Are Sex and Gender Binary?

"Sex", "gender", and "sexuality" aren't binary, the binary model (which gives only an A or B choice and expects all three to align) is wrong.

Non-Binary Sex and Gender: Is Sex Binary? Is Gender Binary?

Studies have shown there is no single determinant of sex, gender, or sexuality. Thus, a binary distinction of “male or female” fails to describe a wide range of humans in practice. Simply put, Gender/Sex/Sexuality is not binary, it is a spectrum… and we can prove it with science (by proving that sex isn’t binary).[1][2][3][4][5]

First, we will cover genetics and sexual differentiation, since that is the foundation from which sex, gender, and sexuality arises; then we will move on to how genetics create a myriad of genders and sexualities due to gene expression and hormones, then discuss how nurture plays a role, and finally muse on how that all relates to modern culture.

Before we get to any of that, let’s clarify what “non-binary” means, clarify what sex, gender, and sexuality mean, and then offer a summary of the science that shows that the sexual differentiation process leads to “intersex” (non-binary gender).

Gender Binary Theory and An Example of Binary Choices in Sex, Gender, and Sexuality

  • Gender binary theory says we can classify sex and gender into two distinct groups.
  • Non-binary gender theory says we cannot simply classify sex and gender into two distinct groups.

Sex, Gender, and SexualitySex refers to biological differences like chromosomes, hormonal profiles, internal and external sex organs. Sex is not binary (by most measures), and we can prove it. Gender describes the characteristics that a society or culture deems as masculine or feminine (since sex isn’t binary, we shouldn’t as a society force a binary view on people). Sexuality describes sexual preference (clearly, not everyone is straight, instead sexuality is “non-binary”). What we are saying here is that “sex” isn’t binary (despite some binary distinctions, like chromosomes being either XX or XY on average; see XO), therefore we can make a strong case that despite social constructs “gender” isn’t binary either, and by logical extension we can reasonably assert that human sexuality is also non-binary as well. This is described in greater detail below, but before we get to that, let’s examine a summary of why sex is non-binary by looking at how the sexual differentiation process works.[6]

A Summary of Why Sex (and therefore Gender) isn’t Binary

All the cells in a human body start as one cell, a fertilized egg containing 1/2 the mothers DNA and 1/2 the fathers.

On average that cell will either contain XX or XY genetic code (with exceptions being things like a missing X chromosome).

Then, for the first 5-6 weeks of gestation, only the X gene will express. Despite only the X gene expressing in these weeks, the developing human (technically an embryo) has no anatomic or hormonal sex (where the “all humans start as female” half-myth comes from).

An embryo in this stage develops undifferentiated bi-potential structures like the the gonadal ridge, undifferentiated gonad, bi-potential Wolffian ducts, and bi-potential Müllerian ducts which, on average, develop into either “male or female” internal and external sex organs depending on gonadal steroid hormones released starting at 5-6 weeks when sexual differentiation begins (so the anatomy is the same in both XX and XY in this stage, and that anatomy is part a result of the X gene only, and thus it is no surprise that later each “gender” still shares many of the same basic anatomical structures and that sometimes things can come out somewhere in-between giving us “intersex”; see some similarities and differences here and here and here and here).

Next, at 5-6 week mark when sexual differentiation occurs (spurred on by gonadal steroid hormones like androgen and estrogen), the results are a mater of genetic expression and hormonal development (and a host of other complex factors described below, including the expression of the very important SRY and DAX1 genes).

In the average XY-“male,” the Y gene starts expressing at about 5-6 weeks, it releases androgens like testosterone, represses some X gene expression (and estrogen development), and expresses specific Y genes. If everything goes smoothly, this leads to those undifferentiated structures developing into male internal and external sex organs.

The same basics are true for a XX-“female,” the X gene continues to express, and hormonal development leads to the undifferentiated structures developing into male internal and external sex organs.

Complexities aside, in this phase the undifferentiated gonad develops into ovaries in genetic females (on average), and develops into testes in genetic males (on average).

However, that is just on average. The reality is that, to our point, many different things can happen in this stage.

For example the Y gene can fail to express in a XY “male” and fail to produce some or all “male” structures and features, or the X gene can fail to expressed properly in XX “females” and fail to produce some or all “female” structures and features, or a mix of these things could happen.

Furthermore, there can be a lack of hormone receptors, or receptors could not receive hormones properly, and this could affect gonadal development, the development of external genitalia (which occurs from the 9th to 12th week), and other aspects of the sexual differentiation process (which continue up until the the 28th week).

Furthermore, key genes like SRY, the Sox9, Fgf9, Wnt4 add complexity to the story (as each plays a role in development).

