Is sex a spectrum?

Question

At the moment I'm especially interested in the question of whether biological sex is really a spectrum. Unfortunately I could not find any good answers on the Internet. Therefore I hope that some of you might be able to present the current state of science here and whether it is already generally accepted in biological science that sex is a spectrum.

What I am confused about here is for something to be a spectrum it must have some some sort of distribution along a parametrised scales. For example, it would be daft to think height to be binary even though there are tall people and short people. Tall and short are relative; height is defined by a continuous factor, length, measured (for example in cm) from foot to head. There is no such parameter that can be used to show that male and female are relative in the way that tall and short are. There is no scale. What I thought is that it relies on the binary to classify gonads as either male or female. So, people with disorders of sex development are still either male or female due to the presence of gonads (which define sex) if I understood correctly. There are only two gonad types since this is the fundamental feature of what sex is. I hope someone can correct me here, if this is wrong.

Answer 1

Short answer: it's messy, and probably no answer will satisfy everyone.

I'm only going to consider sex (I'm not going to mess with "gender") in humans.

• It's reasonable to consider sex as multidimensional rather than one-dimensional; see the various definitions listed below. The same individual could have different karyotypic, morphological, and endocrinological "sex".
• Some but not all of the definitions refer to continuous scales, i.e. if you pick one of the continuous measures below (Quigley scale, circulating testosterone level) you can probably find at least one individual in any not-too-narrow interval on that scale.
• It's reasonable to say that many of the continuous measures below are bimodal, i.e. in some random sample of humans there are more people close to 1 or to 6/7 than in the 2-5 range on the Quigley scale; similarly, there would be more people in the ranges of 10-30 ng/dl or 200-500 ng/dl in their testosterone than in between.

Here are some of the possible definitions of sex, drawing on the Wikipedia article mentioned in the comments:

• karyotype (chromosal type): this is discrete (XY, XX, XYY, XO, XXY, ...) but not necessarily easy to split into "male" vs "female" except on the basis of what the phenotype (morphology, endocrinology, etc.) looks like. Would "male" mean "has a Y chromosome" (XY, XYY, XXYY)? or "has only one X chromosome" (XY, XYY, X0)? Would female mean "has no Y chromosome" (X0, XX) or "has 2 X chromosomes" (XX, XXY, XXYY)? Non-standard sex karyotypes are rare: e.g X0 1 in 3000 live births, XXY [Klinefelter's syndrome] 1 in 1000 live births; XYY 1 in 1000 live births; XXYY 1 in 20,000 live births (Jarzembowski, J.A. “Sex Chromosome Abnormalities.” In Pathobiology of Human Disease, 185. Elsevier, 2014. https://doi.org/10.1016/B978-0-12-386456-7.01505-7 ; https://rarediseases.org/rare-diseases/xyy-syndrome/)

Below the level of the chromosome, there's lots of genetic and environmental variation that can change the phenotype (e.g. here)

• morphology (gonads or internal/external genitalia): the Quigley scale referenced in comments above is one way to describe this variation. The Quigley scale could be used to classify phenotype in cases of
  • androgen insensitivity syndrome, i.e. someone with at least one Y-chromosome who doesn't respond developmentally in the typical way to androgens such as testosterone
  • someone with an XX karyotype and congenital adrenal hyperplasia who has become masculinized

(the section on intersex conditions in the Wikipedia article on sexual differentiation links to these among others) (these are cartoons of external genitalia, in case that's not obvious)

Intermediate morphology can occur in the gonads as well as in the genitalia or internal organs; according to MedLine Plus (US National Library of Medicine), people with "true gonadal intersex" have "[both] ovarian and testicular tissue ... in the same gonad (an ovotestis), or the person might have 1 ovary and 1 testis".

• endocrinology (levels of various circulating hormones): researchers most typically consider testosterone (e.g. in this article). This article reports on total serum testosterone in a US health survey. The distribution is indeed bimodal (there's a peak around 10-30 ng/dL for women and 250-600 ng/dL for men), but this article only reports the 10th and 90 percentiles; there could certainly be someone with a circulating testosterone level right in the middle. It's also important to note that a person's phenotype/morphology (musculature etc. as well as gonadal morphology) is not necessarily closely related to their level of circulating testosterone, depending on whether they are androgen sensitive or insensitive ...

