Gender Spectrum: A Scientist Explains Why Gender Isn’t Binary

February 24, 2023 By Cade Hildreth 22 Comments

The gender spectrum is an understanding that gender is not binary, but rather a spectrum of biological, mental and emotional traits that exist along a continuum.

In contrast, the gender binary—also called gender binarism or genderism—is a belief that gender is composed of two distinct and opposite genders (women/men) in which there is not overlap.

Unfortunately for those who believe in a gender binary, it is not scientifically or medically correct. Gender can’t be binary, because it is a personal identity and is socially constructed.

Sex, which refers to one’s biological characteristics, also exists as a spectrum, because intersex people exist. A person’s sex can be female, male, or intersex—which can present as an infinite number of biological combinations.

Today, numerous scientific fields, including biology, endocrinology, physiology, genetics, neuroscience, and reproductive science, have confirmed that both biological sex and gender exist as a spectrum.

This is true for humans and across the animal kingdom.

Gender Spectrum vs. Sex Spectrum

When using the terms sex and gender, it is important to note that “sex” (female/male/intersex) describes biological traits. In contrast, “gender” is a broader term that reflects how a person lives within society. One’s gender identity could be woman, man, transgender, nonbinary, or an infinite number of other possibilities.

Because gender is a personal identity, is socially constructed, and has limitless possibilities, it takes no further explanation to explain why it is a spectrum.

Therefore, when people question the existence of a gender spectrum, what they are usually questioning is the existence of a sex spectrum.

Sex (and Gender) are Bimodal, Not Binary

For all too long, the government, the medical system, and even our parents have assumed that sex is binary. Based on science, this is not biologically or medically accurate.

What is true is that sex characteristics tend to be bimodal, meaning there are clusters of characteristics that tend to be associated with people that we call “female” or “male.”

On average, males do have penises, and on average, females do have vaginas. This is what allows for reproduction. However, there are many examples where this is not the case, such as intersex people. External genitals (a biological marker of sex) present across a spectrum from full-size penis to small penis to micro-penis to clitoromegaly to enlarged clitoris to standard-sized clitoris.

Similarly, most men do have testes and produce sperm, while women do have ovaries and produce eggs. However, intersex people exist who have both male and female gonadal tissue, called an ovotestis. This tissue can present as a spectrum, with both testicular cells and ovarian cells present in the same organ, known as a streak gonad.

Additionally, there are females, males, and intersex people who do not produce gametes (egg/sperm/neither), which means gametes are not binary. In particular, people with duel or ambiguous gonadal tissue coupled with no gamete production eliminate the ability to use gametes to create a sex binary.

Similarly, males tend to have XY chromosomes and females tend to have XX chromosomes. However, sex chromosomes (another biological marker of sex) come in a wide variety as well, with at least 16 different naturally occurring variations (see below). This means that chromosomal presentation is not binary either.

On average, males do have higher testosterone levels than females, but there are females who have higher testosterone levels than some males (called hyperandrogenism). For sex hormones to be binary, then males would have to have higher androgen levels than all females, which of course, they do not.

On a day-to-day basis, we tend to identify each other’s sex using what is called “secondary sex characteristics.” The most common secondary sex characteristics we notice are body and facial hair, as well as chest contour and development.

On average, males do have more facial and body hair than females, but there are also females with facial hair and males who can’t grow a full beard. Similarly, there are men who have substantial breast development (called gynecomastia) and women who have a nearly flat chest. Thus, the secondary sex characteristics aren’t binary either.

As explained by these examples, sex is not binary, because people cannot be grouped into two separate, non-overlapping groups.

However, bimodal sex characteristics are not uncommon.

Bimodal means the presence of two (“bi”) statistical modes, which can be seen as peaks in a graph. The two modes represent probability clusters.

Biological Sex is a Spectrum with Clusters

With regard to human sex, this means that for some sex characteristics, there may be common norms among people whom we tend to assign as “male” and “female.” However, there are also clearly overlaps present between the peaks. This is what makes sex bimodal, and not binary.

Finally, at risk of getting too mathematical, a bimodal distribution is by definition, a continuous probability distribution with two different modes.

In other words, biological sex is a spectrum that has clusters.

If sex is a spectrum, then gender is unquestionably a spectrum, because gender includes aspects of biological sex, interwoven with how a person lives within society and self identifies.

