This comes up a lot – here’s my take:
The distinction between sex and gender seems straightforward. Sex refers to biology, e.g., chromosomes and genitalia. Gender refers to psychosocial processes, e.g., roles and expression. However, they are more complex than this neat division. Like happiness and pain, gender is partly ontologically subjective. Gender identity is ‘deeply felt’ and ‘not necessarily visible to others’ (American Psychological Association, 2015, p. 862).
Sex has social facets. The classic sociological work Doing Gender by West and Zimmerman (1987, p. 127) illustrates how a sex category is assigned at birth:
Sex is a determination made through the application of socially agreed upon biological criteria for classifying persons as females or males. […] Placement in a sex category is achieved through application of the sex criteria, but in everyday life, categorization is established and sustained by the socially required identificatory displays that proclaim one’s membership in one or the other category. In this sense, one’s sex category presumes one’s sex and stands as proxy for it in many situations […].
This assigned sex category affects children’s social life in a variety of arbitrary ways, e.g., how they are dressed and expectations about behaviour, and feeds into the development of gender. For the majority of social contexts where sex category is applied, chromosomes and genitals are concealed and irrelevant.
Gender has biological facets: minds do gender identity and are embodied in the brain and nervous system, so even the phenomenology of gender identity has a biological correlate somewhere (Serano, 2013, pp. 138–168). There is emerging evidence that gender identity is a complex trait that is part-heritable and polygenic. A recent systematic review suggests that its heritability is likely in the range 30−60% (Polderman et al., 2018).
The interwoven biopsychosocial nature of sex and gender has led some scientists to use the combined concept sex/gender (e.g., Rippon et al., 2014). This does not mean that the multiplicity of facets blend into an amorphous blob. It does mean that it is important to clarify what particular facets are intended when discussing measurement and theory: chromosomes, genitalia, gender identity, socialisation, etc. The view of sex as only biological and gender as only psychosocial is too simplistic to progress theorising.
Fugard, A. (2020). Should trans people be postmodernist in the streets but positivist in the spreadsheets? A reply to Sullivan. International Journal of Social Research Methodology, 23, 525–531. [Preprint here]