Biological sex is neither binary nor a spectrum—a biologist explains how it's multidimensional

Biological sex is a complex, multidimensional concept that defies the simplistic notion of being binary or existing on a spectrum, according to a biologist. This perspective challenges the common belief that a person is either biologically female or male, with no middle ground. The article argues that sex-related biological traits are not distinctly divided but rather overlap among individuals.

For instance, physical characteristics such as the amount of coarse hair on the body or the length of genitals can range from typically female to typically male, or somewhere in between. Even the primary sex traits like sex chromosomes, gonads, internal reproductive ducts, and external genitals can exhibit intermediate traits, complicating the idea of a straightforward sex spectrum.

The article uses the analogy of a color spectrum to illustrate that each person occupies a unique position based on various sex-related biological traits. For example, someone with a Y chromosome but complete androgen insensitivity—meaning their cells do not respond to androgens—might develop testes hidden in their abdomen along with typically female genitals, resulting in a typically feminine appearance.

This person's biological makeup cannot be precisely placed on a single-sex spectrum due to the variability across different traits.

Moreover, intersex variations, which occur in 0.2% to 6% of the population, further complicate the matter. These variations involve differences in the maleness or femaleness of various biological traits. Physicians often determine a person's sex at birth based on genitals, which may not align with their gonadal sex. This discrepancy highlights the inconsistency in defining human biological sex, as society has not reached a consensus on the criteria for categorizing biological sex.

Societal definitions of biological sex have also evolved. For example, the International Olympic Committee has changed its sex tests for women's competitions over the years, from focusing on genitals to sex chromosomes and finally to testosterone levels. Yet, having a Y chromosome does not inherently create a masculine body or provide athletic advantages, as most advantages are linked to higher androgen levels during puberty. Some individuals may have little or no athletic advantage despite having a Y chromosome.

Secondary sex characteristics that develop during puberty also demonstrate significant overlap between individuals categorized as female or male based on chromosomes, gonads, or genitals. For instance, while females generally have larger breasts than males, there is considerable overlap, with some males having larger breasts than some females.

Similarly, males typically have more coarse body hair than females, but some females exhibit more coarse body hair than some males. The extent of this overlap varies for each biological trait, with less overlap after puberty for voice pitch, intermediate overlap for waist-to-hip ratio and muscular strength, and almost complete overlap for brain region sizes.

In conclusion, the article emphasizes the complexity of biological sex, suggesting that multiple sliding scales are necessary to accurately represent the diverse reality of individuals' biological traits.