Why Biological Sex Is Neither a Strict Binary Nor a Simple Spectrum

Biological sex is often assumed to be binary, with every person falling neatly into one of two non-overlapping categories, female or male. But a biologist writing in a new book, "The Binary Delusion: How Biology Defies the Myth of Two Sexes," argues that decades of research on human anatomy, hormones and genetics show something more complicated: sex-related traits overlap between females and males, and many people are more typically female in some traits, more typically male in others, and intermediate in still others.

Even primary sex traits, sex chromosomes, gonads, internal reproductive ducts and external genitals, are not always consistently female-typical or male-typical, the biologist writes. All human embryos start with identical reproductive tissue. High levels of hormones called androgens, such as testosterone, produce a penis and scrotum; lower levels, or cells that do not respond to androgens, produce a clitoris and labia; and intermediate levels can produce genitals of intermediate form. Collectively, such intersex variations occur in an estimated 0.2% to 6% of the population, depending on which variations are counted.

The biologist cautions that thinking of sex as a spectrum, a single line running from typically female to typically male, is also misleading, because different traits can point in different directions in the same person. Someone with a Y chromosome and complete androgen insensitivity, for instance, develops internal testes yet typically female genitals, and after puberty typically has a high-pitched voice, breasts and little body hair, making that person impossible to place at a single point on one axis.

The overlap extends to traits that develop at puberty, such as breast size, body hair, hip and shoulder width, and voice pitch. Females have larger breasts than males on average, the biologist notes, but some males have larger breasts than some females; the amount of overlap differs by trait, from little overlap in voice pitch to almost complete overlap in the sizes of nearly all brain regions.