This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
A graph illustrating this idea of how many adult females and males fall within a certain range for traits such as the amount of coarse hair on their bodies 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. 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 to 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.
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