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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.

Some_Guy
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Tetragrammaton
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    Welcome to Bio.SE! What attempts to answer this question have you already taken? We ask that all question posters here attempt to search for an answer to their own question and explicitly indicate what research they've already done, what they learned, and what is still confusing or unknown to them. Our goal is not to simply be an answer site, but rather a site that promotes self-learning with some expert help along the way :). Please take a moment to edit your post with this additional detail, and it will likely be received more positively by our community. Thanks! – theforestecologist Dec 22 '19 at 03:40
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    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 – theforestecologist Dec 22 '19 at 03:41
  • @theforestecologist So, I edited the question and added from what I understood to be a spectrum. I hope, you can help me out there. – Tetragrammaton Dec 22 '19 at 03:46
  • As requested, the assumption in your edit is wrong :p. See the links in my previous comment. Hope this helps. – theforestecologist Dec 22 '19 at 03:46
  • @theforestecologist I did read those articles already. It didn't really help me though, since they also have not mentioned exactly what sex on a spectrum means. Could you help me out here and explain what exactly is wrong in my edit? This is from what I understood so far. – Tetragrammaton Dec 22 '19 at 03:52
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    Also, I am not quite sure why this question was downvoted. :/ It's a fair and crucial question and I hoped that someone could clarify it or at least give a glimpse into what current studies or science says. Just linking to wikipedia articles is not really that much of an helpful answer. – Tetragrammaton Dec 22 '19 at 03:55
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    biological sex is based on presence/absence of sex chromosomes and not on anatomical gonads as you mention. Within a species, there is no spectrum (though, there are variations based on chromosomal disorders/abnormalities). – theforestecologist Dec 22 '19 at 03:58
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    It was downvoted because it doesn't follow the guidelines for writing good questions on our website (specifically, see here). I've already described to you in my first comment what improvements you need to make to have the question receive upvotes and to receive the most helpful answer. your question is currently written in a hypothetical inquisitive manner, whereas we require prior demonstrated research to be explicitly stated in a question. Please take our [tour] to learn more. Thanks – theforestecologist Dec 22 '19 at 04:02
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    @theforestecologist But wouldn't these variations essentially be on a spectrum then? Why would it be a "chromosomal disorder" for example being a hermaphrodite and not just another sex on the spectrum? – Tetragrammaton Dec 22 '19 at 04:05
  • See Chromosome Abnormalities Fact Sheet for a review of chromosomal abnormalities. – theforestecologist Dec 22 '19 at 04:08
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    To get an idea about the spectrum as it relates to genital development check out the Quigley_scale and the related links on that page. Also note that while hermaphrodism (defined by having both male and female genitalia) does occur in some animals (e.g. slugs) it does not occur in humans. – tyersome Dec 22 '19 at 04:33
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    You're comparing apples to houses. Some things occur on a spectrum (e.g. height or intelligence) because it is multifactorial (genetics, hormone expression, nutrition/genetics, nutrition, environment, opportunity, etc., each factor contributing something) and some things do not (you are either a chromosomal male or a chromosomal female) excluding uncommon genetic abnormalities. @theforestecologist has given you the answer, and Wikipedia is actually a good resource to start at a basic level. – anongoodnurse Dec 22 '19 at 06:18
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    @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. – De Novo Dec 22 '19 at 06:39
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    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. – canadianer Dec 22 '19 at 09:03