In other words, the complex process of sexual differentiation based on that [generally] binary XX or XY genetic code has many moving parts, and a number of different possible “abnormalities” can lead to a wide range of non-binary results in practice (including the very rare “true hermaphrodite“).

The bottom line here is that the genetic code a person has is far from the only determining factor in sexual differentiation (how genes express, hormones, and the related aspects of sexual differentiation like gonadal development play a large role).

Given the realities of the process of genetic expression, it is very helpful to be able to acknowledge the non-binary nature of sex and gender (and sexuality) and the related concept of “intersex” (where intersex describes a human born with a reproductive or sexual anatomy that doesn’t seem to fit the typical definitions of female or male.)

TIP: October 26th is Intersex awareness day and November 8th is Intersex Day of Solidarity. These days are dedicated to spreading awareness of the fact that gender isn’t binary. See intersexday.orgIn Recognition of Intersex Awareness Day by the U.S. Department of State, and the Equal Rights Coalition.

TIP: For more reading see: Sex Determination and Differentiation by or see a timeline of sexual differentiation in humans.[7]

The Difference Between Sex, Gender, and Sexuality

To show that sex, gender, and sexuality aren’t binary, it is important to understand what we mean by sex, gender, and sexuality. Although we covered this above, let’s return to it quick before moving on so we can clarify some semantics.

  • Sex is both genetic (what genes a person has and how they express) and epigenetic (how genes express and how environmental factors effect genes). Biological sex depends on the genes a person has and how those genes express to create physiological differences (like a penis or vagina). NOTE: Environmental factors are not thought to influence the sexual determination process, so when we say “epigenetic” we generally mean “how genes express and the related hormonal development in the fetus during the primary and secondary sexual differentiation process.”
  • Gender refers to either “genetic sex” (male, female, or at least intersex) thus acting as a synonym for sex in common language, or the societal constructs of gender identity (how a culture perceives a person and how a person perceives themselves) and gender roles (a set of societal norms based on actual or perceived sex, gender, or sexuality). NOTE: Societal constructs are their own issue. This page asserts that since sex isn’t binary, we shouldn’t force a binary view on society, but one can’t prove a social construct (a form of opinion) wrong in the same way one can shed light on biological factors.
  • Sexuality refers to personal preference regarding human sexuality (which comes in a variety of flavors based on nature and nurture). NOTE: It is fairly clear that sexual preference isn’t binary, but we can’t prove it in the same way we can prove sex is non-binary.

Despite the complexities illustrated in the notes above, we will ultimately make the case that neither sex, gender, nor sexuality are binary alone, and that they are even more complex (and less binary) when combined.

Thus we can say sex isn’t binary, gender isn’t binary, and sexuality isn’t binary either (but with the note that genetic sex is probable with natural science, and gender and sexuality informed by natural science, but provable only with social science; phrased another way, it is a scientific fact that sex isn’t binary… but unfortunately the other two aren’t as easy to prove directly, one is cultural, and the other requires us to rely on testimony, inductive evidence, and social science studies).

However, despite us showing/making the case that neither sex, gender, nor sexuality is binary, we must note that some aspects of sex, gender, and sexuality ARE binary. See arguments against here: “Gender is not a binary, it’s a spectrum”: some problems.

The Binary Distinctions

While the research shows that binary gender theory is wrong (and that non-binary gender theory is right), we can still make some binary distinctions (which add complexity to the topic). For example:

  • A person either has a Y gene or they don’t. It is the expression of the X Y and related genes, and other physiological factors, that makes sex and gender complex. In other words, humans (like many other animals) have a XY sex-determination system. Humans specifically have forty-six chromosomes, including two sex chromosomes, XX in females and XY in males [in general, remember there is oddities like XO]. A single fertilized egg is either XX or XY. On average XX will express as female, and XY as male, but while genetic code is rather binary, the end results are not.
  • Or, as a commenter noted, if you look at it from the point of reproduction alone you only get four options: Male (Sperm), Female (Egg), Hermaphrodite (Both), Androgyne (Neither). While this isn’t exactly a binary either/or choice, we can divide it into binary groups like “sperm producing” and “non-sperm producing”, or “egg carrying” or “non-egg carrying”.
  • Similar examples can be given for gender (for instance “has testes” or “doesn’t have testes”) or for sexual preference (for instance “likes both sexes” or “only prefers one sex”).

Thus, we get binary choices, but these many binary choices are part of what underlies a non-binary theory (just like a scant few properties underly the standard model of particle physics or mathematics; but from there literal spectrums os possibilities arise).

