Direct link to apology:

https://www.amnesty.org.uk/latest/an-apology-a-growing-threat-briefing-published-on-8-july-2026/

Response from Rowling:

https://xcancel.com/jk_rowling/status/2083280764934746228#m

https://www.sciencedirect.com/science/article/pii/S0092867424001223

The history of sex research demonstrates an ongoing coexistence of multiple, conflicting meanings of sex. This history raises questions for scientists about the deployment of a research variable that lacks precision. Cross-disciplinary collaboration between scientists and science and technology studies (STS) scholars offers a way to find solutions to this problem.

https://www.mdpi.com/2077-0383/11/6/1582

A short critique of the paper:

https://xcancel.com/FondOfBeetles/status/2077389417291845705

I’ve requested the subject-level Brain Sex Index data behind Kurth et al., 2022. Brain sex in transgender women is shifted towards gender identity. JCM.

While I wait, some open questions on the published methods and conclusions.

(Using the paper language)

The key comparison (transgender women vs. cisgender men) was “one-tailed”(seeking a statistical difference in only one direction, which provides a more generous boundary to reach significance).

There is no stated correction for multiple pairwise comparisons across multiple groups.

Two-tailed, their own reported p=0.016 becomes 0.032. Why one-tailed, and why no correction across the 3 group comparisons?

The cisgender men’s group has one data point flagged as a statistical outlier (marked “+” on the plot, and apparently excluded from the box’s standard stats display).

But the reported comparison test shows 24 cisgender men were included.

If one cisgender man is such a clear outlier, does the headline finding hold with that point removed?

Effect sizes (d): briefly, how large a difference are we looking at?

Transgender women vs. cisgender men, d=0.64 (moderate).
Transgender women vs. cisgender women, d=1.87 (large).

That’s a big asymmetry: is that gap large enough to justify the authors’ conclusion of “hovering between”.

And since this calculation includes the outlying man, would removing it make “hovering between” a nonsensical conclusion?

As an analogy: the cisgender men are at the 100m finish line, the transgender women are about a 25m behind them, and the cisgender women are still at the start.

The authors report their transgender women cohort (n=24) as 6 androphilic (attracted to men) and 18 gynephilic (attracted to women).

Orientation has been linked to brain differences in other studies.

Why wasn’t orientation controlled for or stratified, especially since the authors recommend this for future studies? 😂

You have the data, my friends.

What does their dataset look like when disaggregated for orientation?

Anyway, I’ve made a formal request; I know other researchers have too. I’ll share what I get.

(And again, I’m repeating the language of the paper for ease of cross reference to it)

Frankly, to make the claim that sexuality might impact analysis in TIMs, look at the TIM plot that clearly shows two groups - as opposed to the smooth bubble of other males and the female cohort - and fail to disaggregate is bonkers.

There is a reason this is a different shape.

https://xcancel.com/SwipeWright/status/2081789091046182975

My scholarly article "Why There Are Exactly Two Sexes" has been accessed over 50K times, placing it in the top 8 of all articles in the journal's history.

Activists refuse to cite it or acknowledge it exists.

I'm still waiting for someone to publish a scholarly scientific takedown of it. You'd think this would be a top priority for those asserting "sex is a spectrum" who believe my work is bigoted, pseudoscientific, and actively harming a vulnerable community.

Until then, you can read it and arm yourself with a thorough understanding of the 5 major frameworks that activists and activist scientists use to undermine the binary nature of the sexes and why they all fail.

https://link.springer.com/article/10.1007/s10508-025-03348-3

One reply to his paper, Response to Wright’s (2025) “Why There Are Exactly Two Sexes”, is probably why he had to qualify takedown with "scholarly" and "scientific". Nicely put, it's neither. He published a response to that, Response to Mahr’s (2026) Response to Wright’s (2025) “Why There Are Exactly Two Sexes”

https://xcancel.com/SwipeWright/status/2059728010253828274

ANNOUNCEMENT: WE’RE SAVING SCIENCE!

We’re often told that science is “self-correcting.”

But that’s not really true.

Science doesn’t correct itself like a thermostat adjusting the temperature in your house. Science is a human institution run by human beings. And human beings are vulnerable to career incentives, groupthink, moral fads, political pressure, and fear.

And when those forces capture academic journals, peer review stops being a filter for bad ideas and starts becoming more of a credentialing system for fashionable nonsense.

This isn’t exactly new.

In 1996, the physicist Alan Sokal managed to publish a totally gibberish article in the journal Social Text full of trendy postmodern jargon. His point was simple: if you flatter the ideological commitments of certain academic editors, nonsense can pass as real scholarship.

Two decades later, @ConceptualJames, @HPluckrose , and @peterboghossian pulled off the “grievance studies” hoax, placing over a half dozen absurd papers in peer-reviewed journals. One paper used dog parks to analyze rape culture and queer performativity. Another rewrote parts of Mein Kampf in the language of feminist theory.

The problem wasn’t just that fake papers got published. It was that they were completely indistinguishable from the real thing.

And today, the problem is even worse.

We now have serious SCIENCE journals publishing papers about feminist lesbians marrying brine shrimp. We have disturbing papers that aim to “queer” and sexualize infants. We have scholarship on “lesbian-queer-trans-canine relationalities” and “trans-dog intimacies.”

But while Clown World papers are concerning because it makes a complete mockery of academia, the same broken, ideologically captured system is also publishing research in legitimate science and medical journals that pushes sex and gender pseudoscience, relies on deeply flawed data, and influences policies on the medical transition of children and young adults.

That’s not funny. That affects real people. It affects medicine. It affects law. It affects children.

And when critics try to respond, they often discover there’s no serious mechanism for correction. Submitted Letters to the Editor often go completely ignored. Contrary evidence is rejected without comment. As a result, the best critiques are often relegated to personal blog posts, social media threads, or newspaper op-eds, while the original paper remains in the literature wearing the armor of “peer review.”

That is untenable.

So Kevin McCaffree, editor-in-chief of Theory and Society (@Theory_Society), and I decided to do something about it.

Today, in the Wall Street Journal, we announced a first-of-its-kind article type called “Peer Review.”

The idea is simple: publication should be the beginning of academic scrutiny, not the end of it.

A Peer Review article can critique a paper from any scholarly journal. It can address problems with methods, evidence, logic, definitions, theory, or interpretation. But it has to focus on the claims and arguments, not personal attacks.

Submissions are capped at 2,500 words and go through a straightforward merit review instead of endless gatekeeping and ideological screening. We ask just one basic question: Is this critique coherent, serious, reasonable, or even popular enough to deserve scholarly attention?

