Trans Women and Female Identity

“Are trans women women?” is a question that frequently arises in public debates, online comments, and parliamentary halls. It is often posed as if the answer were a matter of opinion. It is not. The scientific evidence accumulated over thirty years of research in neuroscience, genetics, endocrinology, and psychology converges on one point: gender identity has solid biological bases and is not determined exclusively by chromosomes or genital anatomy.

This article examines what science actually says, what the positions of major international medical organizations are, and why the question itself, as formulated, betrays an incomplete understanding of human biology.

What determines gender: much more than chromosomes

When discussing “biological sex,” common sense tends to reduce everything to a pair of chromosomes: XX or XY. This simplification, while useful in basic education, does not reflect the complexity of real biology.

Biological sex is a set of characteristics that includes chromosomes, gonads, hormones, internal anatomy, external genitalia, and—a frequently overlooked aspect—brain structure and function. As documented in a 2015 article published in Nature, these components are aligned in most people, but they can vary independently of each other [8]. Biological sex is not a binary switch, but a complex system with many layers.

Gender identity—the deep, internal sense of being a woman, a man, or a nonbinary gender—is one of these biological components. It is not a choice, a whim, or the result of social influences. Scientific research has identified multiple biological factors that contribute to its formation, all rooted in the prenatal development of the brain [3][7].

Neuroscientific evidence

Zhou’s pioneering study (1995)

The first study to identify a neuroanatomical correlation with gender identity was conducted by Zhou and colleagues in 1995 and published in Nature. The researchers examined the central subdivision of the bed nucleus of the stria terminalis (BSTc), a sexually dimorphic brain area—meaning it differs in size between males and females. The study found that in trans women, the volume of the BSTc was consistent with that of cisgender women, not with that of cisgender men. This result was independent of sexual orientation and hormonal treatment [1].

Kruijver’s confirmation (2000)

Five years later, Kruijver and colleagues confirmed and expanded upon these results with a study published in the Journal of Clinical Endocrinology & Metabolism. Analyzing the number of somatostatinergic neurons in the BSTc of 42 subjects, they found that men had almost twice as many neurons as women. Trans women exhibited a neuron count consistent with that of cisgender women (p = 0.83), while a trans man exhibited a neuron count in the male range [2]. Hormonal treatment in adulthood did not influence these results, suggesting that differentiation occurs during prenatal development.

Sexual differentiation of the brain: Swaab (2004)

Dick Swaab, one of the pioneers of research on the sexual differentiation of the brain, synthesized the biological model in a 2004 paper [3]. According to this model, during fetal development, the brain differentiates in a masculine direction through the direct action of testosterone, and in a feminine direction in the absence of this hormone. Since the differentiation of the genitals occurs in the first two months of pregnancy, while that of the brain begins in the second half, the two processes can be influenced independently of each other. This explains how a person can develop genitals typical of one sex and a gender identity consistent with the other.

Luders and gray matter (2009)

Luders and colleagues at UCLA analyzed the brains of 24 trans women not yet on hormone therapy using MRI, comparing them with 30 cisgender men and 30 cisgender women. The study, published in NeuroImage, found that trans women had a significantly larger volume of gray matter in the right putamen compared to cisgender men, a characteristic more typical of the female brain [5]. The results provided new evidence that transgender identity is associated with distinct brain patterns.

Rametti and white matter (2011)

A study by Rametti and colleagues published in the Journal of Psychiatric Research used diffusion tensor imaging (DTI) to examine the microstructure of white matter in 18 trans women prior to any hormonal treatment. The results showed that the white matter pattern of trans women fell halfway between that of male and female controls, differing significantly from both in several brain areas, including the superior longitudinal fasciculus and the right anterior cingulum [6]. According to the researchers, these data suggest that certain nerve fiber bundles do not complete the masculinization process in trans women during brain development.

Guillamon’s systematic review (2016)

A comprehensive review of the literature conducted by Guillamon, Junque, and Gomez-Gil in 2016, published in Archives of Sexual Behavior, analyzed all available neuroimaging studies on transgender people. The conclusion is that the brains of trans women present “complex mixtures of masculine, feminine, and demasculinized regions,” with a specific brain phenotype that differs from both cisgender male and female phenotypes [9]. These results confirm the existence of a neurobiological basis for gender identity and indicate that the brains of trans people have their own characteristics, not reducible to a simple “anomaly.”