  • @canadianer Just saying, that's not really a fair and reasonable critique since I specifically wrote: "biological gender". So, there has been no conflation between the term sex and gender. But thanks anyway for your input. – Tetragrammaton Dec 22 '19 at 09:31
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    This Q/A might be interesting: Whats the difference between male and female – fileunderwater Dec 22 '19 at 13:17
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    @DeNovosupportsGoFundMonica - The OP specified "biological sex". That is the correct answer to "biological sex." I think you read (as we are wont to do these days) too much into "sex". To quote the OP: "There are only two gonad types since this is the fundamental feature of what sex is." Without defining sex as gender but as "biological", I answered what I thought the OP was asking. N.B. Gender is not necessarily sex. It's all about definitions, wic the OP has not accurately specified. – anongoodnurse Dec 22 '19 at 16:17
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    I, too, commented assuming that the OP had conflated "sex" and "gender" and chose to respond as though the OP simply was referring to biological sex. If that is the case (i.e., if the OP did conflate the two), then (as @anongoodnurse suggests above), we mustn't complicate the biology with conflated social constructs, and we need to emphasize the strictly biological aspect of this question. Which is what I have tried to do. – theforestecologist Dec 22 '19 at 16:34
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    However, since the OP has suggested in their comment that they intentionally used "biological gender" (gender normally referring to "social and cultural differences rather than biological ones" (Definition 1 here), then I would suggest that either the OP needs to define what they specifically mean by "biological gender" or otherwise this question is off-topic b/c it is not about biology. – theforestecologist Dec 22 '19 at 16:35
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    @anongoodnurse as i was taught, karyotype is not sufficient for determining biological sex. I mean those terms precisely. Gender is a behavioral category, and not at all what this question or my comment were about. Biological sex is a (complex) question of physical phenotype that depends on a great deal more than the simple presence or absence of a Y chromosome on karyotype. – De Novo Dec 22 '19 at 17:42
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    What is "biological gender"? – Bryan Krause Dec 22 '19 at 19:19
  • @BryanKrause - Exactly. Definitions matter, and the OP provided none. I think, though, you can differentiate between a simple definition of biological sex (female, male, neuter, hermaphrodite, etc.) and phychological/emotional gender. – anongoodnurse Dec 22 '19 at 20:02
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    @DeNovosupportsGoFundMonica - Feel free to tell me your definition of "biological sex." I am not too old to learn. And since I have no dog in this race (not trying to push or support my particular views/biases), I'm open to learning more. – anongoodnurse Dec 22 '19 at 20:04
  • Your question uses the word I very much... sci pubs and research dont communicate journalistically about themselves. – bandybabboon Dec 22 '19 at 22:52
  • @anongoodnurse as I understand it, "biological sex" = reproductive phenotype, which is determined by the (variable) expression of a heterogeneous set of genes on multiple chromosomes, including autosomes. – De Novo Dec 22 '19 at 23:13
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    @anongoodnurse I'd say whether reproductive phenotype is binary or not depends more on philosophy, politics, and whether you're a lumper or splitter. – De Novo Dec 22 '19 at 23:18
  • Possible duplicate https://biology.stackexchange.com/questions/17327/does-any-animal-species-have-two-sexes-and-more-genders – John Dec 23 '19 at 05:57
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    related question https://biology.stackexchange.com/questions/52149/how-is-trangender-female-different-from-normal-female-biologically – John Dec 23 '19 at 15:53
  • Isn’t that rainbow logo confusing you? – David Dec 23 '19 at 19:13
  • Is the question only referring to the human species or the general case? This is a bit muddled, both in the Q and some comments (and the current answers). – fileunderwater Dec 25 '19 at 21:49
  • @BryanKrause Biological gender is sex (and I even wrote it above in brackets. It's absolutely clear what I meant with biological gender). Fairly obvious. – Tetragrammaton Dec 31 '19 at 00:14
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    @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. – theforestecologist Jan 01 '20 at 18:12