If yes, it gets published.

And the authors of the original paper get a built-in right of reply, so readers can see the critique and the response in a legitimate academic venue.

That’s how science is supposed to work.

Science becomes self-correcting only when real people build the mechanisms that allow correction to happen.

That’s what we’ve done.

Now it’s time for academics to use it.

Read our announcement on the @WSJ below.

Background

Alcohol use disorders (AUD) associate with structural and functional brain differences, including impairments in neuropsychological function; however, reviews (mostly cross-sectional) are inconsistent with regards to recovery of such functions following abstinence. Recovery is important, as these impairments associate with treatment outcomes and quality of life.

Objective(s)

To assess neuropsychological function recovery following abstinence in individuals with a clinical AUD diagnosis. The secondary objective was to assess predictors of neuropsychological recovery in AUD.

Methods

Following the preregistered protocol (PROSPERO: CRD42022308686), APA PsycInfo, EBSCO MEDLINE, CINAHL, and Web of Science Core Collection were searched between 1999–2022. Study reporting follows the Joanna Briggs Institute (JBI) Manual for Evidence Synthesis, study quality was assessed using the JBI Checklist for Cohort Studies. Eligible studies were those with a longitudinal design that assessed neuropsychological recovery following abstinence from alcohol in adults with a clinical diagnosis of AUD. Studies were excluded if participant group was defined by another or co-morbid condition/injury, or by relapse. Recovery was defined as function reaching ‘normal’ performance.

Results

Sixteen studies (AUD n = 783, controls n = 390) were selected for narrative synthesis. Most functions demonstrated recovery within 6–12 months, including sub-domains within attention, executive function, perception, and memory, though basic processing speed and working memory updating/tracking recovered earlier. Additionally, verbal fluency was not impaired at baseline (while verbal function was not assessed compared to normal levels), and concept formation and reasoning recovery was inconsistent.

Conclusions

These results provide evidence that recovery of most functions is possible. While overall robustness of results was good, methodological limitations included lack of control groups, additional methods to self-report to confirm abstinence, description/control for attrition, statistical control of confounds, and of long enough study durations to capture change.

During the past twenty years, the prevailing clinical approach to child and adolescent gender dysphoria changed dramatically. Before then, most clinicians considered youth gender dysphoria a mental health problem best addressed through psychological management. Typically, childhood gender dysphoria resolved prior to adulthood. Recently, however, the gender-affirming approach has prioritized facilitating social and medical gender transition of dysphoric youth. The ascendancy of gender-affirming care did not arise because of empirical advances. Rather, gender-affirming care triumphed because of the unquestioned ideological beliefs that treating gender dysphoria as a psychological problem is unenlightened, unethical, and akin to conversion therapy for sexual orientation. Clinical referrals for youth gender dysphoria skyrocketed. Adolescent-onset dysphoria among natal females, previously unknown, became the most common subtype, and there are indications it is socially transmitted. Concerns about these developments led to systematic research on the claims that gender affirmation is necessary for improvement and to prevent suicide. Comprehensive reviews failed to support these claims, and gender transition has serious social and medical complications. Thus, gender-affirming care is increasingly seen as a dubious approach. Psychotherapy, including cognitive-behavioral therapy, holds promise as a tool to manage gender dysphoria, ideally until it desists. Gender transition should be considered only after alternatives have been explored. Three distinct pathways are discussed: childhood-onset, autogynephilic, and rapid-onset gender dysphoria. Their optimal treatment may differ.

For twenty years a single therapeutic approach has predominated in the treatment of gender dysphoric children and adolescents: gender-affirming care [1,2]. This approach is based on several propositions, listed in Table 1, and the most important are as follows: Children and adolescents who feel they may be transgender should not be questioned or doubted. The goal of mental health professionals, doctors, and parents should be to facilitate social and medical gender transition as soon as possible. Delaying, or worse, opposing gender transition is courting suicide. Recently, systematic research has examined these propositions and found them either lacking evidence or false (see Table 1).

Abstract

Background

Identifying groups at increased risk for political violence can support prevention efforts. We determine whether “Make America Great Again” (MAGA) Republicans, as defined, are potentially such a group.

Methods

Nationwide survey conducted May 13-June 2, 2022 of adult members of the Ipsos KnowledgePanel. MAGA Republicans are defined as Republicans who voted for Donald Trump in the 2020 presidential election and deny the results of that election. Principal outcomes are weighted proportions of respondents who endorse political violence, are willing to engage in it, and consider it likely to occur.

Findings

The analytic sample (n = 7,255) included 1,128 (15.0%) MAGA Republicans, 640 (8.3%) strong Republicans, 1,571 (21.3%) other Republicans, and 3,916 (55.3%) non-Republicans. MAGA Republicans were substantially more likely than others to agree strongly/very strongly that “in the next few years, there will be civil war in the United States” (MAGA Republicans, 30.3%, 95% CI 27.2%, 33.4%; strong Republicans, 7.5%, 95% CI 5.1%, 9.9%; other Republicans, 10.8%, 95% CI 9.0%, 12.6%; non-Republicans, 11.2%, 95% CI 10.0%, 12.3%; p < 0.001) and to consider violence usually/always justified to advance at least 1 of 17 specific political objectives (MAGA Republicans, 58.2%, 95% CI 55.0%, 61.4%; strong Republicans, 38.3%, 95% CI 34.2%, 42.4%; other Republicans, 31.5%, 95% CI 28.9%, 34.0%; non-Republicans, 25.1%, 95% CI 23.6%, 26.7%; p < 0.001). They were not more willing to engage personally in political violence.

Interpretation

MAGA Republicans, as defined, are more likely than others to endorse political violence. They are not more willing to engage in such violence themselves; their endorsement may increase the risk that it will occur.

Marked as culture wars even though it's within academia. It's a critique of the guy that discovered SRY, in the context of the controversy around Caster Semenya.

https://xcancel.com/FondOfBeetles/status/2079910911603511412#m

On Sinclair, SRY and the destruction of a legacy.

In 1990, Andrew Sinclair was the first author on a paper that reported the discovery of SRY as the gene that drives male development in mammals.

He was the first author on a paper led by Peter Goodfellow, and they published side-by-side with a paper led by Robin Lovell-Badge.

This pair of papers showed that the “mysterious make male mammals molecule” was the product of the SRY gene.

Unequivocally.

Their joint discovery was hailed, including by Sinclair himself, as the answer to one of the most fundamental questions about human life: what makes us male or female?

This is, IMO, an entirely reasonable lauding of the impact.

