Being Transgender Is Not a Choice: What Biology Shows

The Beauty of Biology

When people talk about trans people, sooner or later someone will claim that gender identity is merely a social construct, a matter of upbringing, or even a freely chosen way of life. Some go further and describe being transgender as a trend caused by social media, liberal parents, or supposedly premature access to medical care.

These ideas may sound plausible to some because, in everyday life, sex is primarily classified according to visible physical characteristics. At birth, people generally look at the external genitalia and assign a legal sex on that basis. From then on, the expectation is that the body, social role, and internal gender identity will all continue to develop in the same direction.

Human biology, however, is more complex.

The Endocrine Society, one of the world’s leading medical organizations specializing in hormones and metabolism, states in its position on transgender health that substantial scientific evidence points to a lasting biological component of gender identity, while external forces do not appear capable of genuinely changing a person’s gender identity.¹ At the same time, the organization emphasizes that the precise mechanisms are not yet fully understood.

That is an important distinction. Science has not discovered “the trans gene” or a clearly identifiable “trans structure” in the brain. What it has found, however, is significant evidence that gender identity cannot be arbitrarily created, taught through upbringing, or changed through willpower.

The Endocrine Society’s position has not gone entirely unchallenged. In 2021, a critical letter to the editor appeared in one of the society’s own scientific journals. The authors argued that describing gender identity as having a “durable biological underpinning” went too far. Instead, they proposed a complex interaction of biological, environmental, and cultural factors that may change across the lifespan.

I will return to this controversy later. It shows that experts continue to debate the relative importance of biology and environment. What is not seriously disputed, however, is that gender identity can have biological roots.

Sex Does Not Develop in a Single Biological Step

In political and public debate, biological sex is often reduced to chromosomes. XX means female, XY means male, and supposedly that is all anyone needs to know.

Chromosomes, however, are only one part of a long chain of biological development. This chain includes chromosomal configurations, genes and their regulation, the development of the gonads, hormone production, the sensitivity of cells to those hormones, the development of internal and external sex characteristics, the sexual differentiation of different areas of the brain, and each person’s individual physical and psychological development.

These processes are connected, but they do not all occur at exactly the same time. The development of external sex characteristics and the sex-related development of the brain take place during different stages of pregnancy. As a result, they can be influenced at least partly independently of one another.²

For that reason alone, it is problematic to suggest that one characteristic, such as a person’s chromosomes, can conclusively determine every aspect of that person. Biological sex is not a single box. It is a collection of different characteristics that largely align in most people, but do not necessarily lead to the same classification in every case.

Brain Development Begins Before Birth

One central scientific hypothesis is that gender identity is shaped at least in part during prenatal development. Sex hormones, the body’s response to those hormones, genetic factors, and epigenetic regulatory processes may all interact during this development.² ³

Hormones such as testosterone and estrogens influence more than the development of genitals, gonads, and secondary sex characteristics. They also affect the developing nervous system. What matters is not only how much of a hormone is present, but also when it acts, where it acts, and how sensitive the relevant cells are to it.

Receptors work like biological receiving stations. A hormone may be present in the body, but its effect can remain limited if the relevant receptor is altered or does not respond sufficiently to the signal. Enzymes, transport mechanisms, and the regulation of the genes involved can also affect development.

Reviews of the neurobiology of gender identity therefore do not conclude that one particular hormone “causes” someone to be transgender. Instead, they describe a complex developmental model in which prenatal hormones, genetic predispositions, and other biological factors may contribute together to the development of gender identity.³ ⁴

What Variations in Sex Development Can Teach Us

People with variations in sex development, often referred to as intersex variations or Differences of Sex Development, offer particularly valuable insights. In these individuals, chromosomes, gonads, hormones, receptors, and external sex characteristics may differ from one another in various ways.

These variations must not be equated with being transgender. Most trans people are not intersex, and most intersex people are not trans. Even so, these variations provide important insights into the biological development of sex and gender identity.

Complete Androgen Insensitivity

People with complete androgen insensitivity generally have XY chromosomes and produce androgens. However, because of a variation in the androgen receptor, their cells cannot respond, or respond only minimally, to these hormones.⁵

Despite having XY chromosomes, they usually develop external sex characteristics that appear female and are generally raised as girls. Most develop a female gender identity.

