Is Sex Bimodal?

Description

When discussing biological sex, there is disagreement over whether sex is a spectrum (with a range of possibilities) or whether it is binary (with only two possibilities). Seeing the potential extremes of either side, many decide to strike a compromise by saying that sex is bimodal.

Does it work?

Sources

Contributors:

[1] This video was adapted from a Twitter thread by evolutionary biologist Colin Wright (@SwipeWright).

Sources:

[1] Wright, C. (2020). Is Sex Bimodal? Twitter. (https://threadreaderapp.com/thread/1233299010456174593.html)

[2] Sax, L. (2002). How common is lntersex? A response to Anne Fausto‐Sterling. Journal of Sex Research.

[3] Marinov, G.K. (2020). In Humans, Sex is Binary and Immutable. Acad. Quest. 33, 279–288.

[4] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

[5] Cox, P., Togashi, T. (2011). The Evolution of Anisogamy, A Fundamental Phenomenon Underlying Sexual Selection. New York Cambridge University Press. 17.

[6] Loof, A. (2018). Only two sex forms but multiple gender variants. Communicative & Integrative Biology, 11(1).

[7] Witchel, S. (2017). Disorders of sex development. Best Practice and Research in Clinical Obstetrics and Gynaecology, 48, 90-102.

[8] Schmitt, D. (2016). Sex and gender are dials (not switches). Psychology Today.

[9] Hyde, J. (2014). Gender similarities and differences. Annual Review of Psychology, 65, 373-398.

Transcript

When discussing biological sex, there is disagreement over whether sex is a spectrum (with a range of possibilities) or whether it is binary (with only two possibilities). Seeing the potential extremes of either side, many decide to strike a compromise by saying that sex is bimodal (a distribution of traits with an average for typical males and a distribution of traits with an average for typical females). Here, intersex conditions are used to argue that a bimodal distribution is the most accurate representation of sex. Since intersex people are often viewed as having a mix of 'male' and 'female' traits, they are therefore placed in between the two averages of male and female.

While it is good to recognize the full diversity of human body types, is the 'sex is bimodal' position the correct approach? To answer this question, we first need to ask: how does such a placement work? In other words, what variables do we use to place people on a bimodal graph for biological sex?

Like any bimodal distribution, there needs to be a quantitative x-axis, comprised of numbers, which defines the range. For example, sex differences in height are bimodal, with an average for males and an average for females. The x-axis is quantitative, with each mark representing a value for height. We can see that there is considerable overlap in height between men and women. But, does this overlap create new sex categories? Or does it simply show where men and women share a common set of values of a given trait? For instance, there are plenty of men who are the same height as many women, and therefore, share the same height value. And yet, this overlap does not form new sexes, since the x-axis is not 'sex' but 'height.'

And this brings us to the major problem with defining sex itself as a bimodal distribution--with the x-axis being defined, not by a trait like height, but by sex. As evolutionary biologist Colin Wright says, "The implication of the 'sex is bimodal' position is that some males are more male than others, and some females are more female than others. For instance, is male A 'more male' than male B? Is female D 'more female' than female C?"

And, what exactly is on the x-axis? As Wright adds, "Is a male with a thicker beard, deeper voice, and larger penis more male than males with light beards, higher voices, and smaller penises?" Or, is a female with a higher voice and larger breasts more female than females with lower voices and smaller breasts? Such a categorization would be both offensive and scientifically wrong. A breast cancer patient, for instance, does not become 'more male' or 'less female' if she removes her breasts. On the other hand, a testicular cancer patient does not become 'more female' or 'less male' if he removes his testes.

Thus, considering that sex category cannot be defined through a quantitative variable, like height, the bimodal nature of sex is a misnomer. The 'sex is bimodal' argument conflates sex-related traits, such as facial hair, voice pitch, height, and muscle mass, with sex category itself. These traits, such as voice pitch, are highly bimodal, with an average for males and an average for females. And yet, this variation does not mean someone falls out of their sex category for having traits atypical of their sex. A bimodal distribution covers every outlier, as each outlier can be found on the tail ends of the graph. For instance, there are plenty of really tall women, and they are still women; there are plenty of really short men, and they are still men. In this way, sex *differences* are bimodal, but sex itself is not.

Sex category is binary, defined by the two *and only two* gamete types that bodies can be structured for. Within the two categories, there is a spectrum of body types for males and a spectrum of body types for females, and this spectrum includes intersex individuals. A graph on the percentage of infants born with differences in sex development shows us that 99.8% of births are unaffected males or females, with typical chromosomal arrangements and typical body structures. Of the 0.2% of births with intersex conditions, most of these infants are also unambiguously male or female.

With such a stark contrast between the bimodal variety of sex *differences* (seen here) and the binary nature of sex *category* (seen here), it is clear that sex is not bimodal, but is rather a binary system that is consistent across our species, with plenty of variation within the two categories. Thus, it is the variation of traits within sex category that is bimodal, not sex category itself.

So, do not be fooled by arguments that sex category is bimodal. While traits like height, weight, voice pitch, muscle mass, and even hormonal differences are highly varied, and are therefore bimodal, this variation does not create new sex categories, and it does not make someone more male and less female, or more female and less male. Perhaps we should stop trying to create new categories for people's bodies, and instead, allow for the spectrum of body types within all males and all females.

I'm Zach, author of the Gender Paradox, for the Paradox Institute.

END

© 2020 Zachary A. Elliott, All Rights Reserved.