SRY is, unequivocally, the molecule that makes male mice and men.

Any researcher who wishes to identify the sex of a (mammalian) animal or tissue (or cell line, whatever) they are working with looks for SRY.

Any researcher in the intersection between developmental biology and clinical genetics (👀) uses SRY to understand how a genetic disorder has caused fetal death.

Any geneticist detecting the sex of a pregnant woman’s baby uses SRY as their marker.

Any embryologist trying to prevent the most appalling sex-linked genetic disorders in Petri-dish embryos uses SRY.

Any forensic scientist trying to profile a murderer uses SRY.

Any archaeologist trying to sex a skeleton will try SRY as a first pass.

Thousands upon thousands of SRY assays run daily for various reasons.

But all to flag “male”.

That SRY is the “make male” gene (at least in mammals) is uncontroversial, GCSE-level (maybe lower), textbook knowledge.

It is so thoroughly assimilated into - and crucial to, for good reason - our knowledge regarding sex development, it’s hard to over-exaggerate.

I don’t think the researchers will get Nobels, although developmental biologists have an outsized impact here.

We do good stuff. Like answering some of life’s most fundamental questions. 😉

And more, in developmental biology, there are few genuine “master switches”.

These are genes that control entire developmental processes, overwriting everything else that could have or should have emerged.

SRY is a master switch. Concept.

So maybe they’ll get a Nobel. I would fully support, TBH. Their impact is impossible to estimate.

So then, to the opening premise: the destruction of a legacy.

Sinclair, back in 1990, has this “switch” nailed down. The answer to a fundamental question. The single gene that is necessary and sufficient to determine (dev bio, not lay, meaning) male development.

And in the 2020s, he chucks this away.

He doesn’t chuck it away in the dev bio literature.

He has a thriving research programme in clinical dev genetics, all good.

He’s not saying in that literature “that discovery no longer holds”. Why would he?

But he’s saying “this gene doesn’t determine sex” in the sports/philosophy literature.

And that needs examining.

Because thousands upon thousands of people have been, for nearly 40 years, been routinely, with good reason, using SRY to “find male”.

He evokes uncertainty.

Of course, technology moves. Of course, we are more alert to exceptions.

He “hedges”. “SRY doesn’t always lead to testes development”.

I mean, OK 🤷‍♀️ It almost always does, but genetic disorders exist. This is not news.

“It’s a complex network and we can’t predict outcomes.”

That something is not guaranteed (because genetic disorders and environment insults) does not mean we can’t predict that almost all people with SRY have testes.

My friend, have you ever heard of a master switch? I feel you have experience here.

“You can’t tell sex by SRY status.”

Ok, if you prefer, you can think of these individuals as SRY+ with testes and all the stuff downstream of that, but don’t call them male/men.

Have you written to the textbook editors to change this record?

“It’s not a good way to call sex.”

I can’t even 🤦‍♀️ enough.

He is a clever guy (with SRY, I’d bet my house on it).

He knows - he knows - the difference between markers, screens, SOPs for medical workflows.

I don’t care if he doesn’t want to say Caster Semenya is male.

He certainly knows Semenya’s DSD, Semenya’s biology and what having SRY triggered for Semenya’s development.

And this is where I will stop. For the time being, at least.

This thread has been therapeutic 😆

I have been struggling with the collision of a scientific legacy, rightfully owned, then cast aside so easily.

He is the person I’d most love to sit down with and get to the bottom of “what’s happening here?”

Footnote: People in the labs that were historically and currently involved with this research, who have seen the various “names” prostrate themselves, have emailed me at various points.

None of you are alone.

The post image comes from a response to that thread:

https://xcancel.com/runthinkwrite/status/2079939020767850835

The paper from that image:

https://www.researchgate.net/publication/378759740_World_Athletics_regulations_unfairly_affect_female_athletes_with_differences_in_sex_development

And the response paper:

https://www.tandfonline.com/doi/full/10.1080/00948705.2025.2522809

Any other good posts/links?

Women’s preferences for penis size may affect men’s comfort with their own bodies and may have implications for sexual health. Studies of women’s penis size preferences typically have relied on their abstract ratings or selecting amongst 2D, flaccid images. This study used haptic stimuli to allow assessment of women’s size recall accuracy for the first time, as well as examine their preferences for erect penis sizes in different relationship contexts. Women (N = 75) selected amongst 33, 3D models. Women recalled model size accurately using this method, although they made more errors with respect to penis length than circumference. Women preferred a penis of slightly larger circumference and length for one-time (length = 6.4 inches/16.3 cm, circumference = 5.0 inches/12.7 cm) versus long-term (length = 6.3 inches/16.0 cm, circumference = 4.8 inches/12.2 cm) sexual partners. These first estimates of erect penis size preferences using 3D models suggest women accurately recall size and prefer penises only slightly larger than average.

Trying out some non-academic, dramatic culture war critical analysis. There's an ongoing kerfluffle involving Amnesty UK and J.K. Rowling. Here's a timeline that provides some context.

• 2022: Beira's Place is launched in Edinburgh as a "women-only support service for victims of sexual violence".
  https://www.bbc.com/news/uk-scotland-edinburgh-east-fife-63943766

• 2024: Employment tribunal finds Roz Adams was unlawfully discriminated against / constructively dismissed by Edinburgh Rape Crisis Centre (ERCC) over "gender-critical" beliefs.
  https://www.bbc.com/news/articles/c6248z383x7o

• 2024: ERCC CEO Mridul Wadhwa stands down after a review into failings, including protection of women-only spaces.
  https://www.bbc.com/news/articles/c2edeyzz0xmo

• 2024: Rape Crisis Scotland chief executive Sandy Brindley apologises to survivors and says women-only spaces must exist alongside trans-inclusion in rape crisis services.
  https://www.bbc.com/news/articles/cj310jvzpd8o

• 2025: Amnesty UK starts publishing "The anti-rights movement" as a framing/analysis of "anti-rights" actors.
  https://www.amnesty.org.uk/knowledge-hub/all-resources/the-anti-rights-movement/

• 2025: UK Supreme Court decision For Women Scotland v Scottish Ministers: "sex/man/woman" under the Equality Act are interpreted as biological sex.
  https://www.gov.scot/publications/supreme-court-judgement-for-women-scotland-vs-scottish-ministers-social-justice-secretary-statement-22-april-2025/

• 8 Jul 2026: Amnesty UK publishes "A Growing Threat: The Anti-Rights Movement in the UK", then removes it, saying the language didn’t reflect Amnesty’s position and that it was uploaded without normal checks.
  https://www.bbc.com/news/articles/cp8r8l84p8yo
  https://web.archive.org/web/20260709084603/https://media.amnesty.org.uk/documents/Report_-_A_growing_threat__the_anti-rights_movement_in_the_UK_July_2026.pdf

• 15 Jul 2026: Beira’s Place demands an apology and threatens court action over being labelled in the Amnesty briefing.
  https://www.bbc.com/news/articles/cp9l9dg5k8ko

The tweet in the post:

Beira's Place exists primarily to exclude trans women. It's a part of your long-term agenda to remove rights of non-discrimination, equality, safety, & bodily autonomy from trans women. It's as if you'd set up a school for white kids only because you think immigrants violent.