What Should Go Along the X-Axis of this Chart?

Of course, the graphic above is simplified for the sake of understanding. To make a bimodal distribution, we would need to assign a numerical value along the X-axis of this chart (the horizontal axis).

This leaves us with the question, what should we put along the X-axis? The answer is simple: You could chart many of the presently known sex markers along the X-axis (discussed below).

Examples of this would be plotting genital length, sex hormone levels, quantity of androgen receptors, volume or cell count of gonadal tissue, quantity of gametes, and so on.

However, no sex marker is relevant to plot in isolation, because biological sex is created by a constellation of traits.

Some people who believe in “binarism” try to claim that only one sex marker should be assessed, which is gamete production. However, gamete production is not binary, which you can read about in great detail here.

Thus, the bimodal graphic above communicates the concept that there are common frequencies (“clusters”) of sex marker traits—and none of them are binary (female/male).

Scientifically speaking, you could plot a wide number of traits along the X-axis, which is why it has not been labelled.

Biological Sex is Not Binary

Most of the biological sex markers (of which there are at least 10) present as a bimodal distribution. A few other, like chromosomes and gametes, are best represented as categorical “bar” graphs with three or more options.

Because there is no sex trait that is limited to two options, biological sex by definition, is not binary.

With this understanding, let’s assess why the most commonly used methods of determining biological sex—including (1) genitals, (2) chromosomes, and (3) reproductive organs (gonads and gametes)—cannot be separated into a “female/male” binary.

1. Why Genitals Do Not Determine Sex

It is common for people to say things like, “Gender is determined by what is in your pants. If you have a penis you are a man. If you have a vagina, you are a woman.”

With regard to assigning sex to people by their external genitalia, it is an inaccurate system at best. There are several reasons for this, as described below.

1) External Genitalia Are Diverse

In newborn humans, genitals are extremely diverse in size and shape. Until about week 7 to week 8 of pregnancy, all fetuses have what’s known as a “genital ridge.”

This genital ridge is the tissue that eventually becomes the sex organs.

At the time of birth, a newborn’s genitals are usually labeled by a physician as male or female, even if the newborn presents with sex organs or characteristics that are intersex, ambiguous, or undefined. In a few places, such as Ontario (Canada), 19 U.S. states, and Washington, DC, “nonbinary” or “gender unspecified” options now exist, but this is not yet the norm.

All sex organs come from the same genital ridge, with the testes in men being equivalent to labia and ovaries in women and the penis being equivalent to the clitoris.

This is why the penis and vagina do not exist as a binary, but rather, as a spectrum that includes the following:

Full-size penis
Small penis
Micro-penis
Clitoromegaly, also called a “Pseudopenis”
Enlarged clitoris
Standard-sized clitoris

2) Intersex People Exist

Intersex means that a person was born with variations in their sex characteristics, such as the biological markers described above. These can include: internal genitals, external genitals, gonads, chromosomes, gene expression, hormone levels, receptor sensitivity, and brain structure.

Current research estimates that intersex people compose 1.7% of the population, which makes being intersex about as common as having red hair.

However, this metric is understated for the following reasons:

Most doctors, parents, or individuals don’t release this confidential medical information.
There are forms of sex variations that do not show up until later in life which go undocumented.
Definitions of what intersex is have not reached consensus.
There are at least 10 biologically relevant markers of sex (described below), and all but one (external genitals) are not routinely assessed.

The following examples explain this lack of consensus:

- How small does a penis have to be before it counts as intersex?
- Do you count sex chromosome variations if there’s no external sexual ambiguity?
- Do unusually high or low sex hormone levels (estrogen, progesterone, testosterone) make someone intersex?
- If so, how high or low must these hormones levels be and where is the “cut-off?” (The Olympic Committee has struggled mightily with this question.)
- How do you classify someone whose secondary sex characteristics (body hair, facial hair, or muscle mass, for example) don’t match their genitals?

As these questions illuminate, sex may present as a spectrum for people who have not been classified as intersex, as well as those who have.

2. Can We Use Chromosomes?

While chromosomes are another biological trait that some people try to use to explain the sex binary (male/female), chromosomes are also varied and diverse across the human species. On average, most people assigned male at birth have XY chromosomes, while most people assigned female at birth have XX chromosomes.

