The concept of gender is widely understood to be nuanced, encompassing how individuals identify, present themselves, and are perceived. However, the notion of biological sex is often oversimplified as a binary system, assuming everyone is either female or male with no overlap or intermediacy. This perspective is fundamentally misleading, as biological sex is far more complex and varied.
In the book The Binary Delusion: How Biology Defies the Myth of Two Sexes Ari Berkowitz delves into the scientific evidence that challenges the binary view of sex. The assertion that sex is binary suggests that females and males are sexually dimorphic, meaning their biological traits do not overlap. However, biologists and physicians have long known that this is not the case. Instead, there is significant overlap in sex-related biological traits between females and males.
The Spectrum of Intersex Traits
Primary sex traits, such as sex chromosomes, gonads, internal reproductive ducts, and external genitals, are not always consistently female-typical or male-typical. Genitals, for instance, can be intermediate between typically female and typically male. All human embryos start with identical reproductive tissues that develop into genitals based on hormone levels. Intermediate levels of androgens or androgen receptivity result in intermediate genitalia, akin to a spectrum of colors ranging from red to violet.
However, the color spectrum analogy can be misleading when considering other primary sex characteristics. For example, someone with a Y chromosome but complete androgen insensitivity develops testes hidden in their abdomen and typically female genitals. After puberty, they may have a high-pitched voice, breasts, and no coarse body hair, making them appear typically feminine. This complexity means that a person’s sex-related biological traits cannot be accurately placed in one location on a single spectrum.
The Multidimensional Nature of Biological Sex
Biologists typically define the sex of animals based on whether they produce small sex cells (sperm) or large sex cells (eggs). However, physicians assign a person’s sex at birth based on genitals, which may not match their gonadal sex. This inconsistency highlights the multidimensional nature of biological sex. For instance, someone can have male-typical gonads while having female-typical genitals, or vice versa.
The International Olympic Committee (IOC) has a complicated history with sex testing, repeatedly changing their criteria from genitals to sex chromosomes to testosterone levels. In March 2026, the IOC returned to using sex chromosomes. However, having a Y chromosome does not necessarily create a typically masculine body or provide athletic advantages. Most athletic advantages are due to higher androgens at puberty, but for people whose cells respond less or not at all to these hormones, they may have little or no athletic advantage.
The Overlap in Secondary Sex Characteristics
Secondary sex characteristics, such as breast growth, coarse body hair, widening of hips and shoulders, and voice pitch, also show overlap between groups categorized as female or male. For example, females have larger breasts than males on average, but some males have larger breasts than some females. Similarly, males have more coarse body hair than females on average, but some females have more coarse body hair than some males.
The degree of overlap varies for each sex-related biological trait. There is little overlap after puberty for voice pitch, intermediate overlap for waist-to-hip ratio and muscular strength, and almost complete overlap for the sizes of nearly all brain regions. This multidimensionality means that people cannot be accurately placed at one location along a single biological sex spectrum.
The concept of a biological mosaic, developed by neuroscientist Daphna Joel, helps illustrate this complexity. Each person has a unique combination of sex-related biological traits, which can be thought of as a mosaic. For example, an individual might have female-typical height and coarse body hair, male-typical voice pitch, and intermediate muscle strength and facial structure. This mosaic approach underscores the fact that biological sex is not binary but rather a complex interplay of various traits.
