Biologists correct misinformation about biological sex

Submission on the “Legislation (Definitions of Woman and Man) Amendment Bill”, 2026. (link to PDF version of whole document or scroll to bottom of page)

By: Ad Hoc Working Group on Sex Denialism, 1 July, 2026. Signatories listed at the end.

Executive Summary. We are a group of academics, primarily biologists, from New Zealand and abroad, concerned that public debate about the proposed legislation is being shaped by inaccurate biological claims about sex.

We do not take a position for or against the Bill as a matter of law or policy. Our contribution is scientific: to clarify what biology does, and does not, show about sex.

The central problem is not that sex development can never go awry. It can. The problem is that rare disorders of sex development are being misrepresented as evidence that sex itself is a spectrum. This is a misrepresentation of science: extremely rare edge cases are elevated, while the broader biological reality is obscured.

In humans, as in other mammals, there are two sexes: female and male. These correspond to the two reproductive pathways organized around large gametes, ova, and small gametes, sperm. Sex is not a social assignment, a spectrum, or a matter of identity. It is a developmental and reproductive feature of the body.

Some individuals have Disorders/Differences of Sex Development, or DSDs. These conditions deserve medical accuracy and humane treatment. But they do not show that sex is a spectrum, nor do they justify replacing clear sex-based language for the whole population.

A major source of confusion is prevalence inflation. Activist materials often claim that 1.7%, 2%, or more of people are “intersex.” These figures are not reliable estimates of people whose sex is genuinely ambiguous. They come from expansive definitions that group together very different conditions, many of which do not make a person’s sex unclear. Some involve hormone exposure, late-onset endocrine disorders, or sex-specific developmental anomalies in otherwise clearly male or clearly female individuals. Treating these as evidence that male and female are unstable categories is misleading.

This distinction matters for policymakers. A law can recognize and make provision for rare, medically complex cases without pretending that those cases undermine the sex categories relevant to the overwhelming majority of people. Good policy should not be built by taking exceedingly rare edge cases and using them to rewrite language, data collection, medicine, sport, safeguarding, prisons, or women’s rights for everyone.

We therefore correct the intersex claim, and several more common, but false, claims:

1. False claim: 1.7% of humans are intersex.
   This statistic relies on an overbroad and heavily criticized definition. The rate of births involving significant genital ambiguity is orders of magnitude lower.
2. False claim: sex is a spectrum.
   Many traits vary continuously, but mammalian reproductive organization is binary. Variation within males and females does not create additional sexes.
3. False claim: there is no reliable definition of sex.
   Biology defines sex by reproductive function: the distinction between large and small gametes.
4. False claim: male and female are “assigned at birth.”
   Sex is observed and recorded, usually with high reliability; it is not assigned by social convention.
5. False claim: brain scans prove “female brains in male bodies” or the reverse.
   The neuroimaging literature does not support such simple claims.
6. False claim: drugs and surgeries can change sex.
   Medical interventions can alter appearance, hormones, and some secondary sex characteristics, but they do not change reproductive sex.
7. Very dubious claim: “gender identity” is genetic.
   Current evidence does not justify strong claims of genetic causation.

Scientific accuracy requires acknowledging rare DSDs. It also requires refusing to let rare cases be used to erase the ordinary, objective, and policy-relevant distinction between female and male.

We provide detailed documentation and references in the attached supplement. We ask MPs, journalists, and the public to evaluate claims about sex by scientific merit, not by repetition, institutional prestige, or political usefulness.

Signatories to the submission (in alphabetical order)

Ad Hoc Working Group on Sex Denialism:

Franco Biondi, Professor, Dept. of Natural Resources and Environmental Science (NRES), University of Nevada, Reno

Gary Bowering, MRSNZ, Wellington

Kendall Clements, Professor, School of Biological Sciences, University of Auckland

Mark Collard, Canada Research Chair in Human Evolutionary Studies and a Full Professor of Archaeology and Biological Anthropology, Simon Fraser University

David Curtis, MD PhD FRCPsych, Honorary Professor, UCL Genetics Institute, University College London

PD Dr. Andreas Edmüller, Philosophy, Ludwig-Maximilians University, Munich

Brian Gill, Ph.D., vertebrate zoologist (retired), Auckland

Christine Janis, Emeritus Professor of Evolutionary Biology, Brown University, USA

Nicholas J. Matzke, Senior Lecturer, Biology, University of Auckland

Luana Maroja, Professor of Biology, Williams College, USA

David Policansky, Ph.D. Biology, Retired

Chris Pook, Lead Technologist, Liggins Institute, University of Auckland

Anthony Poole, Professor, School of Biological Sciences, University of Auckland

Jacques Robert, professor (emeritus), University of Bordeaux, France

Julia Schaletzky, Professor, Dept. of Molecular and Cell Biology, University of California, Berkeley

Keith M Vogelsang, Ph.D., Professor of Biology, Ivy Tech Community College, USA