In light of recent (August 2024) Olympic boxing controversies, here's some more data on circulating testosterone distributions from Clark et al (2019)

top solid line for each column is the weighted 95% confidence interval for normal men and women, solid line for each study is 95% confidence interval, dotted line is absolute range. Blank spacer line below a study reference indicates an additional subset for that study

[PCOS = polycystic ovary syndrome, 5ARD2 = 5-alpha reductase deficiency, PAIS-CAIS = partial/complete androgen insensitivity syndrome]

So ... the answer to your specific question about being able to classify people based on gonad type is that although it's rare, people with true gonadal intersex as defined above could not be classified as either (exclusively) male or female on the basis of their gonads. Furthermore, it's not obvious which of the many criteria (karyotype, internal/external morphology, endocrinology) would be the "correct" way to define sex in a particular scenario.

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Clark, Richard V., Jeffrey A. Wald, Ronald S. Swerdloff, Christina Wang, Frederick C. W. Wu, Larry D. Bowers, and Alvin M. Matsumoto. “Large Divergence in Testosterone Concentrations between Men and Women: Frame of Reference for Elite Athletes in Sex-Specific Competition in Sports, a Narrative Review.” Clinical Endocrinology 90, no. 1 (January 2019): 15–22. https://doi.org/10.1111/cen.13840.

Answer 2

First gender and sex are not the same thing; as an old teacher of mine glibly put it, "sex is plumbing, gender is clothing," and even that is a gross generalization. The problem you are running into is using imprecise terms to ask a scientific question (sex, gender, and spectrum are all imprecise terms in biology). Precision in language is important in science to avoid confusion. The more vague your terms the less precise an answer can be.

More importantly gender isn't really a biological term; it is a sociological term, in particular it is a form of social construct (like countries, currency, or social class). The closest you get in biology is mating strategy, in which case there are species with multiple alternative mating strategies, including species with more mating strategies than sexes, some that are quite drastically different. Humans in particular have a very wide range of complex mating strategies, and of course some are more common than others.

Spectrum is also a tricky word in biology; would you call hair color a spectrum even though the distribution is non-uniform? How about handedness or ear lobe shape? They are both multimodal. Quite often in behavior there are evolutionary stable strategies that are only stable as multimodal distributions; are they spectra?

Even sex is tricky in biology. Since you have phenotypic sexes and chromosomal sexes, there are XY females in humans, as well as XX, XO, and XXX females. There are people with neither testes nor ovaries (both are gonads BTW), and humans with both testes and ovaries. So you end up with quite a variety of "sexes". Then you have things like chimerism in which an individual is really two individual cell lines fused together; even the term "individual human" can get tricky if you look close enough because biology is messy and doesn't respect our generalized human categories.

https://www.ncbi.nlm.nih.gov/books/NBK10943/

https://www.who.int/genomics/gender/en/index1.html

Answer 3

Scientists Fred P. Thieme and William J. Schull of the University of Michigan wrote about sexing a skeleton in 1957: “Sex, unlike most phenotypic features in which man varies, is not continuously variable but is expressed in a clear bimodal distribution.” The same is true for chromosomes, sex organs and testosterone.

"Variability in size or composition of gonads, genital morphology, chromosomes and/or hormonal physiology"

Specere in latin means to look, that's why it's used for the color spectrum. The rainbow is often used to represent gender-identity to represent the amorphous nature of the mind.

The distribution of fitness effects for genetic mutations of whole genomes and individual genes is frequently found to be a bimodal distribution with most mutations being either neutral or lethal with relatively few having intermediate effect.

Through the process of meiosis and fertilization (with rare exceptions), each individual is created with zero or one Y-chromosome. The complementary result for the X-chromosome follows, either a double or a single X. Therefore, direct sex differences are usually binary in expression, although the deviations in more complex biological processes produce a range of exceptions, resulting in a bimodal graph.

Gender does means biological sex in the context you use. https://en.wikipedia.org/wiki/Gender

Answer 4

Summary

The claim that sex is a spectrum is not an accurate conclusion supported by current research and understanding of sex in biology. It is perhaps based on misunderstandings of medical terms and anthropocentric ideas of sex. This answer also points out what I regard as some serious problems with the arguments put forward here by other answers.