3 Answers3

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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.

Ben Bolker
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  • You might consider adding a bullet point for anisogamy, a concept that applies across a wide range of species – Ed Hagen Apr 14 '23 at 02:51
  • 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. – Ben Bolker Apr 14 '23 at 12:44
  • 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? – Ed Hagen Apr 15 '23 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

Ben Bolker
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John
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    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. https://en.wikipedia.org/wiki/Gender#History_of_the_concept https://books.google.com/ngrams/graph?content=gender+identity%2Cgender&year_start=1800&year_end=1990&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cgender%20identity%3B%2Cc0%3B.t1%3B%2Cgender%3B%2Cc0#t1%3B%2Cgender%20identity%3B%2Cc0%3B.t1%3B%2Cgender%3B%2Cc0 – bandybabboon Dec 23 '19 at 10:39
  • 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 – Tetragrammaton Dec 31 '19 at 00:00
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    @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. – John Dec 31 '19 at 04:00
  • @John Again, correct terminology was given. It's written in the question above. I wrote: "biological gender (or sex)". Thus, there's no need to ask a new question. But thanks for your input. – Tetragrammaton Dec 31 '19 at 15:29
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    @Tetragrammaton and As I said, biological sex can mean several different things and biological gender is meaningless. – John Dec 31 '19 at 17:56
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    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. – Galen Jul 20 '21 at 14:38
  • @LifeInTheTrees Thank you. At least someone here who pays attention. – Tetragrammaton Sep 15 '22 at 06:40
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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

bandybabboon
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    I'd avoid using the term menagerie to describe intersex individuals – De Novo Dec 22 '19 at 23:48
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    Mais pourquoi donc? – bandybabboon Dec 23 '19 at 04:11
  • Because it contains the implication that intersex people are non-human animals. I'm sure that's not what you intended, but it can read that way. – De Novo Dec 23 '19 at 04:32
  • this really has nothing to do with the question, as the linked article is discussing sex not gender. – John Dec 23 '19 at 05:56
  • @John the OP seems to be misusing the term gender. The edit clarifies the question to be about sex, not gender. – De Novo Dec 23 '19 at 06:00
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    Menage, is a strictly non-zoological term that french proudly use to refer to their household, I 've never heard of a ménage d'animaux, an animal household. Sorry I copied that straight from the wiki because was ona small phone screen. https://en.wikipedia.org/wiki/Sex_differences_in_humans – bandybabboon Dec 23 '19 at 06:13
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    @john https://en.wikipedia.org/wiki/Sex_differences_in_humans#Physiology besides, it's simple maths like 1+1 =2, if you can read a graph you can understand it john. bimodal distribution is not more complex than a bell curve, it has peaks. – bandybabboon Dec 23 '19 at 06:20
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    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. – John Dec 23 '19 at 06:28
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    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: https://books.google.com/ngrams/graph?content=gender&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cgender%3B%2Cc0#t1%3B%2Cgender%3B%2Cc0 and https://www.google.com/search?q=%22gender%22&tbm=bks&tbs=cdr:1,cd_min:1857,cd_max:1995&lr=lang_en – bandybabboon Dec 23 '19 at 10:26
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    @com.prehensible sounds like even more reason not to use the term. Also what do weaver ants have to do with the question? – John Dec 23 '19 at 15:45
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    @John, No, it's fine to use the word as long as you don't claim there is a precise narrow definition for the word based on your imagination. Your claim that gender is not sex is nonsense according to the wiki page gender, if you read the first phrase of that page it puts your old tutor's quip in the bin. – bandybabboon Dec 23 '19 at 19:03
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    @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. – John Dec 23 '19 at 23:47
  • @DeNovo You are incorrect. I did not misue the term gender and properly differentiated it from the term sex, as can be clearly seen above in my post, if anyone dares to read it. – Tetragrammaton Dec 31 '19 at 00:01
  • @com.prehensible Thank you by the way, for answering the question. So, we can conclude that sex is not binary and is in fact bimodal. That's not really on a spectrum, but it definitely means that sex is not that fixed as many falsly claim. – Tetragrammaton Dec 31 '19 at 00:19
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    @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. – Bryan Krause Dec 31 '19 at 18:32
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    Bryan Krause, you are contradicting fact: physical gender is bimodal ... https://en.wikipedia.org/wiki/Sex_differences_in_humans#Physiology There is a list of other bimodal distributions in nature, which includes harvester ants, which are also related to chromosomes. – bandybabboon Dec 31 '19 at 20:01
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    The answer is bad because you assert that sex is bimodal but provide no evidence to support it. – canadianer Jan 01 '20 at 02:31
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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.” Chromosomes, Gamete structure, Hip structure, they graph into two modes. – bandybabboon Jan 01 '20 at 08:06
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    This is improved. Are you willing to include the sources for your two figures? – Ben Bolker Jan 01 '20 at 16:57
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    was confused with the downvotes on this answer, until I looked at the edit history. It's much better now, good job (imo, although I'm by no means an expert) – Some_Guy Sep 09 '21 at 14:45