Can Chemicals Affect Gender Identity? It Seems So, And That Makes It Worth Investigating

Research by Shanna Swan, an environmental and reproductive epidemiologist, suggests that prenatal exposure to endocrine-disrupting chemicals can blur physiological and behavioural sex differences in offspring. However, she and other scientists conducting similar research acknowledged the issue’s political and ethical implications. Still, one scientist said: “I think the science should be done, but with an appreciation and understanding of how it can be manipulated in political ways.”

Robert F. Kennedy Jr. was running for the Democratic Party’s presidential nomination when he sat for an interview with Jordan B. Peterson, a controversial Canadian psychologist, during his eponymous podcast.

About an hour into the conversation, which was published in June 2023, Kennedy pivoted from answering a question about climate change to bringing up a very different subject. He stated that a lot of the sexual dysphoria seen in children, particularly in boys, “is coming from chemical exposures.”

At the time, Kennedy – now the secretary of the U.S. Department of Health and Human Services – singled out atrazine, a common herbicide that’s a frequent contaminant in U.S. drinking water supplies.

Atrazine will “chemically castrate and forcibly feminize” exposed frogs in a tank, he said, adding that “if it’s doing that to frogs, there’s a lot of other evidence that it’s doing it to human beings as well.”

Kennedy, whose criticism of vaccines and chemical additives in processed foods is well known, was referring to research by Tyrone Hayes, a developmental endocrinologist at the University of California, Berkeley, who studies the hormonal effects of chemical pollutants on amphibians.

In 2010, Hayes and his co-authors reported lab findings showing that 10% of genetically male frogs reared in atrazine-spiked water – four out of the 40 grown to adulthood – had developed female reproductive organs.

The frogs were exposed to a level equal to about a drop of atrazine in 5,000 gallons of water, a minuscule amount. Yet two of the animals had grown ovaries and could produce eggs, a finding that Kennedy noted during Peterson’s podcast.

Other right-wing media personalities have echoed similar themes about how environmental chemicals might affect a person’s gender and sexuality.

Alex Jones shouted on his InfoWars show that “I don’t like them putting chemicals in the water that turn the freakin’ frogs gay” – comments that were then remixed into another video that went viral.

When asked during a speech at UC Berkeley about biological and chemical changes that may be affecting how people see gender, the podcaster and right-wing political commentator Matt Walsh referenced what he called “a proliferation of transgenderism” and said there’s “something in our diet and our environment that’s affecting us at a really basic internal level.”

Atrazine is among many endocrine-disrupting chemicals (EDCs) polluting the planet today. These chemicals do appear to interfere with hormones that control early embryonic development, and studies have linked them with cancer, reproductive disorders and other maladies in wildlife. Both high and low dose exposures have also been linked to adverse effects in humans.

Some investigators who study EDCs have speculated that early exposure to these agents might play some role in human gender dysphoria, too. Steven Holladay, a professor at the University of Georgia College of Veterinary Medicine, for example, published a review of evidence in 2023 that, he wrote, may “support an environmental chemical contribution to some transgender identities.”

That EDCs might affect human brains in ways that cause people to reject a gender that conforms with their assigned sex at birth is a possibility that “should not be dismissed,” he wrote.

As part of a multicentre study in Sweden, researchers are looking into whether early exposures to EDCs correlate later in life with a clinical diagnosis of gender dysphoria – meaning the distress and discomfort people feel if their perception of their own gender and their sex assigned at birth don’t align.

The Swedish Gender Dysphoria study launched in 2016 – primarily to better understand the psychosocial and psychic health outcomes of gender dysphoria, but also its underlying causes.

The focus on early EDC exposure is motivated in part by curiosity among scientists, clinicians, and others over the “marked increase in the number of people seeking healthcare for gender dysphoria over the past 10-12 years,” wrote Fotis Papadopoulos, the study’s lead investigator and a psychiatrist at Uppsala University Hospital’s Gender Identity Clinic, in response to emailed questions.

Papadopoulos pointed out that, to his knowledge, “no study has directly examined the association between prenatal environmental exposures and a clinical diagnosis of gender dysphoria.”

“We have to be very careful not to frame gender non-conforming as an adverse effect,” said Shanna Swan, an environmental and reproductive epidemiologist at the Icahn School of Medicine at Mount Sinai.

In recent years, her work has focused on how EDC exposures, particularly during prenatal development, can affect sperm counts, fertility and other developmental outcomes.

Her research in animals and humans suggests that exposure to EDCs in the womb can blur physiological and behavioural sex differences in offspring. Still, Swan emphasized that these studies provide limited insights into gender identity in people.

“How do you decide that somebody’s gender dysphoric?” she asked. “You ask them, right? It’s not something you can measure physiologically.”

When asked to comment on Kennedy’s statements about atrazine and EDCs, Hayes, the Berkeley endocrinologist argued that there are plenty of environmental reasons outside of any purported impacts on human gender identity to phase out the chemical.

