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

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 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.

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

On average, males tend to have more facial and body hair than females (a secondary sex characteristic), but there are also females with coarse and dense body hair and males who can’t grow a full beard.

On average, males tend to be taller than females, but there are most certainly females that are taller than some males. If skeletal structure (a biological marker of sex) was binary, then all males would have to be taller than all females, which of course, they are not.

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.

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.

Why Genitals Do Not Determine Sex

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:

1. Most doctors, parents, or individuals don’t release this confidential medical information.
2. There are subtle forms of sex variations that do not show up until later in life which go undocumented.
3. Definitions of what intersex is have not reached consensus.
4. 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.

What about 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 “binary” sex chromosomes (XY or XX) can present with intersex characteristics.

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.

The 10 Biological Markers of Sex

Often, you will hear people 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.”

As we’ve already addressed, gender incorporates personal identity, so it is unquestionably a spectrum. Biological sex is also complex, and across all species, exists as a spectrum. With regard to this specific statement, external genitals are highly variable, may be ambiguous, and both male and female sex organs can be present.

More importantly, this statement is incomplete from a biological perspective.

While sex has traditionally been assigned based on external genitalia, this approach neglects that there are at least 10 biologically relevant markers of sex (and likely more).

Biological markers of sex include:

1. Chromosomes – Types of chromosomal expression.
2. Gonads – Organs that produce gametes (testes or ovaries).
3. Hormones –Types and level of hormone secretion, which vary within and across the sexes.
4. Secondary Sex Characteristics – Features that appear during puberty, but are not involved with reproduction.
5. External Genitalia – Genitals visible outside the body.
6. Internal Genitalia – Genitals present within the body.
7. Gametes – Reproductive cells in humans (eggs, sperm, none, or potentially both).
8. 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.
9. Brain Structure – Brain structure characteristics (including the ratio of white matter to grey matter) and brain activation patterns vary by sex.
10. 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.

Given that there are 10 biologically relevant markers of human sex, using external genitalia is at best, a partial assessment. In some cases, it is wholly incorrect.

For example, it would be easy to identify a person with a vagina as female, but this person could also present with “male” gene expression or androgen insensitivity or an intersex brain structure. The same is true for someone who presents with a penis.

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 both sex and gender 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 more sexes and genders to become integrated within this neuroscience research.

The Role of the Mind versus the Brain

Of course, the brain and the mind are also two different things.

The brain is the physical structure in your head that is composed of grey and white matter. It has neurons firing within it and uses neurotransmitters as chemicals messengers.

The brain can be thought of as your central processor, because it integrates and facilitates all of the functions within your body.

As noted by Julie Bakker (lead researcher in the MRI study above) and others, brain structure and activation patterns present along a spectrum.

The mind, on the other hand, is the conscious product of that biological activity that creates emotions, ideas, memories, and creative thought. It determines your personality and impacts how you interact with the world.

The mind plays a central role in your gender identity.

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 biologically 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, skeletal structure, 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:

1. 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.
2. 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).
3. That sex being a permanent identification marker, when that child’s other biological markers of sex (described above) are yet unknown.

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 male individuals also described within 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 represented within its population.

Gender Diversity Across the Animal Kingdom

Finally, gender diversity is 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 that contain fat and connective tissue 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 7.8 billion strong!

Do you have questions about the gender spectrum? Ask them in the comments below. View the Portuguese translation here.

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*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:

• • Sex Isn’t Binary: Olympic Committee Confirms Sex To Be a Spectrum

  • Biology Rejects a Sex Binary: Gametes (Egg/Sperm) Aren’t Binary, After All

  • 4 Ways that Biologists Define Biological Sex

  • Does the Y Chromosome Create a Sex Binary? Science Says No.

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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 Cade was 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 a media expert on stem cells, Cade has been interviewed by the Wall Street Journal, Los Angeles Business Journal, Xconomy, and Vogue Magazine, as well as quoted in Tony Robbin’s his best selling book, Life Force. 

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