However, other sex chromosomal variations frequently exist as a result of the loss, damage, or addition of one or both of the sex chromosomes.

In humans, the following sex chromosome variations are naturally occurring:

45, X, also called Turner syndrome
45,X/46, also called XY mosaicism
46, XX/XY
47, XXX, also called Trisomy X
47, XXY, also called Klinefelter syndrome
47, XYY with normal phenotype
48, XXXX
48, XXXY
48, XXYY
49, XXXXY
49, XXXXX
XX Male Syndrome
XX Gonadal Dysgenesis
XY Gonadal Dysgenesis

Where Gonadal Dysgenesis is listed above, it refers to reproductive tissue (gonads) being replaced by non-reproductive fibrous tissue during prenatal development.

Furthermore, even a newborn born with the “common” sex chromosome set (XY or XX) can present with sex characteristics typical of the “opposite” sex.

For example, either the fetus or the mother’s adrenal glands can produce elevated levels of androgens. When this happens, an XX (“female”) child can be born with a phallus (small penis). In some cases, these newborns may will appear to have a scrotum, due to the labia fusing together.

Similarly, an XY (“male”) child can be born with an enzyme deficiency, such as 5-alpha deficiency and 17-beta dehydrogenase deficiency. When this happens, that “male” infant can be born without a penis and labelled “female” at birth.

Other biological conditions can create similar incongruities between chromosomes and genitals. This is why it’s unfortunate that federal and state-issued documents use external genitalia to make a sex assignments.

It is also why people are medically incorrect when they say, “Anyone with an Y chromosome is male, regardless of how many Y’s they have” (XY, XXYY, XXY, XYY, etc.).

The Olympic Committee Rejected Chromosomal Analysis

Interestingly, chromosome testing was introduced by the International Olympic Committee in 1968. At that time, the committee tested for a Y-chromosome in an attempt to classify competitors. Unsurprisingly, this method of testing was ended in 1999 because it was shown to be “inconclusive” at sex determination.

Specifically, the committee ran into problems because:

People can have XX chromosomes and a penis (called an “XX male”).
People can have XX chromosomes and a vagina, but possess male internal organs, commonly an undescended testes. (This was the debate around 800m star, Caster Semenya, for example.)
People can can have XX chromosomes and a vagina but “overproduce” testosterone, called hyperandrogenism. (In recent years, this has been common among mid-distance runners at the Olympic level.)

Given this, how should these individuals be “sexed”? As these examples illuminate, the presence of one (or more) “Y” chromosomes does not create a sex binary.

3. What About Gonads and Gametes (Reproductive Organs)?

Next we move onto to the reproductive organs, which have two primary components, the gonads (ovaries or testes) and the gametes they produce (eggs or sperm).

As mentioned above, men do tend to have testes and produce sperm, while women do tend to have ovaries and produce eggs. However, intersex people exist who have both male and female gonadal tissue, sometimes called an ovotestis.

Additionally, intersex people can present with ambiguous gonadal tissue ranging from a underdeveloped (“hypoplastic”) to abnormal (“dysplastic”) gonads to streak gonads. Streak gonads are named after their unclear morphological shape.

When testicular and ovarian cells are present at the same time it creates a gradation between female and male.

In many of these individuals, no gametes are produced. For example, no gametes are produced in 85% of individuals with streak gonads.[1] It is also common for people with ovaries and testes to not produce gametes.

As these example illustrate, people who have ambiguous gonads coupled with no gamete production defy our ability to use gametes to create a sex binary.

Because there is the third possibility of “no gametes”, gametes cannot create a sex binary. Perhaps we should suggest a sex trinary for this category?

However, even that understanding would be incomplete, because newborns born with the common gametes (eggs/sperm) can present with sex characteristics typical of the “opposite” sex.

Specifically, gametes (egg or sperm) can mix-and-match with other sex markers—commonly, chromosomes, genitals, or sex hormones—to create a constellation of sex traits.

To provide an example, people with Swyer syndrome have XY (“male”) chromosomes, but develop female reproductive organs, including a uterus and vagina. However, they lack ovaries. In place of ovaries, they develop streak gonads that are unable to produce gametes.[3] Therefore, these people have:

XY Chromosomes = “Male” Sex
External Genitals (Vagina) = “Female” Sex
Streak Gonads = “Undefined” Sex
No Gametes = “Undefined” Sex

Finally, testing a person’s gametes to determine their biological sex is not used in a single country worldwide, making it poor criteria for sex determination from a functional perspective.

