Over the last decade, a growing chorus of scientists has declared that biological sex is not binary. Properly assessing human diversity requires us to apply a broader definition of this word, the argument goes. “The term ‘biological sex’ has been used to sort all people into one of two groups,” a May open letter from hundreds of scientists to the UK’s Minister for Women and Equalities asserted. “However, a strict, binary categorization is an over-simplification.”

The letter – signed by more than 350 academics, clinicians, and activists – is carefully argued, meticulously footnoted—and fundamentally mistaken about a basic biological reality. “Such statements … are most astounding as they ignore or even reject the well-established biological concept of sex and, thus, they ultimately deny fundamental principles of biology,” a team of evolutionary biologists wrote in 2023, responding to similar statements published by several academic journals.

Evolution, a scientific concept as well established as gravity, excludes the idea of a sex spectrum by confirming that biological sex is a binary trait in humans and most mammals. Evolution has shaped organisms to maximize reproductive success, resulting in two distinct sexes—male and female—defined by their roles in producing sperm or eggs. Males produce small, mobile gametes (sperm), while females produce larger, resource-heavy gametes (eggs). This binary system, driven by the need for genetic recombination, is a fundamental mechanism of sexual reproduction, observable across species. “With a few exceptions, all sexually reproducing organisms generate exactly two types of gametes that are distinguished by their difference in size,” the 2023 essay  added.

Intersex conditions, which are rare (approximately 0.018% of the population) and often involve ambiguous genitalia or chromosomal variations, are medical anomalies, not evidence of new sexes or a spectrum. They do not alter the binary framework, as they typically derive from disruptions in male or female developmental pathways.

The notion of a “sex spectrum” conflates sex with gender identity or secondary sexual characteristics, ignoring the objective, gamete-based definition of sex. Evolutionary biology shows no mechanism for a continuum of sexes. Scientists who endorse the sex spectrum often prioritize social or ideological considerations over empirical evidence, denying the binary reality established by evolutionary pressures.

By rejecting these basic facts, sex-spectrum advocates undermine the rigor of biology, misrepresenting a field that relies on testable, reproducible evidence. They also fuel mistrust of science, making it even harder for experts to convince the public that their scientific and medical claims are grounded in solid evidence.

Join Dr. Liza Dunn and Cam English on this episode of Facts and Fallacies as they discuss the science of reproduction and raise some critical questions about the sex spectrum.

Dr. Liza Dunn is a medical toxicologist and the medical affairs lead at Bayer Crop Science. Follow her on X @DrLizaMD

Cameron J. English is the director of bio-sciences at the American Council on Science and Health. Follow him on X @camjenglish