Male and female

Across the natural world, the words “male” and “female” reference the two specific reproductive functions within a system of sexual reproduction that involves two differently-specialised gamete types. These terms describe cells, tissues, organs and/or entire individuals that have a physical role in the contribution of small gametes (like sperm) or large gametes (like ova), respectively, to the next generation.

In humans (and indeed, in almost all animals and many plants), the two reproductive functions are divided between two classes of individual (‘gonochorism’), a state described as: “an almost inevitable consequence of sexual reproduction in complex multicellular organisms.”1 Each class of individual possesses a distinct and specialised molecular and anatomical pattern corresponding to one of the two reproductive functions. In humans, there are two sexes. In most gonochoristic species, including humans, sex is fixed during early embryonic development and remains stable throughout the organism lifespan.2 Thus, human sex is immutable.

Mine is a functional account of sex; it is a system-level understanding of the evolutionary history, the causal mechanisms and the coordinated, sequential, hierarchical development that begins in early embryogenesis. Being male or female is not simply a descriptive checklist of characteristics; it represents an integrated set of interrelated features comprising a functional biological system.3

During embryonic development, males and females develop sex-specific primary sex characteristics that have evolved to facilitate function during future reproduction. In humans, healthy male anatomy comprises gonads in the form of external testes (also called testicles) that will make sperm, internal genital structures like the vas deferens (that carries sperm from the testicles to penis) and external genitalia in the form of a penis and scrotum. In contrast, healthy female anatomy comprises gonads in the form of internal ovaries that will make eggs, internal genital structures like a uterus and vagina, and external genitalia in the form of a vulva, incorporating the clitoris.

The various parts of the reproductive anatomy of a healthy baby (gonad type, internal genitalia, external genitalia) develop as a system in a regulated and coordinated sequence of events. First, genetic information inherited by the fertilised egg determines4 gonad type: testes or ovaries.5 Then, the developing gonads produce a sex-typical hormone profile that drives development of male- or female- internal and external genitalia congruent with healthy gonadal function at maturity.

In 1989, J. D. Wilson discussed Alfred Jost’s pioneering experiments from the 1940s and summarised their significance, writing: “Sexual differentiation is an ordered process in which chromosomal sex determines gonadal sex and gonadal sex in turn directs the development of phenotypic sex.”6 While I could nitpick over the language, this principle is widely accepted and has been consistently confirmed in mammalian developmental biology, forming the foundation for our understanding of sex differentiation.

The sex of a baby is routinely and reliably observed and recorded—not “assigned”, which implies an element of choice and/or arbitrariness—at birth by visual and palpable assessment of external genitalia. Given the tightly-coupled developmental processes that include development of the external genitalia, this serves as a highly-sensitive marker7 for the entire reproductive system.

The above descriptions of biological sex are standard, appearing in dictionaries,8 key biology textbooks,9 academic publications10 and medical consensus statements considering ‘sex as a biologically variable’ like that by the Endocrine Society in 2021.11

By these standard descriptions of sex, “transgender women” are biologically male and not biologically female. This cohort is more accurately described as “trans-identifying men.”

Trans-identifying men may profess discomfort with their male sex (‘gender dysphoria’) and claim a sense of identification with the female sex (via ‘gender identity’).12 I am neutral to the possibility that a sense of ‘gender identity’ has a biological basis. It is logically incoherent to assert gender identity as a component of sex, yet defined as an identity that exists in reference to one’s sex.