Biological Sex: The Gamete-Based Definition and Why "Assigned at Birth" Is Inaccurate Language

The question of what biological sex is — as opposed to what it means socially, legally, or psychologically — has a scientific answer that does not depend on politics, ideology, or current cultural fashion. That answer is: sex is defined by gamete type.

In sexually reproducing species, reproduction requires two types of gametes — reproductive cells. The biological definition of sex follows from this:

• Female: the sex that produces large, immobile gametes (eggs/ova). In humans, females are born with all the eggs they will ever have — a finite number, present before birth.
• Male: the sex that produces small, mobile gametes (sperm). In humans, males produce millions of sperm continuously from puberty through old age.

There are exactly two gamete types. No third gamete type has ever been described in any sexually reproducing species. There is no intermediate gamete. The existence of infertile individuals, or individuals who never reproduce, does not change this definition any more than sterility changes a person's sex. A female who cannot produce eggs is still female; a male who produces no viable sperm is still male. Sex is defined by the type of gamete the reproductive system is organized to produce, not by whether it succeeds in doing so.

In humans, sex is encoded in every somatic (non-reproductive) cell of the body via sex chromosomes:

• XX chromosomes: female developmental pathway
• XY chromosomes: male developmental pathway

Every cell in a human body carries that person's sex chromosomes. A male who undergoes surgical removal of his testicles still has XY chromosomes in every cell of his body. A female who takes cross-sex hormones still has XX chromosomes in every cell. No medical intervention currently available changes sex chromosomes.

The chromosomal system is not the definition of sex — the definition is gamete type — but chromosomes are the mechanism by which gamete type is encoded throughout the organism. This is why sex is a whole-body phenomenon, not a characteristic of any single organ.

The phrase 'sex assigned at birth' is now standard in activist, clinical, and media contexts. It replaced the previous standard phrase 'sex at birth' or simply 'sex.' The change is not neutral.

'Assigned' implies that a decision was made — that a person or institution chose to label a newborn as male or female, in a process that could have gone differently and might have been wrong. This framing suggests that the designation is administrative and arbitrary, like a case number.

This is not what happens. When a midwife, physician, or ultrasound technician observes or determines a baby's sex, they are observing an objective biological fact that was established at the moment of fertilization — months before birth. In many cases, sex is observed before birth via ultrasound (week 16–20) or via amniocentesis (analysis of chromosomes). A newborn does not get 'assigned' a sex any more than a newborn gets 'assigned' a height or blood type.

The purpose of the language change is to imply that sex is a social construct that can be revised, rather than a biological fact that was observed. This is an ideological claim embedded in a vocabulary choice. A factual site uses factual language: sex is observed, determined, or recorded — not assigned.

Intersex conditions — more precisely called Differences of Sexual Development (DSDs) — are sometimes cited as evidence that biological sex exists on a spectrum or that a third sex exists. This argument does not hold up.

DSDs are a range of conditions in which chromosomal, gonadal, or anatomical sex development diverges from the typical male or female pattern. The commonly cited figure is that 1.7% of the population is intersex (Fausto-Sterling 2000). This figure is widely used in advocacy contexts. It is misleading.

The 1.7% figure includes a broad range of conditions, including late-onset congenital adrenal hyperplasia, which does not affect reproductive anatomy and is not typically recognized as intersex even by the people who have it. The rate of DSDs that involve ambiguous genitalia, atypical chromosomes, or atypical gonadal development is approximately 0.018% — less than 1 in 5,000.

More importantly: no DSD produces a third gamete type. No DSD produces an intermediate gamete type. All DSD conditions involve a body that developed atypically within the male or female developmental pathway, or a body in which both pathways began and one did not complete. They are variations within the binary sex system, not evidence against it.