This does not disprove the relevance of chromosomes. It does show, however, that chromosomes do not operate independently of hormones, receptors, and the other stages of development. An XY chromosome pattern does not automatically and completely determine every aspect of physical and psychological sex development.

Congenital Adrenal Hyperplasia

In certain forms of congenital adrenal hyperplasia, or CAH, XX fetuses are exposed to higher levels of androgens before birth. Studies show that affected girls and women are, on average, more likely to display behaviors and interests that their particular culture considers masculine.⁶

Some also experience doubts about their female assignment or develop a male gender identity more frequently than the female population overall. Most, however, continue to identify as female.

This does not produce a simple equation such as “more testosterone makes a person male.” Instead, the findings show that prenatal hormones may influence later gender development without determining it completely. This is a recurring pattern: biological factors alter probabilities. They do not mechanically produce predetermined identities.

Why Unsuccessful Sex Assignments Matter

For many decades, numerous medical professionals believed that a child’s gender identity could be shaped during the first years of life. This led to the assumption that surgery, upbringing, and social treatment could permanently adapt a child to an assigned gender.

This thinking was especially apparent in the treatment of children with ambiguous genitalia or severe genital injuries. In some cases, children were assigned a gender that medical professionals believed would be easier to create surgically or manage medically.

The results of these interventions were nowhere near as reliable as the theories of the time suggested. Some individuals developed a different gender identity despite being consistently raised in the gender assigned to them.⁷ The Endocrine Society therefore explicitly notes that attempts to align the gender identities of intersex people with their genitals or chromosomes were frequently unsuccessful.¹

These experiences do not prove one specific biological cause. They do, however, strongly contradict the idea that gender identity can be produced at will through upbringing. Parents can give a child certain clothes, choose a name, and communicate particular expectations. They cannot train an arbitrary gender identity into that child.

What Twin Studies Show

Twin studies are a traditional method of distinguishing genetic influences from environmental ones. Identical twins share almost all of their genetic material, while fraternal twins share, on average, around half of their variable genetic characteristics. When a trait appears together more often in identical twins than in fraternal twins, that suggests a genetic contribution.

It does not prove an exclusively genetic cause, however, because identical twins also share similar prenatal and social environments.

An older review of published cases found substantially greater concordance for gender dysphoria among identical twin pairs than among fraternal twin pairs. The number of cases was small, however, and the data came mainly from individual case reports.⁷

A methodologically much stronger study published in 2025 refined this picture considerably. An Australian research team combined new data from 27 twin pairs recruited through two specialized clinics with previously published twin data, resulting in a pooled sample of 463 twin pairs.

Assuming a population prevalence of one percent, the researchers calculated a relative risk of concordance of 21.2 for identical twins and 8.7 for fraternal twins. This points to a substantial genetic contribution to gender diversity.⁸

A second result from the same study is also noteworthy. When researchers compared same-sex and opposite-sex fraternal twin pairs, who differ in aspects of their prenatal hormonal environment, they found no difference in concordance. In this specific context, that finding argues against a major influence of prenatal sex hormones through that particular mechanism. It suggests that genetic and hormonal factors may not act with equal strength or in the same way.⁸

The fact that identical twins do not show complete concordance remains scientifically important. If gender identity were determined exclusively by one simple genetic variation, identical twins would almost always be expected to develop the same identity. They do not.

An accompanying systematic review by the same research group concluded that most studies find both genetic and environmental contributions to gender identity and gender-typical behavior. Depending on the study, estimates of genetic influence ranged from around 10 to 77 percent.⁹

This enormous variation primarily demonstrates how differently the existing studies were designed. It also calls for caution when interpreting individual percentages.

A complex interaction among many genetic variations, prenatal conditions, epigenetic regulation, and individual development is therefore more likely than any single cause.

Is There a “Trans Gene”?

No. Researchers have not discovered a single gene that determines whether a person is trans. Given the complexity of human development, such a discovery would be surprising.