He and Kennedy “agree on one aspect,” said Hayes, “and that is that atrazine should be banned.”

In lab experiments, researchers can expose animals to EDCs and observe the effects, whereas human studies are limited to evaluating effects from real-world exposures to the chemicals. Swan and her colleagues in Sweden and the United States reported in 2018 that in-utero EDC exposures compromise language skills in preschool-aged children.

The scientists measured traces of EDCs called phthalates in mothers’ urine and found that increasing levels predicted a worsening ability among boys to understand 50 words or more, based on questionnaires filled out by their mothers. The results for girls were inconclusive.

Humans and many other mammals also show sex-specific differences in play behaviour from an early age. Research suggests that among many species of rodents and non-human primates, for example, males tend to engage in more play fighting than females, although that’s not the case across all animals. This type of play tends to be more common in human boys as well.

After controlling for social factors, including parental attitudes toward what Swan described as atypical play choices, she found in a study published in 2010 that EDCs affected play behaviour in boys, who showed less interest in ball sports and toys like guns, swords and tool sets.

Swan said the results of her study suggest that the EDC-exposed boys were “less male-typical because they’re less likely to engage in these male-typical play behaviours.”

But she insisted her findings can’t speak to other sex-linked behaviours or to the boys’ experience of gender. “It just, at this point in their life they are playing more like girls than boys.”

In what may be the only effort to have published research investigating EDCs’ effects on human gender identity specifically, a French-led team studied a cohort of 253 adults who were born males, and whose mothers had been treated with diethylstilbesterol, or DES, a synthetic oestrogen, during pregnancy.

DES – which indirectly reduces testosterone production in the body – was used to prevent miscarriage and other pregnancy complications decades ago, but its uses fell off after research linked it with a higher risk of a type of vaginal cancer among women who were exposed to the drug in utero.

The French-led team noted in 2024 that four people from the DES-exposed cohort – or 1.58% of the cohort overall – reported having female identities since childhood and adolescence.

The researchers wrote that the finding “strongly suggests that DES plays a role in male to female transgender development,” adding that the rate uncovered in the study was much higher “than in the general population.”

Moving toward proof that EDCs and transgender identities are related might require a deeper, longitudinal study, said an expert who did not want to be identified due to the sensitivity of the research question and the lack of data available.

Studies that aim to explore any potential biological underpinnings of gender identity can face considerable hurdles, however, in part because of the “fear, I would say, of this kind of research,” said Ivanka Savic, a neurologist and neuroscientist at Sweden’s Karolinska Institute.

Savic has spent years doing brain imaging studies in transgender people that, in her words, point to decreased connection between brain circuits that mediate “perception of our one’s body and the perception of self.”

She’s also an adjunct professor at the University of California, Los Angeles, where she previously worked with collaborators on investigations partly funded by the National Institutes of Health into the neurobiology of gender dysphoria.

In 2020, her team was working on a study on transgender identity that the lead researcher paused the following year after transgender researchers and other academics wrote to members of the university and trans advocacy groups warned their local constituencies against participating.

According to Phil Hampton, senior director of communications at UCLA Health, following a review, changes were made to the design and execution of the study. It closed in 2023.

An earlier study of the origins of transgender identity planned by a consortium of five research institutions in the United States and Europe similarly faced challenges.

Savic said her studies of brain differences between trans- and cisgender people are motivated not by politics but rather “pure science” and her position as a medical professional.

Her aim, she explained, is merely to discover more about how the brain perceives the body’s physical appearance, and whether these insights shed any light on why someone might view their gender identity as being different from their biological sex.

Papadopoulos, the lead of the Swedish Gender Dysphoria study, acknowledged the risk of research into the biology of gender identity and wrote that “this is why we need to report in a scientific sound and ethically correct way.”

But to some in the trans community, these kinds of studies pose an existential threat. Those who called out the study Savic was working on at UCLA expressed worry that the findings might be applied to medical gatekeeping, or the potential denial of gender-affirming care to people who don’t meet clinically defined thresholds.

Grounding discussions of transgender identity “solely in the terrain of neuroscience and biology,” they wrote, “undercuts this critical point: we are who we say we are, regardless of biological evidence.”

Meanwhile, back at UC Berkeley, Hayes is still busy studying the environmental cues that change hormones and sex differentiation. Asked if scientists should investigate potential associations between EDCs and transgender identity, he responded, “Yes, absolutely.”

But Hayes also cautioned against using the research in ways that could demonize or lead to the discrimination of certain groups of people.

To say we shouldn’t do the science “would be like me saying we shouldn’t do genetics because of this aspect called eugenics,” he said. “I think the science should be done, but with an appreciation and understanding of how it can be manipulated in political ways.”

 

yogaesoteric
January 13, 2026

 

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