Given these truism, we can confidently say there is no single determining factor, including the Y chromosome, and we can conclude that it is incorrect to ignore the many binary (and non-binary) aspects that create complexity.

NOTE: It is very important to note that we are acknowledging the binary distinctions and simply saying that, despite this, the end result is non-binary.

Human Sexuality is Complicated… Indeed, human sex, gender, and sexuality are all complicated. It isn’t just XX or XY.

Genetic Sex Versus the Other Determinants

We used to think that genetic sex was determined by the presence or absence of a Y chromosome alone (where XX is male and XY female, and where Y turns on hormones that cause masculinization and inhibits hormones that cause feminization).

However, recent studies (mostly on mice, a type of mammal) show that sexual differentiation and determination is much more complex.

This is true for both primary sex determination and secondary sex determination.[8]

  • Primary sex determination: the ongoing process of gonadal differentiation between about 6 – 12 weeks which results in the development of the testes or ovaries.
  • Secondary sex determination: Usually determined by hormones secreted from the gonads, and which affects the biological phenotype outside the gonads, where a male mammal develops a penis, seminal vesicles, and prostate gland and a female mammal a vagina, cervix, uterus, oviducts, and mammary glands.

FACT: Nettie Stevens discovered the sex determination system in 1905. Today we know that the truth is more complex than she originally thought. However, her theories still form a pretty accurate foundation and helped lead us where we are today.

Complex Primary Sex Determination and Differentiation: SRY, DAX1, Sox9, Fgf9, Wnt4, Etc.

It turns out we have to consider factors like the SRY gene, which is the sex-determining region on the Y chromosome. We also have to explore the DAX1 gene, which represses the SRY, the Sox9 gene, Fgf9 gene, Wnt4 gene, etc. when considering primary sexual determination and differentiation. Differentiation is the development of the gonads, which turn into either testes or ovaries.[9][10][11][12]

  • The SRY acts as a facilitator or blueprint for “masculinization” and can be present on a Y or X chromosome.
  • The SRY can turn an XX into a male, but only if androgen receptors are present. The release of male hormones must not only be facilitated by the SRY, but those hormones must be received by the body.
  • The DAX1 is essentially the opposite. It inhibits a Y and can turn an XY into a female by blocking hormones.
  • The Sox9 plays a role too, as does the Fgf9. The Fgf9 activates the Sox9 in a loop of sorts and represses the Wnt4.
  • Meanwhile, other un-noted, and perhaps undiscovered, genetic factors add even more complexity to primary and consequently to secondary differentiation.[13][14]

If you want an introduction to the complexities of genetics, I suggest reading this study: The Genetics of Sex Determination: Rethinking Concepts and Theories.

And Thus, Via Epigenetics, A Complex Genetic Spectrum of Sexes Arises

As clearly noted by this point, the basic genetic code that determines sex is more complicated than just the absence or presence of a Y chromosome. Gene expression, which is also known as epigenetics, adds a layer of complexity. Thus we have a significant genetic range of sexes. Some sexes are recognized as “disorders of sexual differentiation” and some are not.

Given the complexity we can see how many different sexes, genders, and sexualities would be present in our society.[15]

FACT: Mutations in Sox9 or any associated genes can cause a reversal of sex or cause a person to be born intersex. If Fgf9, which is activated by Sox9, is not present, a fetus with both X and Y chromosomes can be converted into a female; the same is true if Dax1 is not present. A person may be born intersex because of an unusual activity of the SRY, usually when it’s translocated onto the X-chromosome and its activity is only activated in some cells.[16]

Physiological Factors of Sexual Determination

Now that we know genetics aren’t binary, we can take the next logical step and explore the way this creates non-binary physiological factors.

Oddly enough (or not), anatomical factors that we normally attribute to boy or girl aren’t binary either. This includes primary gonadal differentiation which creates the testis and ovaries and secondary differentiation which creates organs like enlarged breasts, the penis, and the vagina. You should already understand that none of these things are binary determinants, but let’s discuss each quickly.[17]

Gonadal Differentiation: Do Testis or Ovaries Determine Sex?

In mammalian primary sex determination, there is no “default state.” The formation of ovaries and testis from the gonads are both active, gene-directed, processes.

Despite this A or B choice of sorts, since the development of both are an active process, a wide range of results may occur. What happens then affects secondary sex determination factors (like whether the child will have a penis, vagina, multiples of either or both).

FACT: Although modern science thinks of the gonads as a blank slate, not as female sex organs that turn male, we can say all mammals are essentially born female (meaning Y related genes don’t express right away and only X genes are active initially). Around 6 weeks, male hormones released by genes and received by the body cause sexual differentiation. To get a traditional boy or girl where sex, gender, and sexuality align perfectly requires a complex cascade of gene products in certain dosages and at precise times.

