more genders, allowing three or more times the extraction of more information from the evolutionary environment, thus becoming more adaptable?

  17. #17 See Noevo
    February 19, 2015

    To Daulnay #13:

    With all the fatal mutations being passed on (and, I’ve read, accumulating over generations), it’s a wonder anything is alive after billions of years of this.

  18. #18 See Noevo
    February 19, 2015

    To Lucid #14:

    “There are dozens other thinkable reasons why no third sex develops, e.g. because it would be even more trouble to get 3 persons in bed together than two.”

    But it was noted earlier, I think, that single-celled, asexual organisms can fuse and exchange genetic material. No need to get even two in bed, because you have do-it-yourself “sex.” One “gender” sounds a lot less problematic, evolutionarily-speaking. Why would evolution make problems for itself, “problems” that haven’t gone away for billions of years?

  19. #19 Lucid
    February 20, 2015

    “But it was noted earlier, I think, that single-celled, asexual organisms can fuse and exchange genetic material.”

    Yes, the world of unicellular live seems rather strange from our view. But as far as I know most replication of bacteria and archea is sex-less and solitary. The cell just divides itself into two cells and both contain the same genome except for some mutation. Fidelity of replication is rather low, so there are many errors (=mutations); and this may indeed be a neccessary consequence of not having sex (i.e. recombination), because they still need a way to adapt to a changing environment. Most mutations are harmful but for an unicellular species as a whole they are essential to survive, because the few cells with good mutations are better adapted to the changed environment and so their offspring form the majority of the population a few generations later. Without mutations the whole species wouldn’t survive against the also steadily adapting immune system of their hosts or newly used antibiotics.
    Sex, i.e. recombination = mixing of the genome of two individuals, on the other hand is usually not that harmful, It creates more viable variation per harmful error so to speak. So humans can evolve mostly by mixing their genes (and these are already working gene variants, at least they got each partners into puberty). Mixing functional variants seems to be better than making random changes. Still, recombination is quite like cut and paste and if the cut happens during a critical part also damage occurs (but of course also the potential of adaption). But due to having two parents humans have two copies of each gene and if one is damaged often the second copy is enough to survive.
    A cell has only one parent and one copy per gene, so there is neither mixing nor a backup copy. However there are indeed some other ways of gene exchange for unicellulars. There are viruses that pray on bacteria to inject their own genome into them (using a kind of penis even). If successfull the victim will produce new viruses instead of its own offspring. If the cell has defenses against the virus it might survive and even gain a few new resistance genes from the virus. Also some bacteria exchange genes using plasmids, small packets of RNA, which might carry resistance genes. Of course this has dangers, the RNA could be harmful. Actually the function of cells hulls (and thus the design of the cell) is probably to keep foreign RNA out and their own in. Also RNA might not get integrated into their genome, so the next generation does not participate. I guess these ways of so called ‘vertical gene transmission’ have a much smaller effect than recombination has in e.g. humans. Even if it is as effective, this would not be feasible for multicellular live like we are, because it would only affect single cells and out immune system would usually kill the changed cells. Every single cell in our bodies sharing the same genome enables the cooperation between them. The only way to change all of them is during conception, when there is only a single cell.

    Also note that bacteria etc don’t have sexes; concerning vertical gene transmission they are rather like equals in a peer2peer network. So this does not help in finding a third gender.
    Getting these plasmids into each other might well be more difficult than getting a girl into bed.

    Comparing our live with unicellular live might produce some interesting ideas, but in the end we are not cells but mega-colonies of cells and so reproduction is a wholly different problem for us and of a much larger scale.

    Sex evolved by chance in the same sense as the whole evolution is a product of chance. That said, there are some good theories how splitting into sexes in a series of small steps, each beneficial, is possible for cells. It most likely starts with very small sex differences,that get larger with time as long as there is some benefit in stronger specialization. This explains the different roles in society and child rearing of men and women; they are optimized for different parts. And thats obviously much more efficient than if each partner would have the same talents. Imagine a pregnant women having to take part in hunting mammoths, etc.

    On the other hand there is a maximum possible distance between both sexes, because they still need to have largely compatible genomes to successfully breed. Together these boundaries define what kinds of successful relationships are possible between current men and women. Violation of such boundaries by politicians, state actors and anti-natural education will be harmful to either one or both sexes, at least until evolution evolves us some new adaptions. Unfortunately human adaptions evolve in timescales of thousands to millions of years.

  20. #20 Lucid
    February 20, 2015

    I don’t quite agree: harmful mutations can only accumulate to a certain degree, because their carriers would either die early or otherwise be unable to procreate.

    If it doesn’t affect the number of offsprings the mutations are – by definition – not harmful. Evolutionary fitness is the expected total number of offspring. If it does affect this number negatively the mutations end up in a evolutionary dead end sooner or later.

    Consider innovations like social security: many fear fitness reductions because the state prevents natural selection from happening. But this is not really true. Fitness in the above sense is not harmed at all. However the state intervention modifies the evolutionary environment in the sense that today other abilities are necessary for procreation than in the past. Being stupid or lazy may be fitness enhancing trait now. These new virtues might seem foreign to us because we are used to something else; but evolution will gladly optimize everything that increases the number of children – as long as the environment sustains.

  21. #21 See Noevo
    February 20, 2015

    To Lucid #20:

    If you’re in anyway implying that Social Security was a harmful mutation to begin with, I think I would agree.

  22. #22 geronimo
    February 20, 2015

    To #15, 20.
    You’re associating Social Security with promoting laziness or stupidity? .. and women’s subservience with happiness?
    What a delightful character you are.
    Oh, wait, you mentioned ‘christian’. That explains it.

  23. #23 tlsmith
    February 26, 2015

    I was completely blown away when I found out about ring species. Herring gulls and less black backed gulls in the UK are two ends of a polar circumverence ring species. Awesome

  24. #24 C
    February 27, 2015

    >> Drag this article out next time someone tries to argue that biology supports their simplistic version of a discrete sexual dichotomy.

    Sure. The problem is that the kind of people with limited capacity for complex ideas are the kind of people who reject science. You got a Bible verse we could whip out for them? 😉

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