J.K. Rowling's response:

Trans-identified men in Edinburgh are fortunate enough to be welcome at the Edinburgh Rape Crisis Centre, which was headed for a while by a trans-identified man who believed, and said publicly, that women who wanted a female-only service were bigots. That same rape centre admitted a male as a client because he called himself non-binary. The man was subsequently convicted for rape.

Beira’s Place was set up by me to provide women in Edinburgh and the Lothians with the female-only service they were being denied. Internal emails between ERRC staff were revealed during an unrelated court case, which showed them to be angry and dismayed that there was now an alternative service for women. One might have thought, reading those emails, that ERRC staff’s priority was the ideological conversion of rape survivors rather than the provision of a service that traumatised women required to feel safe after rape.

Beira’s Place has been exceptionally busy since it opened. We’ve taken on more staff and are currently looking for larger premises. We’ve helped women who’ve admitted that they didn’t want to use the alternative rape centre because of its very loud and public ‘trans women are women’ stance. We are also open to any and all trans-identified women who need support after rape.

In short, you’ll be relieved to hear that the trans-identified men for whom you feel such empathy and compassion aren’t losing out. It was biological women who were being shamed for their entirely legitimate desire for a male-free space. Apparently you’d have preferred that situation to continue, but I think you’ll find that most people, whether male or female, will see that as evidence of your cruelty, not mine.

Here's someone's opinion on why Beira's Place is anti-rights:

https://rejserin.medium.com/beiras-place-anti-rights-28a8a3830507

The actual letter sent from Beira's Place to Amnesty UK:

https://xcancel.com/jk_rowling/status/2077366128620323213

Any other good links?

In 1985, Thomas and French published results of a meta-analysis that examined sex differences in grip strength in children 5 years of age and older. Their analysis included results from only four studies, and no update has been published. The purpose of the current study was to use meta-analysis to examine sex differences in grip strength from birth to age 16. The analysis included 808 effects from 169 studies conducted in 45 countries between 1961 and 2023. The total sample was 353,676 (178,588 boys, 175,088 girls). From birth to 16 years of age, grip strength was consistently greater in boys than girls. Between 3 and 10 years old, the effect size was small-to-moderate, with female grip strength equaling 90% of male grip strength (Hedges g = 0.33–0.46). At age 11, the effect size decreased slightly, likely due to girls reaching puberty before boys (g = 0.29, 95% confidence intervals (CI) [0.22, 0.35]). At age 13, the effect size increased markedly likely due to male puberty (g = 0.63, 95% CIs [0.55, 0.70]). By age 16, the sex difference in grip strength was substantial, with female grip strength equaling 65% of male grip strength (g = 2.07, 95% CIs [1.86, 2.27]). Secondary analyses revealed that the sex difference in grip strength is broadly similar between countries and has been mostly stable since the 1960s, except for a narrowing of the difference among 5–10-year-olds after 2010. Various biological factors explain why, on average, boys are stronger than girls from birth onward.

Key Points

Question Are puberty blockers associated with lower odds of diagnosed mood disorders and suicidal thoughts and behaviors for transgender compared with cisgender youths?

Findings In this cohort study among 231 783 youths identified in claims data, prescription of a puberty blocker was associated with decreased adjusted odds of mood disorder and suicidal thoughts and behavior diagnoses for transgender youths compared with cisgender youths.

Meaning In this study, puberty blocker prescription was associated with partial attenuation of mental health disparities for transgender youths, suggesting mental health benefits.

It has been suggested human female breast size may act as signal of fat reserves, which in turn indicates access to resources. Based on this perspective, two studies were conducted to test the hypothesis that men experiencing relative resource insecurity should perceive larger breast size as more physically attractive than men experiencing resource security. In Study 1, 266 men from three sites in Malaysia varying in relative socioeconomic status (high to low) rated a series of animated figures varying in breast size for physical attractiveness. Results showed that men from the low socioeconomic context rated larger breasts as more attractive than did men from the medium socioeconomic context, who in turn perceived larger breasts as attractive than men from a high socioeconomic context. Study 2 compared the breast size judgements of 66 hungry versus 58 satiated men within the same environmental context in Britain. Results showed that hungry men rated larger breasts as significantly more attractive than satiated men. Taken together, these studies provide evidence that resource security impacts upon men’s attractiveness ratings based on women’s breast size.

Introduction

Animal and clinical studies suggest complementary effects of magnesium and high-dose pyridoxine (vitamin B6) on stress reduction. This is the first randomized trial evaluating the effects of combined magnesium and vitamin B6 supplementation on stress in a stressed population with low magnesemia using a validated measure of perceived stress.

Methods

In this Phase IV, investigator-blinded trial (EudraCT: 2015-003749-24), healthy adults with Depression Anxiety Stress Scales (DASS-42) stress subscale score >18 and serum magnesium concentration 0.45 mmol/L–0.85 mmol/L, were randomized 1:1 to magnesium–vitamin B6 combination (Magne B6 [Mg–vitamin B6]; daily dose 300 mg and 30 mg, respectively) or magnesium alone (Magnespasmyl [Mg]; daily dose 300 mg). Outcomes included change in DASS-42 stress subscale score from baseline to Week 8 (primary endpoint) and Week 4, and incidence of adverse events (AEs).

Results

In the modified intention-to-treat analysis (N = 264 subjects), both treatment arms substantially reduced DASS-42 stress subscale score from baseline to Week 8 (Mg–vitamin B6, 44.9%; Mg 42.4%); no statistical difference between arms was observed (p>0.05). An interaction (p = 0.0097) between baseline stress level and treatment warranted subgroup analysis (as per statistical plan); adults with severe/extremely severe stress (DASS-42 stress subscale score ≥25; N = 162) had a 24% greater improvement with Mg–vitamin B6 versus Mg at Week 8 (3.16 points, 95% CI 0.50 to 5.82, p = 0.0203). Consistent results were observed in the per protocol analysis and at Week 4. Overall, 12.1% of Mg–vitamin B6 treated and 17.4% of Mg-treated subjects experienced AEs potentially treatment related.