Does a Penis or Vagina Determine Sex?

Just as the organs resulting from primary sex determination aren’t binary, organs that occur as a result of secondary sex determination aren’t binary either. Consequently, The idea that people with penises are male and those with vaginas female is not reliable in real life.

People can be born with multiple genitalia or genitalia that doesn’t match their other attributes or organs or hormones depending on how their genes expressed in development.

A Person may have multiple vaginas, cervixes, and uteri, or may have both a penis and a vagina. People can be born with two penises, which is known as being diphillatic. People can be born with genitalia that is ambiguous.[18]

Gene expression leads to a wide range of physiological features, and of course a wide range of corresponding genders, identities, and sexualities.

TIP: See Does having a Y chromosome make someone a man?

Science doesn’t support the sex binary | Riley J. Dennis.

The Sexual Spectrum: Non-Binary Gender and Sexuality

To keep this page at a readable length we can summarize the above by saying, “genetic expression results in such a wide spectrum of sexes that in turn when paired with factors of nurture, result in an even wider array of gender identities and sexualities.”

Since we can’t discuss all the social issues related to the sexual spectrum here, I’ll point you to a few key related pages.

For more information see “some people are born gay” (which discusses an facet of sexuality) and “Athletes Can Determine their Own Sex in International Sports” (which discusses some aspects of the way in which our culture is dealing with advances in science and the spectrum of sex, gender, and sexuality).


We can draw from the natural sciences related to genetics and biology, or we can draw from the social sciences, or we could just look at our friends, family, and culture, wherever we look we can see that an A or B choice doesn’t cut it when it comes to “sex”, “gender” and “sexuality”. There is very obviously a range of choices, despite this, some aspects are binary. So, while it may not be a quantum system, it is at least a complex binary system which results in a wide array of choices. This much seems simple, but figuring out how that informs our culture is not.


  1. Sex and gender distinction
  2. Sex differences in humans
  3. Gender binary
  4. Defining Sex, Gender, and Sexuality Source: Boundless.
  5. Brain sex differences and hormone influences
  6. What is the difference between sex and gender?
  7. Sex Determination and Differentiation –
  8. Chromosomal Sex Determination in Mammals. Primary and secondary sex determination
  9. Does having a Y chromosome make someone a man?
  10. Is the Y chromosome all that is required for sex determination?
  11. Disorders of sexual differentiation: I. Genetics and pathology
  12. The Genetics of Sex Determination: Rethinking Concepts and Theories
  13. FGF9
  14. Fgf9 and Wnt4 Act as Antagonistic Signals to Regulate Mammalian Sex Determination
  15. disorders of sexual differentiation
  16. SOX9
  17. Chromosomal Sex Determination in Mammals
  18. Ambiguous Genetalia

"Sex and Gender Differences in Humans Aren’t Binary" is tagged with: Cells, Complexity Science, DNA, Human Rights, Sex. Gender. and Sexuality

Vote Fact or Myth: "Sex and Gender Differences in Humans Aren’t Binary"

Your Vote: {{ voteModel || 'no vote' | uppercase }}

R on

If you look at from the point of producing (or capable of producing) sperm & eggs (at least during part of the individual’s life) you only get four options: Male (Sperm). Female (Egg). Hermaphrodite (Both). Androgyne (Neither). Of course, it can get complicated when you add appearances, but it basically still breaks down into the four same catergories.

On some humor site (I can’t remember which), there’s the claim that there’s more and that kind of thinking, I believe, is problematic at best. Lucky I never personally encountered such thinking so I don’t know how real the “joke” is. But even as a joke, such deception troubles me.

On a related note (since I don’t feel like posting twice), I hope those that can choose their gender in the Olympics are those where the distinction is ambiguous and clearly male or female.

Thomas DeMichele
Thomas DeMichele on

Good comments. I’ll take those into account. Both pages include difficult topics, I don’t think we have a perfect answer beyond what our research has presented above. But comments help uncover more bits of the puzzle. Thanks!

Bob on

100% myth. Anyone saying otherwise is a liar.

Louise Goueffic on

You are spreading a myth, The sexes in (hu) MANity and said to be opposites. And they are,
Fem was renamed (wo)man in 11th C. This made her a man-that’s-not-a-man. Man is the man that’s a man due his make genitals. The feme was renamed fe+male in the 13th C by a poet needing rhyme!

Opposite sexes: one with a penis,+1. and opposite having no penis, -1. Fact: (hu) MANity’s binary.

Thomas DeMichele
Thomas DeMichele on

Appreciate the viewpoint, but kindly disagree.