To summarize, we can’t create a sex binary using the reproductive organs, because:

People can have both ovarian and testicular tissue (an ovotestis or gradation of cells)
People can have ambiguous gonadal tissue
It is common for all types of gonads (female/male/intersex) to lack gamete production

Because neither gonads nor gametes are binary, yet again, sex is best understood as a mosaic of traits, none of which none are binary.

In aggregate, there are at least 10 biologically relevant markers of sex, described below.

Biological markers of sex include:

Chromosomes – A person’s sex chromosome complement.
Gonads – Organs that produce gametes (testes or ovaries).
Hormones –Types and level of hormone secretion, which vary within and across the sexes.
Secondary Sex Characteristics – Features that appear during puberty, but are not involved with reproduction.
External Genitalia – Genitals visible outside the body.
Internal Genitalia – Genitals present within the body.
Gonads – Reproductive cells in humans.
Gene Expression –Levels and types of gene expression. Genes dictate the proteins made by the body. Known genes that impact sex include DMRT1, SRY (produces Testis-Determining Factor), and Foxl 2.
Brain Structure – Brain structure characteristics (including the ratio of white matter to grey matter) and brain activation patterns vary by sex.
Hormone Receptor Sensitivity – The response to sex hormones can vary, depending on receptor sensitivity. Some individuals may be partially or completely insensitive to hormones, negating their effect.

Of course, if you want to assess gender instead of sex, then you also have to add an 11th marker, which is personal identity. Personal identity is how a person self-identifies. This is often a result of many of the other factors interacting, making it a valuable scientific marker as well.

Gender and the Brain

The brain is another biological marker of sex that presents with great diversity, further supporting the concept of a sex (and gender) spectrum.

In a fascinating study published May 2018 by the European Society of Endocrinology researchers discovered, “Brain activity and structure in transgender adolescents more closely resembles the typical activation patterns of their desired gender.”

When MRI scans of 160 transgender youths were analyzed using a technique called diffusion tensor imaging, the brains of transgender boys’ resembled that of cisgender boys’, while the brains of transgender girls’ brains resembled the brains of cisgender girls’.

Put simply, transgender kids’ brains resemble their gender identity and not their biological sex.

Cisgender means that a person’s gender identity aligns with the sex assigned to them at birth, while transgender means a person’s gender identity does not align with the sex assigned to them at birth.

As stated by Julie Bakker, lead researcher from the University of Liege, “We now have evidence that sexual differentiation of the brain differs in young people with [gender dysphoria], as they show functional brain characteristics that are typical of their desired gender.”

This study explored the brains of cis girls, cis boys, trans girls and trans boys. The next step is for nonbinary identities to be included.

Sex and Gender Identification at Birth

As described above, external genitals are not an accurate marker of sex to use at birth, because they are one of at least 10 relevant markers of sex. External genitals are also highly variable, may be ambiguous, and can have both male and female sex organs present.

Furthermore, performing this gender assignment at birth based on external genitals does allow a physician to integrate information about the child’s internal genitals, gonads, chromosomes, gene expression, sex hormones, secondary sex characteristics, brain structure, hormone receptivity, and most importantly, how the child will grow up and express themselves within society.

While I am not opposed to the option to note sex on a child’s birth certificate, I am opposed to:

The requirement that parents select a sex for their child. Parents should be allowed not to indicate a sex for their child if this is their preference.
Binary options for sex, when it is clear that sex exists along a spectrum and intersex people exist. At the very least, there should be the option to choose Female (F), Male (M), or Other (X).
That sex being a permanent identification marker, when that child’s other biological markers of sex (described above) are yet unknown and may not be known until puberty or beyond.

For example, this father of three didn’t know until age 67 that he had a ovary, uterus, fallopian tubes, and cervix. As this proves, regardless of your age, genitals, and reproductive status you could be intersex and not yet know it.

Sex versus Gender

Although it has been useful to consider sex and gender separately for the sake of this analysis, it should be mentioned that they are not fully separate from each other. This is because a physical organ (the brain) directly impact gender identity, a phenomenon which is culturally experienced.