Conclusions

These findings suggest oral Mg supplementation alleviated stress in healthy adults with low magnesemia and the addition of vitamin B6 to Mg was not superior to Mg supplementation alone. With regard to subjects with severe/extremely severe stress, this study provides clinical support for greater benefit of Mg combined with vitamin B6.

In 2015, we conducted a cross-sectional, Internet-based, U.S. nationally representative probability survey of 2,021 adults (975 men, 1,046 women) focused on a broad range of sexual behaviors. Individuals invited to participate were from the GfK KnowledgePanel®. The survey was titled the 2015 Sexual Exploration in America Study and survey completion took about 12 to 15 minutes. The survey was confidential and the researchers never had access to respondents’ identifiers. Respondents reported on demographic items, lifetime and recent sexual behaviors, and the appeal of 50+ sexual behaviors. Most (>80%) reported lifetime masturbation, vaginal sex, and oral sex. Lifetime anal sex was reported by 43% of men (insertive) and 37% of women (receptive). Common lifetime sexual behaviors included wearing sexy lingerie/underwear (75% women, 26% men), sending/receiving digital nude/semi-nude photos (54% women, 65% men), reading erotic stories (57% of participants), public sex (≥43%), role-playing (≥22%), tying/being tied up (≥20%), spanking (≥30%), and watching sexually explicit videos/DVDs (60% women, 82% men). Having engaged in threesomes (10% women, 18% men) and playful whipping (≥13%) were less common. Lifetime group sex, sex parties, taking a sexuality class/workshop, and going to BDSM parties were uncommon (each <8%). More Americans identified behaviors as “appealing” than had engaged in them. Romantic/affectionate behaviors were among those most commonly identified as appealing for both men and women. The appeal of particular behaviors was associated with greater odds that the individual had ever engaged in the behavior. This study contributes to our understanding of more diverse adult sexual behaviors than has previously been captured in U.S. nationally representative probability surveys. Implications for sexuality educators, clinicians, and individuals in the general population are discussed.

Does a woman’s cognitive performance change throughout her menstrual cycle? Menstruation continues to be a taboo topic, subject to myths about how it affects women. Despite the considerable number of empirical studies, there have been few quantitative summaries of what is known. To address this gap, we conducted a meta-analysis of cognitive performance across the menstrual cycle, including the domains of attention, creativity, executive functioning, intelligence, motor function, spatial ability, and verbal ability. We included studies that measured women’s performance at specific points in the cycle for tasks that have objectively correct responses. Our analysis examined performance differences across phases using Hedges’ g as the effect size metric. Across 102 articles, N =  3,943 participants, and 730 comparisons, we observe no systematic robust evidence for significant cycle shifts in performance across cognitive performance. Although two results appeared significant with respect to differences in spatial ability, they arise from a large number of statistical tests and are not supported in studies that use robust methods to determine cycle phase. Through the use of Egger’s test, and examination of funnel plots, we did not observe evidence of publication bias or small-study effects. We examined speed and accuracy measures separately within each domain, and no robust differences across phases appeared for either speed or accuracy. We conclude that the body of research in this meta-analysis does not support myths that women’s cognitive abilities change across the menstrual cycle. Future research should use larger sample sizes and consistent definitions of the menstrual cycle, using hormonal indicators to confirm cycle phase.

The current study explored a form of femmephobia (specifically, negative attitudes toward femininity in men) as a predictor of anti-gay behaviors among a sample of heterosexual men (N = 417). Additional predictor variables included hierarchical worldviews (i.e., social dominance orientation, right-wing authoritarianism, narcissism) and prejudicial attitudes (i.e., old-fashioned and modern homonegativity). Femmephobia emerged as a robust predictor, accounting for 23% of the variance in anti-gay behavior, surpassing the explanatory power (15%) of all other considered variables combined. Moreover, social dominance only predicted anti-gay behavior when femmephobia levels were high. Future research on discrimination and violence related to sexual identity and gender expression should incorporate femmephobia as a key predictive factor.

https://xcancel.com/NeuroSGS/status/2073447631917818301

Why are male brains larger on average?

One of the most robust findings in human neuroscience is that males have 10–12% greater total brain volume than females (on average). This has been replicated across hundreds of MRI studies and large population cohorts.

The difference is substantial across tissue classes. In the UK Biobank dataset, males exhibited larger total brain volume, cortical surface area, grey matter volume, white matter volume, and cerebrospinal fluid (CSF) volume. Mean differences in overall brain measures were typically 8–15%, whereas average cortical thickness was slightly greater in females.

Although body size explains part of the variance, the difference is not fully eliminated after adjustment for height, weight, lean body mass, or allometric scaling, suggesting that brain growth is, at least partly, an independently sexually dimorphic developmental process.

Developmental evidence strongly supports this. For instance, fetal MRI studies demonstrate that male fetuses already exhibit significantly larger intracranial and brain volumes by mid-gestation, before substantial divergence in height or weight. Similar findings have been reported in neonatal MRI cohorts and longitudinal developmental studies, indicating that sex differences in brain growth emerge prenatally and persist throughout development.

The difference also extends beyond gross anatomy. Using unbiased stereological methods, it’s been estimated that males possess approximately 23 billion neocortical neurons, compared with 19 billion in females (approximately 16% more cortical neurons). Males also exhibit 10% greater cortical surface area, consistent with the larger neuron count, whereas females tend to have slightly thicker cortex after correcting for brain size.

The prevailing mechanistic model is that sex chromosome complement and prenatal gonadal hormones interact to regulate early neurodevelopment. Experimental and clinical evidence suggests several complementary mechanisms:

• Sex chromosome effects: X- and Y-linked genes influence neurodevelopment independently of hormones. Studies of sex chromosomes show direct genetic effects on neural proliferation, regional brain growth, and developmental timing.

• Prenatal androgen signalling: Testosterone influences neural progenitor proliferation, neuronal differentiation, programmed cell death, axonal growth, dendritic arborisation, synaptogenesis, and trophic signalling, thereby altering overall brain growth trajectories.

• Developmental scaling: Early prenatal differences are likely amplified during infancy and adolescence through sexually dimorphic trajectories of cortical surface-area expansion, white-matter maturation, myelination, synaptic pruning, and intracranial growth.

Taken together, the evidence suggests that males and females follow partially distinct neurodevelopmental growth trajectories, beginning before birth and driven by interacting genetic and endocrine mechanisms, rather than brain size simply scaling as a passive consequence of adult body size.