Researchers have examined genes involved in the production, conversion, and effects of sex hormones. Some studies have found statistical associations between particular variations in androgen or estrogen receptors and gender dysphoria or transgender identity.¹⁰ ¹¹ Other studies have used exome sequencing and found rare variations in individual trans people in genes that may be involved in hormonal signaling pathways or the sex-related differentiation of the brain.¹²

These findings are interesting, but they do not demonstrate a clear genetic mechanism. Many studies involve relatively small groups. Some findings have not yet been independently replicated. And when researchers examine a very large number of possible genetic variations, there is always a risk that some statistical associations will occur by chance.

The current state of research therefore points more toward polygenic involvement. Many genetic variations may each contribute a small amount and interact with hormonal, epigenetic, and other developmental conditions.

Genes would not function as a switch that flips a person from cis to trans. They would be components within a broader biological developmental network.

What Brain Scans Show, and What They Do Not

Few areas of research are simplified as heavily in public discussion as the neurobiology of trans people. It is repeatedly claimed that trans women have “female brains” and trans men have “male brains.” The idea is easy to understand, but scientifically far too crude.

There are statistical differences between the brains of men and women, but the distributions overlap substantially. There is no single, uniform male brain and no single, uniform female brain. Each person has an individual combination of many different characteristics.

Studies involving trans people have found differences in regions involved in body perception, self-perception, sensory processing, and the processing of sex-related characteristics.¹³ ¹⁴ Some findings were, on average, closer to those of people who shared the trans participants’ gender. Other characteristics were closer to those associated with their sex assigned at birth. Still others could not be clearly assigned to either group.¹⁵

A large pooled analysis of several datasets by the ENIGMA Transgender Persons Working Group concluded in 2021 that the trans people studied were not simply shifted along one axis from a “male” brain to a “female” brain. Instead, they showed a complex and partly distinct neuroanatomical pattern.¹⁶

That finding is significant, but it must not be overinterpreted. Many brain-imaging studies use small and inconsistently composed samples. They employ different measurement methods, and factors such as age, sexual orientation, psychological distress, and hormone treatment can also affect the results.

Differences between groups do not allow reliable conclusions about an individual person. The brain itself also changes continually through experience and social conditions.

A brain scan therefore cannot reveal whether a particular person is trans. One study that attempted to classify biological sex using structural MRI data found a considerably higher misclassification rate among trans women than among cis people. This also argues against a simple binary model of the brain.¹⁷

More recent reviews explicitly emphasize that neither genetic variations nor neuroanatomical measurements can predict or diagnose an individual person’s gender identity.¹⁸

Neuroscience reveals biological relationships. It does not provide an identity test.

The Brain Is Not a Seal of Legitimacy

Searching for biological foundations may seem politically useful. If being transgender has innate or early developmental biological components, claims about upbringing, social influence, or conscious choice become easier to refute.

But this argument also carries a danger.

Trans people must not be considered real and worthy of protection only after a brain scan, genetic test, or hormone measurement confirms their identity. Such a requirement would replace self-determination with a new form of biological control.

Some neurobiological explanatory models are also scientifically and socially controversial. A philosophy-of-science paper published in 2024 critically examined three common neurobiological models of transgender identity. It warned against reducing trans identity to the idea of a clearly “wrong” or “switched” brain.

The paper noted that research findings are always influenced by the categories researchers use, the comparison groups they select, and social assumptions about sex and gender. It proposed understanding the body and identity as an ongoing developmental process rather than as fixed facts permanently embedded in the brain.¹⁹

This criticism does not disprove biological influences. It rightly calls for greater scientific precision and more restraint in the use of simplistic formulas.

The more accurate statement is therefore not: “Trans people demonstrably possess the brain of the other sex.”

It is this: The development of gender identity has biological components. It is complex and cannot be reduced to genitals or chromosomes. At the same time, no biological measurement can verify or disprove the identity of an individual person.

Biological Does Not Mean Unchanging in Every Detail

The word “biological” is also frequently misunderstood. Biological does not automatically mean genetic, visible at birth, or entirely unchanging throughout life.

Reading, memory, attachment, language, and personality all have biological foundations, even though they develop and are influenced by experience. The brain is a biological organ whose structures and networks continue to change throughout life.

In the same way, gender identity may be the result of a biological developmental process without being externally recognizable at birth. People may also understand or name their identity only later in life. That does not mean they created the identity at that moment.