Across many cultures, both biological sex and gender are intertwined. Examples of this include:

Two-Spirited People for Native American/First Nations people
Hijra for South Asian people (also known as Kinnar or Kinner)
Māhū for Kanaka Maoli (Hawaiian) and Maohi (Tahitian) cultures
Fakaleiti for Tongans
Ffa’afafine for Samoans
And many others

Gender diversity has existed across all time periods and cultures, with evidence of it being documented as long as 4,500 – 9,000 years ago across Sumerian, Mediterranean, Greece, Phrygia, and Roman cultures. Hijras within Indian and kathoeys in Thailand have existed since ancient times, with the presence of trans males also described in texts that are thousands of years old.

Today, it is estimated that more than a half a million hijras live in India and another half million live in Bangladesh, legally recognized as a third gender. Given that gender is personally and societally expressed, every culture has gender diverse individuals within its population.

Biological sex diversity has also been present all time periods and cultures.

For example, intersex skeletons are now being identified, with the Revolutionary War hero Casimir Pulaski being one fascinating example.

Gender Diversity Across the Animal Kingdom

Finally, gender and sex diversity are widely present across the animal kingdom. For example, seahorses, pipefish, and sea dragons all have pregnancy as a male reproductive process. In these species, the male fertilizes eggs that are deposited within a pouch in his belly and then he carries his developing embryos until they are ready to be birthed.

In another example, female spotted hyenas have a pseudo-penis that is capable of erection and can be as much as 90% the size of a male hyena’s penis. They have two fleshy masses at the base of their pseudopenis which appear analogous to a scrotum. Where you’d expect there to be a vagina, spotted hyena females have fused labia. Female spotted hyenas also dominate males behaviorally.

Chickens can also naturally undergo gender changes. This is because female chickens only use one functional ovary on their left side. However, they have two sex organs that are present from their embryonic stage onward through their lifespan. If the left ovary shrinks within a hen, then its right gonad may start secreting androgens, turning the hen into a rooster.

In short, sex and gender exist as a spectrum for humans and animals (and in fact, plants too). We might as well embrace it, because after all, natural variation has caused the rise of our species to 8 billion strong!

Article Summary: Key Takeaways

Biological sex presents as a constellation of traits.
Biological markers of sex include: chromosomes, gonads, hormones, secondary sex characteristics, external genitals, internal genitals, gametes, gene expression, brain structure, and hormone receptor sensitivity.
None of the biological sex markers are binary, not even not even gametes.
Most of the biological sex markers are bimodal, meaning they present as a spectrum that has probability clusters.
A few of the sex markers are categorical with three or more options.
Gender incorporates personal identity, so it is unquestionably a spectrum.

Footnotes

1. Finlayson, C., Fritsch, M. K., Johnson, E. K., Rosoklija, I., Gosiengfiao, Y., Yerkes, E., Madonna, M. B., Woodruff, T. K., & Cheng, E. (2017). Presence of germ cells in disorders of sex development: Implications for fertility potential and preservation. Journal of Urology, 197 (3 Part 2), 937–943. https://doi.org/10.1016/j.juro.2016.08.108.

AUTHOR BIO: Cade Hildreth attended Dartmouth College & Smith College for Undergraduate Studies in Biology and then acquired a Master’s Degree in Biochemistry and Molecular Biology with Specialization in Biotechnology & Bioinformatics from Georgetown University, where they were Valedictorian. Cade is the Founder/President of BioInformant.com, the world’s largest stem cell industry news site that attracts nearly one million views per year and serves all-star clients that include GE Healthcare, Pfizer, and Goldman Sachs. Cade has authored over one-thousand articles about the stem cells, interviewed hundreds of executives from across the industry, and presented at stem cell conferences worldwide. As an expert on stem cells, Cade has been interviewed by the Wall Street Journal, Los Angeles Business Journal, Xconomy, and Vogue Magazine, as well as cited in Tony Robbin’s his best selling book, Life Force. Cade owns a portfolio of income-producing residential and commercial properties across the U.S.

*This article is part of a Biological Sex and Gender Diversity Series published by Cade Hildreth, B.A., M.S. You can read the other articles here:

Do you have questions about the gender spectrum or the sex spectrum? Ask them in the comments below.

You can view a Portuguese translation of this article here.

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