Importantly, these findings should not be conflated with cognitive ability. Brain volume correlates only modestly with intelligence

• Sex differences in relative brain size: The mismeasure of woman, too?
• Sex differences in brain gray and white matter in healthy young adults: correlations with cognitive performance
• Cerebral asymmetry and the effects of sex and handedness on brain structure: a voxel-based morphometric analysis of 465 normal adult human brains
• A meta-analysis of sex differences in human brain structure
• Sex Differences in the Adult Human Brain: Evidence from 5216 UK Biobank Participants
• Sex differences in gray matter volume: how many and how large are they really?
• Regional gray matter growth, sexual dimorphism, and cerebral asymmetry in the neonatal brain
• Maturational Trajectories of Cortical Brain Development through the Pubertal Transition: Unique Species and Sex Differences in the Monkey Revealed through Structural Magnetic Resonance Imaging
• Sexual dimorphism of brain developmental trajectories during childhood and adolescence
• Sex differences in fetal intracranial volumes assessed by in utero MR imaging
• Neocortical neuron number in humans: effect of sex and age
• Greater male than female variability in regional brain structure across the lifespan
• The organizational-activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues
• Reframing sexual differentiation of the brain
• Sex-chromosome dosage effects on gene expression in humans
• A General Theory of Sexual Differentiation
• Sex Differences in Human Brain Structure at Birth

ABSTRACT

The definition of biological sex has become a renewed focus of societal debate, fuelled by the conflation of biological principles with discussions of human gender diversity. Here, we argue that conceptual clarity critically depends on separating these domains. Drawing on evolutionary theory and empirical evidence, we maintain that biological sex is best defined as a binary classification of male and female reproductive strategies rooted in anisogamy, characterised by the production of two discrete gamete types of different sizes. We stress that gamete size constitutes the ultimate criterion for biological sex and that this definition applies consistently across sexual systems, from separate-sexed species to hermaphrodites, irrespective of variation in karyotype, hormonal profile, somatic phenotype, or behaviour. Further, we emphasise that evolutionary insights offer a coherent explanation for recurring, though not universal, associations between biological sex and patterns of sex-specific selection, sexual dimorphism and parental care. We conclude that the definition of biological sex as a binary classification based on gamete size is a powerful scientific framework compatible with the diversity of sexual phenotypes found in anisogamous organisms and distinct from the concept of human gender.

1 Introduction

Researchers across disciplines have long been confronted with a societal debate about the concept of biological sex that has resurfaced with renewed intensity in recent years (Griffiths 2020; DiMarco et al. 2022; Coyne and Maroja 2023; McLaughlin et al. 2023; Rehmann-Sutter et al. 2023; Arnold et al. 2024; Velocci 2024; Eppley et al. 2026). This controversy stems largely from efforts to reconcile scientific insights and facts in biology with human societal values—an endeavour predestined to misunderstandings and cross-disciplinary tensions. Biology, as a scientific discipline, aims to describe the complexity of life and to understand the mechanisms that generate and govern this diversity. It is not designed to prescribe ethical norms or moral values in human societies, which fall within the domain of other disciplines. Conversely, contemporary societal values or political agendas should not guide the interpretation of empirical observations in biology or the conceptualisation of life's complexity. Doing so constitutes a ‘reverse naturalistic fallacy’ (or ‘moralistic fallacy’): the illogical attempt to derive an ‘is’ from an ‘ought’, thereby conflating the empirical with the normative (Davis 1978). Based on these premises, we call for a more differentiated discourse on sex, one that separates the scientific conception of biological sex from societal discussions regarding the diversity of gender expressions and identities in humans.

The recognition that biological sex is rooted in anisogamy has been longstanding in biology (Minot 1888; Geddes and Thomson 1889), and there have been several recent accounts illuminating its foundations and explanatory power (e.g., Goymann et al. 2023; Hilton and Wright 2023; Griffiths and Spencer 2025; Wright 2025). Our aim here, without claiming novelty on this well-documented topic, is to (i) explain and support the gametic definition of sex, (ii) take a stand on its most prominent criticisms and (iii) elucidate the association between biological sex and sexual dimorphism beyond anisogamy, as this issue appears to lie at the heart of recent debates.

2 The Gametic Definition of Biological Sex and Its Conceptual Value

Biological sex is a binary classification of reproductive strategies for propagating genetic material, defined by differences in gamete size (i.e., anisogamy). The production of large, nutritive gametes produced at a relatively low rate (i.e., macrogametes, such as eggs and ovules) is referred to as the ‘female’ strategy, whereas the production of small gametes produced at a relatively high rate (i.e., microgametes, such as sperm and pollen) is referred to as the ‘male’ strategy. Accordingly, gamete size (as an indicator of per-gamete investment) is the ultimate criterion distinguishing male and female reproductive strategies, making the gametic definition of sex independent of any genetic or phenotypic corollaries that may be present in some systems but absent in others, such as karyotypes in species with differentiated sex chromosomes or secondary sexual traits in sexually dimorphic species. Depending on an organism's sexual system, both strategies can be mapped onto individuals (i.e., males and females in separate-sexed organisms and sequential hermaphrodites), or within individuals as male and female sex functions (in simultaneous hermaphrodites) (Charnov 1982; Schärer 2017). In seed plants, size differences occur not at the level of the gametes themselves but at the level of the multicellular gametophytes. In these organisms, the female strategy produces megagametophytes (ovules), whereas the male strategy produces microgametophytes (pollen grains). For simplicity, and following common usage, we nevertheless refer to them as gametes here.

The definition of biological sex based on gamete size implies that sex is defined only in anisogamous species, which produce gametes of two distinct size classes (Figure 1), but not in isogamous species or species in which reproduction does not involve gametes. It therefore mostly pertains to sexually reproducing organisms, although asexual individuals producing large gametes are sometimes still referred to as ‘females’. Isogamous species, including some fungi and algae, do not have sexes but may possess distinct mating types (sometimes more than two; Lehtonen, Kokko, et al. 2016). Even though anisogamy has multiple evolutionary origins (Kirk 2006; Umen and Coelho 2019), it occurs in the vast majority of sexually reproducing eukaryotes covering some fungi and nearly all multicellular plants and animals. Importantly, evolutionary theory predicts that selection favours the evolution of exactly two, and not more than two, gametic strategies (Lehtonen and Parker 2014)—a prediction that is strongly supported across anisogamous species: with the exception of a few rare lineages showing slightly overlapping yet bimodal gamete-size distributions (e.g., in certain algae; Clifton and Clifton 1999), all anisogamous species exhibit two distinct, non-overlapping size classes of gametes. Thus, despite independently replicated evolutionary origins, the emergence of anisogamy is associated with only two classes of gametes, and never more. It is precisely this highly consistent occurrence of two distinct gamete size classes across anisogamous species that renders the definition of biological sex based on gamete size inherently binary. However, defining biological sex as binary does not imply that individuals necessarily fall into two classes, as they can produce male, female, both, or neither gamete class.

