Intersex Is Not as Common as Red Hair

In discussions about intersex conditions it is common to hear the claim that intersex people make up 1-2 percent of the population and is therefore “as common as red hair.” There appear to be two main goals when forwarding this claim—one laudable, the other insidious. The laudable goal is to normalize the existence of intersex people and thereby help facilitate the societal acceptance of a marginalized community who may experience social ostracism and who have often been victims of unconsentual and medically unnecessary “corrective” cosmetic surgeries. The insidious goal is to plant seeds of doubt in our collective understanding of biological sex and suggest that the categories “male” and “female” may be social constructs or exist on a “spectrum.”

This “intersex is as common as red hair” claim is frequently voiced by leading human rights and LGBTQ+ organizations, Left-wing activists, popular online magazines, and even scientists.

“According to experts, around 1.7% of the population is born with intersex traits - comparable to the number of people born with red hair.” — Amnesty International

“Worldwide, up to 1.7% of people have intersex traits, roughly the same proportion of the population who have red hair.” — The Guardian

“An estimated 98-99 percent of humans are either blond or brunette. Despite this, we have accepted—rather than denying, hiding, or attempting to eliminate—the existence of redheads, who, interestingly, make up the same percentage of the population as intersex people (1-2 percent).” — Hida Viloria and Dr. Maria Nieto in The Spectrum of Sex: The Science of Male, Female, and Intersex (page 24)

Where does this 1.7% figure originate?

This statistic was first forwarded by Anne Fausto-Sterling, professor of biology and gender studies at Brown University, in her book “Sexing The Body: Gender Politics And The Construction Of Sexuality”, published in February of 2000, and shortly after in a review paper published a month later in the American Journal of Human Biology titled “How Sexually Dimorphic Are We?” In this review, Fausto-Sterling and her coauthors set out to refute the purported claim that sexual anatomy in humans is “absolutely dimorphic”, i.e. that all humans are either unambiguously male or female in all sex-related traits. They broadly define an intersex person as “an individual who deviates from the Platonic ideal of physical dimorphism at the chromosomal, genital, gonadal, or hormonal levels.”

To arrive at their 1.7% figure, they asked how frequently humans deviate from this Platonic ideal. In the review, their “ideal male” is defined as someone with XY chromosomes, functional testes located in the scrotal sac, a penis between 2.5 and 4.5 cm at birth, and a completely enclosed urethra that opens at the tip. The ideal male must also have testes that produce Mullerian inhibiting factor as well as testosterone and dihydrotestosterone, and juvenile testicular activity must result in a typical masculinizing puberty. Their “ideal female” has two X chromosomes, functional ovaries that result in normal feminizing puberty, intact oviducts attached to a functional uterus, cervix, and vaginal canal. This ideal female must also have labia minora and majora present, and a clitoris that ranges between 0.20 and 0.85 cm in length at birth.

According to these strict criteria, approximately 1.7% of individuals can be defined as intersex, though the authors suggest this figure could even be as high as 2.27% if the criteria are made even more exacting.

Is this an accurate portrayal of intersex?

Defining an individual as intersex for deviating from their sex’s “Platonic ideal” for any trait stands out as extreme. In a 2002 paper titled “How Common is Intersex? A Response to Anne Fausto-Sterling”, physician and psychologist Leonard Sax pointed out a flaw in Fausto-Sterling’s 1.7% statistic; namely, that it included many conditions that cannot be considered intersex in any clinically relevant sense. Indeed, the conditions making up the large majority of Fausto-Sterling’s 1.7% figure do not result in any sexual ambiguity whatsoever. In Sax’s words:

Many reviewers are not aware that this [1.7%] figure includes conditions which most clinicians do not recognize as intersex, such as Klinefelter syndrome, Turner syndrome, and late-onset adrenal hyperplasia. If the term intersex is to retain any meaning, the term should be restricted to those conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female. [emphasis added]

Fausto-Sterling’s central error was to equate any “differences of sexual development” (DSDs) with “intersex.” But while all intersex conditions may be considered DSDs, not all DSDs are necessarily intersex conditions.

The following figure using Fausto-Sterling’s data (created by Twitter user @zeno001) helps visualize this.

Here we can see that the large majority (88%) of Fausto-Sterling’s 1.7% figure is taken up by one condition: late-onset adrenal hyperplasia (LOCAH). These individuals have completely normal male or female genitalia at birth that align with their sex chromosomes. The sex of these individuals is not ambiguous, so to label LOCAH as an intersex condition is a far cry from what most people and clinicians conceptually envision the term to capture.

The next most prevalent DSD on Fausto-Sterling’s list iclude any chromosomal deviations from classical XX and XY (e.g. Klinefelter syndrome, Turner syndrome, etc.). However, these conditions do not result in ambiguous genitalia and therefore cannot be considered intersex in any clinically relevant sense. (see my previous post about why sex chromosome variants are not their own unique sexes.)

Lastly, vaginal agenesis, the next most common DSD on the list, is not generally considered an intersex condition, as girls with this condition are genotypically XX, possess perfectly normal ovaries, and can even become pregnant and birth their own children following vaginoplasty. They are unambiguously female.

When these common DSDs are removed, and intersex conditions are more precisely defined as “conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female,” Fausto-Sterling’s 1.7% figure drops dramatically. According to Sax, “Applying this more precise definition, the true prevalence of intersex is seen to be about 0.018%, almost 100 times lower than Fausto-Sterling's estimate of 1.7%.”

While the prevalence of intersex conditions, defined in Sax’s clinically-relevant sense, is quite low, this by no means justifies any of the mistreatment, whether socially or medically, that many gender activists hope to prevent when they overstate its prevalence. How we treat people, and the rights afforded to them, should not be predicated on their prevalence in a population. And that is the point we should be trying to normalize, rather than false statistics.

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