When we look at gonadal development, the way hormones work and genes express, and then note intersex and non-binary sexuality in practice, we can see that gender isn’t binary. We can look at the DNA of fertilized eggs and see if they are supposed to have a penis or not, but after all the complex expression is done in the development stages, not everyone fits neatly into an A/B box. It is not skin off my back to recognize diversity, but that is a personal opinion, above i’m commenting on my research of the scientific aspects of this and only looking to the social science and issues of morality and ethics as a secondary device.

Think about it this way, “is a hermaphrodite a man or a woman?” Who decides, you? The Church? The Doctor? The State? Well I think the philosophers would pick a bone with anyone but the person themselves deciding, but if we want to codify it by science then we are much better off calling this grey area what it is “intersex”, respecting that, and then leaving the rest up to individual liberty.

Oblomov on

Not to talk about why he wrote man in “huMANity” in capital letters. I’m afraid of the answer…
I don’t wanna be a “sjw” about it :), it might be about the etymology, but certainly shows if true, as the men build the culture with themselves as the default, when the women had little to no place, and the woman as something different and not themselves rather than another person.
The accent on being opposites, dunno might be reading too much on it and he might just be illustrating the point of them being functional one another from the reproductive stand point and no speculation about what their social role or correct behavior is supposed to be.
Though even in the hypothesis of the existence of complete ermaphroditism, this wouldn’t eliminate the dualism of gametes, with no third way aside from one person simply representing both sexes like some types of slugs, rather than a third sex for example.
Intersex people, as the ethimology illustrates, present combinations of elements typical of both sexes and reproductive roles in varying proportions.

Somehow I saw similar discussion about the voice. People claiming there is no mix between M1 and M2 registers, though they both can present characteristics of either one, and m2 “falsetto” presenting high level of compression and close quotients but despite that always being an “either or”.

Oblomov on

Hi, given that I usually express myself more in musical matters. I’ll try to be as discrete and respectful about this topic.
With the premise that I respect how one feel about something as intimate as one’s own sexuality, regardless of me eventually agreeing or not with scientific explanation and I find unnecessaryly rude and self appeasing being a douche about it just to be “politically uncorrect” (which when sexism and racism dominated used to be being for freedom and civil rights, btw, or satire against the power). I don’t aim this to whoever disagrees, but if one does it with the subtext of attacking the minority subjects of matter as “freaks”, deluded or such.
I can’t give a cut and dry vote, as it is different depending on which aspect of sex we talk of. Plus I have a couple of questions, this time :).
As one said the gametes are in themselves quite binary, sperms and eggs, the chromosomes x and y.
It’s the outcome that in a minority of cases might not be binary, with intersex people.
Not sure if I got it right. Can we have xx people who develope a penis and don’t grow breasts after puberty or have a breast and penis and Xy people who develope breasts, have no penis or somehow have both?
What about the ratio of sterility or fertility of those developing part of the reproductive system of the opposite chromosome combo?

Is transexuality a case of brain intersexuality? Iike xy people who “express” a female brain and xx people who express a male brain? As we know there are slight differences between the brains of the two sexes. Both have the potential to achieve the same results, though, through slightly difference paths.
But there is no legittimate absolute model or expectation of how a male or a female is supposed to act or behave because of his/her brain, at most some statistical likelyness, also filtered through the social context, which intermingles with that starting biological contribution.
And with this I come to the “gender”, which is what has me more skeptical. One might say he/she doesn’t recognize him/herself in the gender assigned at birth. But that sounds like assuming that if you don’t cope with the current (and ever decreasing) expectation of what a typical man or woman is supposed to look, dress or behave, you are any less of a woman or a man or something else.
I might like to paint my nails black and/or wear eyeliner and still be a straight man, many straight women I know like short hair and/or wear no makeup and are still women, in past this would have been regarded as androgynous, even just wearing pants, jeans or trousers. Or long hair for men in the 60’s :). Same for jobs attributed to either sexes, stereotypes of passivity and dominance.

The only natural role is the reproductive role, I might say, though still with some flexibility. We have cis straights who don’t want anything to do with it, gay men or women who want to fertilize or get pregnant.
The only way “gender presentation” can stay in a sliding bar between male and female is intentional crossdressing, which still accepts somehow the codes for which makeup and certain clothes mean female and you want to present as such and “androgynous” if you mix such things a la David Bowie or Tilda Swinton, while they, to me are still men or women, which present (/ed :/) as such the way they like, with no conventional stylistic restriction, relative to an everchanging culture.
So I might see gender presentation as “personal presentation” period :).
Thanks for giving me thought fuel.