Answer

There are a couple of answers here that provide "definitions of sex" and claims about sex being continuous or a bimodal distribution which are not supported at all.

The top answer does not provide any actual definitions of sex and seemingly misunderstands the "definitions" they believe describe sex. To start with, the author gives possible definitions from a wikipedia talk page on sex differentiation in humans, not sex. The claim that definitions of sex were provided here is incorrect. https://en.wikipedia.org/wiki/Sexual_differentiation_in_humans.With regard to sex his first definition, sex has never been defined by karyotype, as plenty of organisms have males and females with different sex chromosome combinations or no sex chromosomes at all. Furthermore, the Karyotypes listed are specific to males or females. His own sources, https://www.sciencedirect.com/science/article/abs/pii/B9780123864567015057?via%3Dihub and https://rarediseases.org/rare-diseases/xyy-syndrome/ not only do not define sex by sex chromosomes but name some of the sex chromosome abnormalities as being sex specific. The Quigly scale in no way or form defines sex, it's just a convenient scale so physicians can describe the appearance of external genitals. There's also the Prader scale. It's categorical with only a set of discrete categories, so I have no idea why it's being claimed to be continuous, let alone why it would define sex. His own citation, https://en.wikipedia.org/wiki/Quigley_scale says does not claim it defines sex and highlights it does not match his description at all. There is no literature that describes a person's sex as "3" on the Quigly scale for example.

Another issue is the answer also provides an "endocrinological" definition using a reference, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6391653/, that doesn't support it. Said reference does not define sex by hormone levels, in fact I think it bizarre to even claim sex is defined by hormone levels as they constantly change and also vary throughout life. I'd be very interested to hear about the testosterone levels of male ginkgos.

I have no idea why it is being argued that humans have some special definition of sex when sex is not a specifically human thing. Humans have male and female sexes for the same reason all other anisogamous species do. Literature on sex establishes that the sexes are two reproductive roles in anisogamy predicated on two differing types of gamete. This understanding is well demonstrated both in taxonomy and literature (Parker et al. 1972, Lehtonen et al. 2014, "The Biology of Reproduction" pg 112 by Fusco and Minelli, 2019.)

A second answer talks about sex having a bimodal distribution. It includes a graph (Fig.1) from Blackless (2000), but there are several problems with this. First, the graph in question is conceptual in nature and has no real data nor any clear methodology. In fact, looking at the axes, what's being measured doesn't even make sense. Given it says "Variability in size or composition of gonads, genital morphology, chromosomes and/or hormonal physiology", the authors don't even appear to be sure of what is being measured. I've never seen any graph with an "and/or" in it. Also keep in mind this isn't even a conceptual graph of sex but sexual dimorphism. The authors do not argue that sex has a bimodal distribution. Another issue is that it actually isn't even a bimodal distribution but just two normal distributions plotted on a single graph. A bimodal distribution does not have separate curves. Statistically, bimodal distributions require two well separated local maxima, which means the random variable must be quantitative. Sex chromosome combinations therefore cannot have a bimodal distribution, contrary to the assertions in that answer. The distribution the posted of hormones is clearly not a measurement of sex, as it readily recognizes that that there are two separate distributions.

I think a lot of confusion is misunderstandings of medical diagnostic terminology(Money, John; Hampson, Joan G; Hampson, John (October 1955). "An Examination of Some Basic Sexual Concepts: The Evidence of Human Hermaphroditism". Bull. Johns Hopkins Hosp. 97 (4). Johns Hopkins University: 301–19) for describing patients with atypical sex development, etc. You have terms such as "chromosomal sex", etc that are not and never were definitions of sex. There are no such things as "chromosomal sexes" and "phenotypic sexes". Strictly speaking, spectra are measurements along a continuous variable. A continuous variable has infinite possible values, thus if sex was a "spectrum", then each of the infinite points are a different sex. Biology literature does not describe infinite sexes, let alone more than two in anisogamy. Therefore the claim that sex is a spectrum is not supported by research. It is important to distinguish sex from characteristics associated with sex in humans and that while we can describe how those characteristics individually vary that is not the same as sex being a spectrum.