Biological sex at a glance. The binary classification of biological sex applies to anisogamous sexually reproducing organisms, in which the male strategy produces small gametes and the female strategy produces large gametes. Depending on the sexual system, one or both gametic strategies can be mapped onto individuals, making the gametic definition of sex applicable to separate-sex organisms, sequential hermaphrodites, simultaneous hermaphrodites and mixed systems. The sexual cascade provides the theoretical framework for explaining the evolutionary drivers of sexual dimorphism. Organisms that reproduce sexually through the fusion of equally sized gametes (isogamy) do not have sexes but may possess two or more mating types. Asexual reproduction via parthenogenesis involves nutrient-rich egg-like gametes such that parthenogenic individuals are also often referred to as females.

The term ‘anisogamy’, which strictly refers to the presence of ‘unequal’ gametes, is most often conceptualised in terms of differences in their size. In fact, the theoretical foundations for the evolution of anisogamy centre on disruptive selection acting on pre-zygotic investment per gamete on the one hand and on gamete number on the other hand, whereby increased provisioning can be achieved only at the cost of producing fewer gametes at a given time (Kalmus 1932; Parker et al. 1972; Bulmer and Parker 2002). Parental provisioning of resources to the zygote is assumed to require the physical packing of resources, such that greater pre-zygotic investment results in gametes of larger volume and therefore larger size.

Another point worth clarifying concerns the biological sex of non-reproductive individuals. Male and female strategies can be mapped onto these individuals according to the developmental trajectories they possess (Griffiths and Spencer 2025). Therefore, an individual producing one of the two gamete types during its lifetime can be associated with the corresponding strategy and classified as a male or a female, whereas an individual producing both types during its lifetime can be classified as a sequential (i.e., male or female at different times) or simultaneous hermaphrodite (i.e., simultaneously having male and female sex functions). This approach allows assigning male and female strategies to, for example, sexually inactive juveniles, post-reproductive adults, or organisms exhibiting seasonal variation in gamete production. However, it is not always meaningful to assign a sex to every individual, as in the case of early embryos of species with environmental sex determination, sequential hermaphrodites in the process of switching sex (Griffiths 2020), or diploid individuals in the life cycle of mosses and liverworts (Bachtrog et al. 2014; Coelho et al. 2018).

It is also worth mentioning that the gametic definition of biological sex is nothing more than a classification of reproductive strategies. These strategies clearly dominate multicellular life and happen to be historically termed ‘male’ and ‘female’ but they could just as well be called the ‘microgamete-producing’ strategy and the ‘macrogamete-producing’ strategy. From this perspective, the debate is in part terminological, and replacing the long-established terms ‘male’ and ‘female’ seems counterproductive. Importantly, defining sex by gamete size does not imply universal differences in any traits other than the type of gamete produced. Whether the male and female strategies are associated with morphological, physiological, or behavioural traits beyond the dimorphism in gamete size is an important question (see Box 1), but this issue is independent of the definition of biological sex.

BOX 1. Sex Differences Arising From Anisogamy.

In light of the most common criticisms of defining biological sex by gamete size, the question arises whether anisogamy has explanatory value. When Darwin (1859, 1871) founded the discipline of evolutionary biology, he was particularly intrigued by sex differences. His assignment of males as primarily competing for mates, whereas females being more often choosy about whom they mate with, was later taken up by Bateman (1948), who hypothesised that this sex difference in the operation of sexual selection is caused by higher reproductive benefits of accessing multiple mates for males than for females. Importantly, Bateman already speculated that Darwin's postulated sex difference in sexual selection is ultimately rooted in anisogamy. These insights were later synthesised in the so-called ‘Darwin–Bateman Paradigm’, which constitutes a central pillar of modern evolutionary biology for explaining the divergence of reproductive strategies grounded in gametic sex (Dewsbury 2005; Janicke 2024a). Bateman's idea was inspired by data he obtained from experimental work on fruit flies, which was pioneering for its time but later subject to severe criticism regarding statistical analysis and experimental design (Snyder and Gowaty 2007; Gowaty et al. 2012; Hoquet et al. 2020). These criticisms continue to be used as key arguments, even in popular science books (Cooke 2022; Fuentes 2025; Kamath and Packer 2025), to dispute the view that sex differences have an evolutionary basis rooted in anisogamy. Yet, the scientific value of Bateman's theoretical conceptualisation of the evolution of sex differences does not hinge on the robustness of one particular empirical dataset, as these merely served as a catalyst for his conceptual reasoning (Morimoto 2020). Importantly, critics of Bateman's work often seem to overlook the fact that repetitions of his experiment with the same fruit fly system, designed to avoid the potential pitfalls he faced, consistently support all the conclusions he drew from his data (e.g., Bjork and Pitnick 2006; Davies et al. 2023), and that his main findings have been verified across many other taxa (Janicke et al. 2016).

Darwin's and Bateman's foundational work has been subject to constant theoretical refinements and extensions. Parker's sexual cascade outlines the evolutionary sequence following the emergence of anisogamy: in external fertilisers, competition among male gametes led to the evolution of greater mobility and intensified pre-copulatory sexual selection, which on land became associated with internal fertilisation and the expansion of Darwinian sex roles (Parker 2014). Formal theoretical models have confirmed that anisogamy can trigger a divergence in reproductive strategies: the production of excess gametes by the male strategy makes it more prone to benefit from additional matings (Lehtonen 2022; Lehtonen and Parker 2024) and to invest more in competition than the female strategy (Lehtonen, Parker, et al. 2016), which in turn affects sex differences in parental care (Fromhage and Jennions 2016). Empirical data strongly support these theoretical predictions. Across diverse animal taxa, the male strategy, defined by gamete size, typically gains more from additional matings than the female strategy, a difference associated with the evolution of more elaborate male secondary sexual traits and female-biased parental care (Janicke et al. 2016).

