Sex

Sex refers to biology. In humans, sex refers to the biological attributes that distinguish male, female, and/or intersex. In non-human animals, sex refers to biological attributes that distinguish male, female, and/or hermaphrodite. In engineering & product design research, sex includes anatomical and physiological characteristics that may impact the design of products, systems, and processes.

Sex is complex, dynamic, context-dependent, and interacts with gender and other social factors (Ritz & Greaves, 2022).

Defining Sex for Biomedical Research: Humans and Lab Animals
Sex refers to the biological attributes that distinguish male, female, and/or intersex according to functions that derive from the chromosomal complement, reproductive organs, or specific hormones or environmental factors that affect the expression of phenotypic traits in sexually reproducing organisms. These attributes may or may not be aligned in any individual (Ainsworth, 2018; Fausto-Sterling, 2012). Sex may be defined according to:

  • 1. Genetic sex determination: chromosomal make-up, generally XX/XY for most mammals. The presence of sex-determining genes means that every nucleated human cell has a sex.
  • 2. Gametes: germ cells. In species that produce two morphologically distinct types of gametes, the egg-sperm distinction is the basis for distinguishing between females and males.
  • 3. Morphology: physical traits that differentiate female and male phenotypes.
    • a. Primary sex characteristics in humans and other mammals include:
      • i. internal reproductive organs and genitalia, which derive from bipotential organs (e.g., indifferent gonads that become ovaries or testes) and dual structures. Usually, one structure is maintained and the other regressed.
      • ii. external genitalia, which generally differentiate towards one of two basic forms: distal vagina, labia and clitoris in females and scrotum and penis in males. Nevertheless, external genitalia may not reflect karyotypical or internal genital sex (Fausto-Sterling, 2000).
      b. Secondary sex characteristics in humans and many other animals are phenotypic traits strongly associated with females or males that become prominent at puberty under the influence of endogenous estrogens in females and androgens in males. Examples of secondary sex characteristics in humans include shorter stature and wider pelvis, breast development, and more fat in the thighs and buttocks in females and broader shoulders, greater muscle mass, more facial and other body hair, and “male pattern” baldness in males. These traits vary within each sex and ranges overlap. For instance, many women are taller than many men and some women are stronger than many men.
    Intersex conditions may be defined as variations or combinations of what are considered XY male-typical and XX female-typical chromosomal, gonadal and genital characteristics. In some cases, intersex individuals (ranging from 1:100 to 1:4,500 depending on the criteria used) have genitalia or other traits not easily categorized as male or female (Jones, 2018; Arboleda et al., 2014; Karkazis, 2008; Kessler, 1998).

    Defining Sex for Biomedical Research: Humans and Lab Animals
    • Sex refers to biological attributes that distinguish male, female, and/or intersex according to functions that derive from the chromosomal complement, reproductive organs, or specific hormones or environmental factors that affect the expression of phenotypic traits in sexually reproducing organisms. These attributes may or may not be aligned in any individual (Ainsworth, 2018; Fausto-Sterling, 2012). Sex may be defined according to:
      • 1. Genetic sex determination: chromosomal make-up (female/male), such as XX/XY, ZW/ZZ (birds and some insects), and XX/XO (insects). Regardless of karyotype, the presence of sex-determining genes means that every nucleated cell has a sex.
      • 2. Non-genetic sex determination: common in many species (Gilbert, 2010). These are diverse and include:
        • a. Social sex determination: For a number of fish, mollusc, and other species, sex is determined through social interactions with other members of a population. In the slipper limpet Crepidula fornicata, all young individuals are male but some later change to female, depending on their position in a mound of snails.
        • b. Environmental sex determination: In the echiuran worm Bonellia viridis, sex is determined by physical environment. 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 all crocodilians, most turtles, and some other reptiles, sex determination is determined partially or entirely by temperature. In certain species, sex is genetically determined within a temperature range but environmentally determined outside that range.
      • 3. Gametes: germ cells. In species that produce two morphologically distinct types of gametes, the egg-sperm distinction is the basis for distinguishing between females and males. In some species (called sequential hermaphrodites), the type of germ cell produced by an individual can change at different stages of life.

    Hermaphrodite describes an individual that is able to 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 and Auld, 2006). Hermaphrodites are classified either as 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 at some point). 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; see Case Study: Marine Science).

    Defining Sex for Engineering and Design:
    In engineering & product design research, sex includes anatomical and physiological characteristics that may impact the design of products, systems, and processes (see portal: Design Thinking). Many devices and machines have been designed to fit 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 are also based on male bodies; while small dummies are now used to represent women, they do not model bodily differences, such as neck strength (see Case Study: Inclusive Crash Test Dummies; Linder & Svedberg, 2019). Office building thermostats, which are based on male metabolic rates, may set temperatures too low for many women (van Hoof, 2015). Workplace safety gear (e.g., police vests) often does not fit women or smaller men.

    It is also important to understand differences within groups of women, men, and gender-diverse people. Many period tracking apps fail users who have irregular cycles (Tiffany, 2018).

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

    Works Cited

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

    Arboleda, V. A., Sandberg, D. E. & Vilain. (2014). E. DSDs: Genetics, underlying pathologies and psychosexual differentiation. Nat. Rev. Endocrinol. 10, 603–615.

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

    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.

    Fine, Cordelia, Joel, Daphna, & Rippon, Gina. (2019). Eight Things You Need to Know about Sex, Gender, Brains, and Behavior: A Guide for Academics, Journalists, Parents, Gender Diversity Advocates, Social Justice Warriors, Tweeters, Facebookers, and Everyone Else. The Scholar & Feminist Online 15 (2).

    Gilbert, S. (2010). Developmental Biology, 9th Edition. Sunderland: Sinauer Associates.

    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.

    Karkazis, K. (2008). Fixing Sex: Intersex, Medical Authority, and Lived Experience. Durham: Duke University Press.

    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.

    McCarthy, M., & Arnold, A. (2011). Reframing Sexual Differentiation of the Brain. Nature Neuroscience, 14 (6), 677-683.

    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. https://doi.org/10.17226/26424

    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.

    van Hoof, J. (2015). Female thermal demand. Nature Climate Change 5, 1029-1030.

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

 

 

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