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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.

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    $\begingroup$ Sex-determination system and Sexual differentiation in humans might be informative for you and are as good a place to start as any. Read up a bit more on the topic, and then please come back to edit your post about what you've learned and by adding a specific biological question about something you still don't understand. Thanks $\endgroup$
    – theforestecologist ♦
    Dec 22, 2019 at 3:41
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    $\begingroup$ @anongoodnurse I don't believe the OP asked about chromosomal sex. Maybe different medical schools teach this differently, but I was taught that the karyotype is not adequate for defining biological sex. $\endgroup$
    – De Novo
    Dec 22, 2019 at 6:39
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    $\begingroup$ For what it’s worth, I don’t think your question deserves as much grief as it has gotten. The only critique I’d make is that you should not conflate sex with gender. $\endgroup$
    – canadianer
    Dec 22, 2019 at 9:03
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    $\begingroup$ What is "biological gender"? $\endgroup$
    – Bryan Krause is on strike ♦
    Dec 22, 2019 at 19:19
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    $\begingroup$ @Tetragrammaton it is absolutely not clear because there is no such thing as "biological gender". As has been made clear multiple times in responses you've received, "gender" (regardless of adjectives you associate with it) is NOT a biological concept. As such, "biological gender" is meaningless and demonstrates a lack of understanding of terminology. Put differently, You cannot equate "sex" and "biological gender" because the latter phrase doesn't make sense. I highly suggest you edit your post to remove all mention of "gender" since it is at-best unclear and at worst off-topic. $\endgroup$
    – theforestecologist ♦
    Jan 1, 2020 at 18:12

3 Answers 3

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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

(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.

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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  • $\begingroup$ You might consider adding a bullet point for anisogamy, a concept that applies across a wide range of species $\endgroup$
    – Ed Hagen
    Apr 14 at 2:51
  • $\begingroup$ But humans are completely anisogamous as far as I know - I'm not aware of any circumstances under which humans produce gametes that are anything other than eggs or sperm. Seems tangential to me. $\endgroup$
    – Ben Bolker
    Apr 14 at 12:44
  • $\begingroup$ Since anisogamy is fundamental to the biological definition of "male" and "female" (e.g., Parker et al. 1972, The origin and evolution of gamete dimorphism and the male-female phenomenon), and since the OP's question was "Is sex a spectrum?", doesn't the fact that humans are completely anisogamous provide one important answer? $\endgroup$
    – Ed Hagen
    Apr 15 at 14:38
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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

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    $\begingroup$ Gender is a corruption of the term "gender-identity", which is also "sex-identity", genre in french means "type" and gendre means spouse, so it's a contradiction if you know the etymology. en.wikipedia.org/wiki/Gender#History_of_the_concept books.google.com/ngrams/… $\endgroup$
    – bandybabboon
    Dec 23, 2019 at 10:39
  • $\begingroup$ Just a small reminder: Again (and I don't like repeating myself in that matter for the third time) I never said that gender and sex are the same thing, nor did I ever claim so. @John $\endgroup$
    – Tetragrammaton
    Dec 31, 2019 at 0:00
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    $\begingroup$ @Tetragrammaton biological gender isn't a thing, even "biological sex" is not a single thing. I have have answered the question given, if this is not what you wanted to know then do some research and ask a new question with correct terminology. $\endgroup$
    – John
    Dec 31, 2019 at 4:00
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    $\begingroup$ @Tetragrammaton and As I said, biological sex can mean several different things and biological gender is meaningless. $\endgroup$
    – John
    Dec 31, 2019 at 17:56
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    $\begingroup$ I don't think a random variable has to have a continuous or discrete uniform distribution in order to be a spectrum. Not that the uniformity isn't important, and could be quantified with information entropy. $\endgroup$
    – Galen
    Jul 20, 2021 at 14:38
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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.

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

enter image description here

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. enter image description here

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

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    $\begingroup$ I know what a bimodal distribution is and human gender is not bimodal, even if sex largely is and confusing them makes your answer straight up wrong. Its bad enough the question is using poor terms there is no excuse for an answer to. $\endgroup$
    – John
    Dec 23, 2019 at 6:28
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    $\begingroup$ Gender is not really an established word, John, it has almost no precedence in scientific literature other than the sense that I use, It's etymology doesn't make sense for science either. Since 1990 there has been a lot of identity hysteria which has seen a new sense attributed to gender, the one that you are militant about: books.google.com/ngrams/… and google.com/… $\endgroup$
    – bandybabboon
    Dec 23, 2019 at 10:26
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    $\begingroup$ @com.prehensible sounds like even more reason not to use the term. Also what do weaver ants have to do with the question? $\endgroup$
    – John
    Dec 23, 2019 at 15:45
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    $\begingroup$ @com.prehensible have you read the wiki? because the first sentence tells you it is a social construct, and the second paragraph goes on to tell you why sex and gender are not interchangeable terminology. $\endgroup$
    – John
    Dec 23, 2019 at 23:47
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    $\begingroup$ @Tetragrammaton It's usually not good to accept answers with so many downvotes. This answer is bad: it has a graph of a bimodal distribution that is just an illustration and has nothing to do with sex. The rest of the answer talks about sexual orientation and not sex. It then talks about morphology of ants which are not related to sex at all since all the worker ants are sterile females. $\endgroup$
    – Bryan Krause is on strike ♦
    Dec 31, 2019 at 18:32

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