I was asked recently on social media if we are to ignore XX versus XY chromosomes as a scientific identifier of gender and sex? My short answer was that science gave up on XX/XY as an identifier of sex a generation ago. My much longer answer follows.
First, though, gender is not biologically determined. It is a social construct that varies immensely over time, from developmental stage to developmental stage, from societal role to societal role, and from culture to culture. Most cultures have recognized multiple genders without stigma. Three - four gender cultures are common and some have as many as 6. Jewish culture and law have recognized 8 genders. ery few cultures recognize only two genders. Western societies are the outlier with their binary concept of gender and their suppression of genders outside the binary. There is simply no biologic marker for gender. V
Though sex IS a legitimate biologic framework (unlike gender), there is still no accepted single scientific identifier of sex. Yes, we were taught in high school in the 1950s and 1960s and beyond that sex is a genetically determined binary and that males are XX and females are XY. We were also taught that atoms were solid (rather than probability cloud-like structures made up of things like quarks), that stress caused ulcers, that there was no continental drift, that there were active volcanoes on the moon, that all dinosaurs were scaly rather than having feathers, and that the vortex of water in a toilet was reversed in the southern hemisphere. In the early 2000s biologic research began to unravel some of the complexity, diversity, and nuance of sexual differentiation. A key part of this was recognition that almost all traits are determined by gene networks and epigenetic influences (things that turn genes on and off over an organism's lifetime) rather than single genes. By 2010 in medicine two things had become clear: (1) sex is a important variable in disease manifestation/treatment and needs to be included as a studied variable in research; and (2) that sex is not binary. At the present time there are over 40 identified patterns of sex outside the standard male/female binary (sometimes referred to as intersex).
Biology, medical science, anthropology and other fields have a simplified working definition, but before I describe it, I will review the candidates often proposed as identifiers.
Genetic/chromosomal sex. Although XX/XY are by far the commonest chromosomal patterns associated with sex, there are also XO, XXO, XXX, XXY, XYY, and XXXY. In addition, it is not the XX/XY distinction that determines sexual development, but a specific gene (the SRY gene) that results in male development. An individual with XY chromosomes but no functioning SRY gene will develop as a typical female, and an individual with XX chromosomes but an SRY gene will develop as a typical male. In addition, not all species have either X or Y chromosomes. Some humans are chimeric with mosaicism, containing cells with different chromosomal content. In birds, butterflies, some fish and reptiles one sees female rather than male heterogeneity: males are ZZ and females are ZW. In crocodiles, some turtles, and some fish the temperature of the incubating egg determines sex differentiation, though the chromosomes do not change. In ants, bees, and wasps, unfertilized eggs develop into haploid males and fertilized eggs develop into diploid females. A number of fish, reptiles and amphibians change sex during their lifetime (sequential hermaphroditism). This includes the parrot fish and clown fish but is also seen in slugs and oysters. Science has long since abandoned XX/XY as an accurate or useful identifier or predictor of sex.
Gonadal sex. The presence of ovaries or testes has been used in some settings to identify biologic sex. However, there are individuals who have both testicular and ovarian tissue/organs and some have neither. This is also not an identifier that can be used at birth without invasive testing.
Genital sex. The presence (at birth or later) of penis/scrotum versus labia/clitoris/vagina. Because genitalia can be surgically altered, and because some infants are born with ambiguous genitalia or with external genitalia that do not match other aspects of sex, this is known to be an unreliable identifier of sex.
Hormonal sex. Levels of testosterone and estrogens and other associated hormones have been proposed as an identifier, but the normal distribution curves of hormone levels change so drastically with age and developmental stage, and because the male/female curves overlap so much, this approach can identify a few outliers (who are not just different from the opposite sex but also very atypical for their own sex) but cannot serve as an identifier for sex. Again, this is not an identifier that could be used at birth.
Gamete sex. The size and number of gametes have been suggested as an identifier. This works reasonably well as a biologic identifier for research about sex in humans (though there are other species where there is only one size gamete) but it would mean we need to legally and socially identify the sex of newborns by obtaining a gamete rather than what we do now, which is to decide based on external genitalia. We would also have to determine how to categorize those who do not make gametes. In addition, gamete type does not correlate reliably with the things generally considered salient for sex identification for athletes: hormone levels, size, muscle power, endurance. Gamete size is useful for science and research but impractical for cultural and legal contexts.
Those working the fields of biology, medicine, anthropology, genetics and allied fields increasingly use 3G sex as a shorthand for an imperfect categorization: The 3 Gs are genes, gonads, genitals. A 3G female has XX 23rd chromosomes, ovaries and a clitoris/vagina/labia. A 3G male has XY 23rd chromosomes with testes and a penis and scrotum. These are different groups of humans, labeled based on some specific parameters, but they are not 'different kinds' of humans. (Any more than blondes and redheads are different 'kinds' of humans.) Science recognizes that this 3G shorthand is a simplification, and that a very significant number of individuals do not fit neatly into ANY attempt to create a binary because a binary does not exist in nature.
A fascinating, detailed, objective, and well documented discussion of this can be found in Sex Is A Spectrum: The Biological Limits of the Binary (2025) by Agustin Fuentes, author, researcher, and Professor of Anthropology at Princeton University