Many years can pass between having an internal experience and gaining the ability to understand it.

Trans people who grow up in environments without the necessary language, role models, or social acceptance may need a long time to make sense of their experiences. A later coming out is therefore not evidence that an identity developed late. It may simply mark the moment when a person finally found words for something that had existed for a long time.

Why Social Influences Still Matter

Accepting biological foundations does not mean that society is irrelevant. Societies determine which clothes, names, professions, postures, and behaviors they classify as masculine, feminine, or nonbinary. They influence how people express their identities, which words are available to them, and how safe they feel coming out.

Social conditions can influence when someone recognizes that they are trans, which label they use, and whether they are able to live openly. That is different from claiming that society causes the identity itself.

Greater visibility may allow more people to recognize and name their transgender identity. It does not mean that visibility is making more people trans.

The growing number of openly left-handed people did not result from visibility creating left-handedness. It happened because fewer people were forced to suppress their natural preference.

Gender Identity Cannot Be Reprogrammed From the Outside

The Endocrine Society summarizes the research by stating that external forces do not appear capable of genuinely changing a person’s gender identity.¹

That does not mean people never experience uncertainty or change the language they use to describe themselves. Some people need time to understand themselves. Labels may change. Some people stop a transition or later choose a different way of living.

These individual developments do not prove that gender identity in general can be reprogrammed through therapy, pressure, or upbringing.

So-called conversion efforts do not treat trans people. They frequently train them to suppress themselves, feel shame, and hide who they are. Research on self-reported exposure to such conversion efforts has found associations with greater psychological distress and a higher risk of suicide attempts.²⁰

Society cannot prevent people from being transgender. It can only influence whether trans people are able to live safely, openly, and in good health, or whether they are forced to deny themselves.

What We Know

The current state of research can be summarized cautiously as follows: gender identity has a biological foundation, or at least substantial biological components.

Several factors are likely involved in its development, including genetic variations, hormonal signaling pathways, receptor sensitivity, prenatal developmental conditions, and the differentiation of the brain.

Twin studies now provide fairly strong evidence of a substantial genetic contribution. At the same time, the most recent study did not confirm a simple association with prenatal sex hormones through the specific mechanism it examined.⁸

Research on variations in sex development shows that chromosomes, hormones, receptors, genital development, and gender identity cannot be treated as one single, inseparable process.

Neurobiological studies identify group differences that may be associated with gender identity, but these findings cannot be turned into an individual test.

None of these fields of research has discovered a single universal marker. There is no trans gene, no trans hormone level, and no definitive trans brain scan.

The evidence points toward diversity, not toward a simple biological test.

What We Still Do Not Know

We still do not understand exactly how the different biological factors interact. Nor do we know whether different people may be trans because of different combinations of developmental conditions.

There may not be one single biological pathway leading to transgender identity. Different genetic, hormonal, epigenetic, and neurological pathways may produce similar forms of gendered experience.

The categories used in previous research also impose limitations. Many older studies examined only trans women or trans men understood within a strictly binary framework. Nonbinary, genderfluid, and other gender identities were often excluded or not recorded at all.

The Australian twin study published in 2025 is among the relatively few studies that attempted to include gender diversity beyond a purely binary classification systematically within a twin registry. This also demonstrates how much methodological work remains to be done.⁸ ⁹

Older publications also sometimes use pathologizing language. That reflects the medical terminology of their time, not necessarily current professional standards or respectful practice. Good science must openly acknowledge these limitations.

Trans People Are Not a Biological Mystery That Must Be Solved

It is legitimate and scientifically interesting to investigate the biological foundations of gender identity. The findings can help challenge outdated assumptions about upbringing, blame, or conscious choice.

But trans people do not have to be explained completely in biological terms before they can be believed.

Cis people are not required to produce brain scans to have their gender recognized. No one demands that a cis woman prove her female identity through particular genes, receptors, or regions of the brain.

The central insight from the research is therefore not supposed proof that every trans person possesses one clearly measurable biological characteristic.

It is that the simple story does not work.

Sex is more than chromosomes. Gender identity is more than upbringing. Being transgender is neither contagious nor a trend nor an arbitrary choice.

It is part of the natural diversity of human development.

Yours, Lizbeth

Sources

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