Many people recognize that gender – how a person identifies, presents themselves and is perceived – has nuances. Some people identify and present as women, others as men, and still others as nonbinary.

But there is a common misconception that biological sex – usually determined by a person’s sex chromosomes, gonads (ovaries or testes) or genitals – is binary. This idea assumes that everyone is biologically either female or male, with no ambiguity or intermediacy.

The assertion that sex is binary is essentially saying that people are what scientists call sexually dimorphic – that is, the sex-related biological traits of females and males don’t overlap. Illustrating this idea on a graph of how many adult females and males measure within a certain range for traits such as the amount of coarse hair on their body would look like two separate bell curves.

But biologists and physicians have long known that sex-related biological traits are not sexually dimorphic. Instead, as I explore in my new book,“The Binary Delusion: How Biology Defies the Myth of Two Sexes,” there is overlap between females and males in each sex-related biological trait. Some people are more typically female in some traits, more typically male in other traits, and intermediate in still others.

Intersex traits

Even primary sex traits – sex chromosomes, gonads, internal reproductive ducts that transport eggs or sperm, and external genitals – are not always consistently female-typical or male-typical. Genitals can be intermediate between typically female and typically male.

All human embryos initially have identical reproductive tissues that later develop into genitals. If an embryo has lots of the hormones called androgens, like testosterone, it will form a penis and scrotum; if an embryo has less of these hormones or has cells that don’t respond to androgens, it will form a clitoris and labia instead.

With intermediate levels of either androgens or androgen receptivity, an embryo will form a phallus – a penis or clitoris – of intermediate length, along with a structure intermediate between labia and a scrotum. One way of thinking about intermediate sex-related biological traits is by analogy of the color spectrum, with each person located somewhere along an axis that goes from red at one end to violet at the other.

But primary sex characteristics other than genitals – such as sex chromosomes; gonads, or the ovaries and testes; hormones and hormone receptors; and internal reproductive ducts that transport eggs or sperm cells – can differ in their “femaleness” or “maleness,” making the color spectrum analogy misleading.

For example, someone who has a Y chromosome but complete androgen insensitivity – meaning their cells do not respond to androgens at all – develops testes hidden in their abdomen as well as typically female genitals. After puberty, they typically have a high-pitched voice, breasts and no coarse body hair. In appearance, most would consider them typically feminine. This person cannot accurately be placed in one location on a sex spectrum because their sex-related biological traits differ in their femaleness or maleness.

Multiple sliding scales are needed to represent this person’s reality.

Collectively, intersex variations occur in 0.2% to 6% of the population, depending on which variations you choose to count.

No consistent definition of human biological sex

Biologists usually define the sex of any animal based on whether it produces small sex cells (sperm) or large sex cells (eggs), which are produced by testes or ovaries, respectively. But physicians assign a person’s sex at birth based on genitals, which may or may not match their gonadal sex (ovaries or testes).

Someone can have male-typical gonads while having female-typical genitals, or the reverse. In fact, because the sex chromosomes, gonads and genitals in a given person can point in different directions with respect to their maleness or femaleness, it’s not clear whether one of these alone should determine someone’s sex category.

Society hasn’t reached general agreement or consistency in how to define human biological sex. For example, regulators of elite women’s athletics such as the International Olympic Committee have repeatedly changed their sex tests for women’s competitions over the past century, going 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 by itself create a typically masculine body or provide athletic advantages. Most athletic advantages that males typically have 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.

Overlap in secondary sex characteristics

There is also overlap between groups of adults categorized as female or male – based on either their chromosomes, gonads or genitals – in secondary sex characteristics that develop at puberty. These include growth of breasts and coarse body hair, widening of hips and shoulders, and lowering of voice pitch.

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 amount of overlap differs 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.

Many people are more typically female in some biological traits, more typically male in others, and intermediate in still others. In such cases, people can’t be placed accurately at one location along a single biological sex spectrum because there are multiple traits that can vary, and an individual’s biological measurements can land in different locations along these different axes.

In other words, biological sex is multidimensional.

Human bodies are mosaics

One way of thinking about the multidimensionality of sex-related biological traits is as a mosaic. Neuroscientist Daphna Joel and her colleagues developed this metaphor to describe how people have brain regions and personality traits that may be relatively female-typical in some, more male-typical in others, and intermediate in still others.

I argue that across the body, each person has a unique combination of sex-related biological traits, which can be thought of as a mosaic. For example, you might have female-typical height and coarse body hair, male-typical voice pitch, and intermediate muscle strength and facial structure.

The fact that people are multidimensional in their sex-related biological traits means that the binary view of sex is fundamentally misleading. Biological sex has variations and nuances, just as gender does.