Sex

text box that says Sex refers to biological characterisitics
Sex refers to biology; however, there is no single, universally agreed-upon set of guidelines for defining sex. Sex is complex, dynamic, context-dependent, and interacts with gender and other social factors (Ritz & Greaves, 2022).

In humans (and non-human mammals), sex traditionally refers to the biological attributes that distinguish male, female, and/or intersex according to functions that derive from the chromosomal complement (generally XX/XY), reproductive organs (internal and external genitalia), or specific hormonal, environmental, or cultural factors that affect the expression of phenotypic traits (Barresi & Gilbert, 2023; Jones 2018; Kessler, 1998). These attributes may or may not be aligned in any individual (see figure below; see also Fausto-Sterling, 2012; Ainsworth, 2015). For excellent recent articles, see the special sections on sex and gender in Cell (2024) and Nature (2024); also (DuBois & Shattuck-Heidorn, 2021).

Graphic by Amanda Montañez, Beyond XX and XY: The Extraordinary Complexity of Sex Determination.
Scientific American, 317 (2017). With permission.

In non-mammals, sex refers to biological attributes that distinguish male, female, and/or hermaphrodite. Sex differentiation may derive from chromosomal make-up (female/male), such as XX/XY, ZW/ZZ (birds and some insects), XX/XO (insects), or from social or environmental conditions. Sex, for a number of fish, mollusk, and other species, may derive from social interactions with other members of a population. In the slipper limpet Crepidula fornicata, for example, all young individuals are male but some later change to female, depending on their position in a snail mound. Sex may also derive form environmental conditions. In the echiuran worm Bonellia viridis, for example, larvae that land on the ocean floor develop as females (~10 cm. long), whereas larvae that are engulfed by a mature female through her proboscis develop as males (~2 mm. long) and live symbiotically. In other organisms, such as crocodiles, most turtles, and some other reptiles, sex depends on the temperature at which their eggs incubate (see Marine Science).

While we use the term “intersex” for humans, we use the term “hermaphrodite” in non-human individuals that produce both male and female gametes during its lifetime. Hermaphroditism is very common in nature, occurring in approximately 30% of animal species (excluding insects) and most plants (Jarne & Auld, 2006). Hermaphrodites are classified as either “simultaneous” (individuals functioning as both male and female at the same time) or “sequential” (individuals first functioning as one sex and then changing to the other (Hermaphroditic Species). The factors determining the timing, direction and frequency of sex change are diverse across nature, and dependent on the species and an individual’s social-ecological context (Munday et al., 2006).

In engineering & product design research, sex includes height, weight, and biomechanical characteristics that may impact the design of products, systems, and processes (see Intersectional Design). Many devices and machines have been designed to fit mid-size, male bodies. For example, military and commercial cockpits were traditionally based on male anthropometry, which made it difficult or even dangerous for some women (or small men) to be pilots (Weber, 1997). Crash test dummies were also historically based on mid-sized male bodies; only recently was the first 50th percent female dummy developed that model the biomechanics of the female body (see Case Study: Inclusive Crash Test Dummies; Linder & Svedberg, 2019). It is also important to understand differences within groups of women, men, and gender-diverse people. Many period tracking apps, for example, fail for users with irregular cycles (Tiffany, 2018).

Data collection. The U.S. National Academies of Sciences, Engineering, and Medicine recommend methods for data collection for the term “sex” (National Academies, 2022).

Take care not to overemphasize sex differences. Sex is not neatly binary; there is considerable within-group variation and between-group overlap (see figure below). Take, for example, height. In the US, women are shorter than men on average, but about 3% of women are taller than the average man, and 6% of men are shorter than the average woman. The height difference between the average woman and man is less than the height difference between a 90th percentile and 10th percentile woman, or the difference between a 90th and 10th percentile man.

graph of height of adult Women and Men overlapping

Works Cited

Ainsworth, Clair. (2015). Sex Redefined. Nature 518 (7539), 288-291.

Barresi, M. & Gilbert, S. (2023). Developmental Biology, 13th Edition. New York: Oxford University Press.

Cell, Editorial Team. (2024). Special Section on Sex and Gender, 187, 1513-1357.

DuBois, L. Z., & Shattuck‐Heidorn, H. (2021). Challenging the binary: Gender/sex and the bio‐logics of normalcy. American Journal of Human Biology, 33(5), e23623.

Fausto-Sterling, A. (2000). Sexing the Body: Gender Politics and the Construction of Sexuality. New York: Basic Books.

Fausto-Sterling, A. (2012). Sex/Gender: Biology in a Social World. New York: Routledge.

Jarne, P. & Auld, J. R. (2006). Animals mix it up too: the distribution of self-fertilization among hermaphroditic animals. Evolution 60, 1816-1824.

Jones, T. (2018). Intersex studies: A systematic review of international health literature. Sage Open 8(2), 2158244017745577.

Kessler, S. (1998). Lessons from the Intersexed. News Brunswick: Rutgers University Press.

Linder, A., & Svedberg, W. (2019). Review of average sized male and female occupant models in European regulatory assessment tests and European laws: gaps and bridging suggestions. Accident Analysis & Prevention, 127, 156-162.

Montañez, Amanda. (2017). Beyond XX and XY: The extraordinary complexity of sex determination. Scientific American, 317.

Munday, P.L., Buston, P.M. and Warner, R.R., 2006. Diversity and flexibility of sex-change strategies in animals. Trends in Ecology & Evolution, 21(2), pp.89-95.

National Academies of Sciences, Engineering, and Medicine. (2022). Measuring Sex, Gender Identity, and Sexual Orientation. Washington, DC: The National Academies Press.

Nature, Editor. (2024). Special Section on Sex and Gender, 629, 7-8.

Ritz, S. A., & Greaves, L. (2022). Transcending the Male–Female Binary in Biomedical Research: Constellations, Heterogeneity, and Mechanism When Considering Sex and Gender. International Journal of Environmental Research and Public Health, 19(7), 4083.

Tiffany, K. Period-tracking apps are not for women. Vox, November 16, 2018.

Weber, R. N. (1997). Manufacturing Gender in Commercial and Military Cockpit Design. Science, Technology, & Human Values 22 (2): 235-253.

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