Despite the prevailing patterns described above, evolutionary theory also predicts that sex differences in sexual selection can vary (e.g., Lehtonen 2022; Lehtonen et al. 2024; for conditions under which sex-specific selection can reverse) and that sex-specific phenotypes can evolve independently of such differences through ecological character displacement (De Lisle 2019; Li and Kokko 2021). Moreover, females have been found to commonly benefit from having multiple partners (Arnqvist and Nilsson 2000; Fromonteil et al. 2023) and in some species this leads them to compete more for mates and invest less in care than males (Hare and Simmons 2019; Fritzsche et al. 2021)—a phenomenon already acknowledged by Darwin (1871). Emblematic examples of species in which ornamented females compete for choosier males include seahorses and shorebirds such as phalaropes and painted snipes (Jones and Avise 2001; Janicke 2024b). Finally, environmental conditions are known to alter sex differences in sexual selection imposed by gamete size leading to flexible sex roles even within species (Cornwallis and Uller 2010; Miller and Svensson 2014; Garcia-Roa et al. 2020).

The outlined secondary modifications indicate that the evolutionary forces driving sex differences beyond anisogamy are diverse and complex. However, they do not refute the theoretical work and empirical evidence showing that anisogamy, as the ultimate defining criterion of biological sex, explains a significant fraction of the diversity in sexual phenotypes we can observe in nature. Empirical observations from across the eukaryotic tree of life are consistent with the predictions of the sexual cascade, of which the Darwin–Bateman paradigm is one important building block. In this sense, beyond the binary definition of biological sex by gamete size, little may appear to be universally binary. However, there are well-documented associations between anisogamy and various forms of sexual dimorphism in most lineages, making the definition of biological sex by gamete size a useful starting point for studying the evolutionary origins of phenotypic diversity.

Finally, we should highlight why the biological definition of sex based on gamete size is useful. There are at least three interrelated reasons: First, the evolution of anisogamy from isogamy represents one of the earliest intra-specific differentiations of reproductive strategies in sexually reproducing eukaryotic lineages. Second, anisogamy is evolutionarily stable and, together with the previous point, is therefore very widespread across the eukaryotic tree of life. Thus, the two gametic strategies can be distinguished across a very broad taxonomic range, which stands in stark contrast to any other biological trait that is sometimes used to assign sex (e.g., not all organisms have sex chromosomes, sex hormones, ornaments, genitalia, courtship behaviour). Third, gamete size dimorphism has far-reaching evolutionary consequences that help us understand the diversity of lifeforms in nature.

3 Robustness of the Gametic Definition Across Sexual Systems

A number of criticisms have been raised regarding the definition of biological sex in terms of gamete size (Table S1). Presumably the most frequent concern is that binary sex based on gamete size does not account for the whole biological diversity of sexual phenotypes beyond anisogamy. Proponents of pluralistic approaches argue that sex is not binary because sex-related characteristics are too complex to be confined to gamete size (McLaughlin et al. 2023; Smiley et al. 2024; Velocci 2024). As mentioned above, defining biological sex by gamete size does not in itself imply any universal relationship with other morphological, physiological, genetic, behavioural, or life-history traits. However, a very long-standing and continuously refined body of evolutionary theory predicts that anisogamy is indeed a precursor that can, under a certain set of conditions, lead to further divergence of reproductive strategies (Schärer et al. 2012). As we explain in Box 1, this framework (referred to as the ‘sexual cascade’; Parker 2014) offers a powerful toolkit for predicting large-scale tendencies in sex differences while acknowledging their diversity.

Other objections relate to particular sexual systems, life histories or rare species-specific character states, such as simultaneous hermaphrodites, individuals unable to produce gametes, species with giant sperm, or cases in which gamete production is restricted to certain life stages. Yet, in all these cases the gametic definition remains applicable if sex is interpreted as a reproductive strategy rather than a fixed property of individuals (see Table S1 for detailed clarifications of individual critiques). Alternative definitions face substantial difficulties. For instance, the multivariate model of sex attempts to incorporate multiple binary and non-binary traits to describe the sexual phenotype (McLaughlin et al. 2023), but it is unclear how different traits should be weighted and combined in a way that is general, biologically meaningful, and grounded in evolutionary theory. Given that many proposed traits, such as morphology, behaviour and levels of sex hormones, are expressed only in certain taxa, the multivariate model precludes cross-species comparisons and therefore does not allow us to understand the evolution of sexual strategies at broader macroevolutionary scales. Similarly, other approaches, such as the definition of sex based on the karyotype, apply only to a small set of taxa or biological contexts (where they largely coincide with the gametic definition), implicitly rely on the gametic definition (e.g., when describing traits as ‘male-like’ or ‘female-like’), or require accepting the existence of sperm-producing females and egg-producing males. These definitions tend to obscure rather than clarify the biological concepts associated with reproductive strategies and their associated traits. Even though the definition of biological sex rooted in anisogamy does not capture all variation in sexual traits, it provides a robust and widely applicable framework for understanding the evolution of reproductive strategies (Box 1).

4 Concluding Remarks

Up to this point, we have intentionally refrained from discussing the implications of biological sex for our understanding of sex in humans. Yet, within the outlined definitional framework, humans are not exceptional. Rather, like all other metazoans, we are anisogamous organisms, and it is this shared reproductive feature that makes the binary definition of biological sex directly applicable to humans. Because human individuals can produce only one of the two gamete types, biological sex can be mapped onto individuals, as in other species with separate sexes. Challenges can arise when applying the binary definition of biological sex in society under the assumption that biological sex is universally tied to other traits, such as karyotype, hormone levels, the morphology of primary and secondary sexual characteristics, or behaviour. This assumption, for instance, ignores the possibility that genetic variants or differences in sex development can result in a divergence between gametic sex and phenotypic correlates. Crucially, however, the definition of biological sex based on anisogamy neither relies on nor supports this assumption. Therefore, the binary definition of biological sex is fully compatible with the diversity of sexual phenotypes beyond anisogamy (Heitzmann et al. 2023; Mank 2023; Loveland et al. 2025) and provides a coherent explanation for a significant part of it. In the context of ongoing societal debates, we emphasise that defining biological sex places no constraints on the diverse spectrum of human gender identities and expressions. While biological insights can inform societal debates on sex, they do not prescribe which gender-related traits society should assign, permit, or encourage for different sexes. Conflating human gender identities and expressions with the binary nature of biological sex is therefore not a scientific inference, but amounts to two fallacies: first, not acknowledging the diversity of human gender identities and expressions because biological sex is considered binary (‘naturalistic’ fallacy), and second, denying that biological sex is binary because human gender identities and expressions are not strictly binary (‘reverse naturalistic’ or ‘moralistic’ fallacy).