The science is settled: biological sex is not a spectrum but a binary, and the evidence is undeniable to anyone willing to fairly consider it. So argues evolutionary biologist Colin Wright in a new peer-reviewed article simply titled Why There Are Exactly Two Sexes. Grounding his argument in Darwinian evolution—a theory universally endorsed by the science community—Wright outlines a few basic facts that no serious person disputed until just a few years ago.

In anisogamous species, including humans, sex is strictly defined by the type of gamete an organism’s reproductive system is designed to produce, small sperm for males and large ova for females. This gametic dimorphism, evolved over a billion years ago from isogamous ancestors through disruptive selection, forms the universal basis for the two sexes. “As with the fact of evolution itself,” Wright notes, “contemporary scientific debates have long moved on from questioning whether the sex binary is a fact to questions about how anisogamy evolved, why it persists, and what its evolutionary consequences are.”

Of course, challenges to this consensus are widespread today; even some major science journals have declared their opposition to the sex binary. “The idea of two sexes is simplistic,” Nature famously asserted in 2015, for example. “Biologists now think there is a wider spectrum than that.” The problem with this and similar counterarguments, Wright adds, is that they’re “seemingly driven by cultural and political debates surrounding the concept of ‘gender identity’ and transgender rights.” They don’t introduce new evidence that improves our understanding of sex, they merely try to redefine it to facilitate certain policy outcomes.

This biased methodology introduces an even greater problem beyond its factual mistakes: lawmakers, physicians and individuals begin making decisions that cause real harm. As Wright puts it:

Join Dr. Liza Lockwood and Cam English on this episode of Facts and Fallacies as they take a closer look at Wright’s defense of the sex binary.

Dr. Liza Lockwood 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