Genetic and hormonal bases

The evidence is not limited to neuroanatomy. Genetic studies have identified significant correlations between transgender identity and specific genetic variants.

Hare and colleagues, in a 2009 study published in Biological Psychiatry, examined 112 trans women and 258 cisgender men, finding a significant association between transgender identity and a variant of the androgen receptor (AR) gene. Trans women had longer CAG repeats in the AR gene compared to male controls [4]. Since longer repeats are associated with decreased sensitivity to testosterone, this finding suggests that reduced masculinization of the brain during prenatal development could contribute to the development of a female gender identity.

A literature review conducted by Saraswat, Weinand, and Safer in 2015 in Endocrine Practice concluded that, although the precise mechanisms remain to be clarified, there is strong support in the literature for a biological basis of gender identity [7]. Studies on patients with disorders of sex development (DSD) and neuroanatomical studies provide the most solid evidence.

The 2017 Endocrine Society guidelines officially recognized that “results of studies from a variety of biomedical disciplines—genetic, endocrine, and neuroanatomic—support the concept that gender identity likely reflects a complex interplay of biologic, environmental, and cultural factors,” and that “considerable scientific evidence has emerged demonstrating a durable biological element underlying gender identity” [10].

The parallel with intersex conditions

An argument that helps to understand why sex cannot be reduced to a rigid dichotomy is the parallel with intersex conditions. There are people with a 46,XY karyotype who are phenotypically female due to complete androgen insensitivity syndrome (CAIS). There are people with a 46,XX karyotype who develop male characteristics due to congenital adrenal hyperplasia. There are chromosomal variations such as Klinefelter syndrome (XXY) and Turner syndrome (X0).

As documented in the 2015 Nature article, intersex conditions affect a significant percentage of the population, varying depending on the definition adopted [8]. These conditions demonstrate that biology does not operate according to rigidly binary categories: chromosomes, hormones, gonads, and anatomy can vary independently.

If biological sex itself admits this variability, there is no reason to be surprised that gender identity—another biological component—might also not align with the sex assigned at birth. Transgender people and intersex people represent different expressions of the same fundamental biological variability.

What international medical organizations say

The world’s leading medical and scientific organizations have taken clear and converging stances.

World Health Organization (WHO)

In 2019, the World Health Assembly approved the ICD-11, the new international classification of diseases. The condition previously known as “gender identity disorder” was reclassified as “gender incongruence” and moved from the chapter on mental and behavioral disorders to a new chapter on conditions related to sexual health [11]. The decision explicitly recognizes that transgender identity is not a mental disorder.

American Psychological Association (APA)

In February 2024, the APA adopted a policy statement affirming: “There is a growing body of research showing that transgender or nonbinary gender identities are normal variations in human expression of gender” [13]. The statement opposes the dissemination of inaccurate information about gender identity and the characterization of transgender identity as a mental disorder.

World Professional Association for Transgender Health (WPATH)

Version 8 of the WPATH Standards of Care, published in 2022, is the international reference document for the care of transgender and gender-diverse people [12]. The guidelines, developed by a multidisciplinary committee of experts, are based on the most up-to-date scientific evidence and fully recognize the gender identity of transgender people.

Endocrine Society

The Endocrine Society, the world’s leading organization for endocrinology, published its clinical guidelines in 2017, recognizing that gender identity has documented biological bases and that transgender people require appropriate, evidence-based care [10].

These are not isolated opinions. The same position is shared by the American Medical Association, the British Medical Association, the Royal College of Psychiatrists, the American Academy of Pediatrics, and dozens of other medical and scientific organizations worldwide.

Legal recognition

The principle that trans women are women is not just a scientific conclusion: it is also a legal principle recognized in many jurisdictions.

In Italy, Law 164 of 1982 allows for the rectification of sex attribution, making Italy the sixth country in the world to legally recognize the right of people to change their gender. Since 2015, following the rulings of the Constitutional Court, it is no longer necessary to undergo surgery to obtain a change of documents.

The European Parliament has adopted resolutions calling for the full recognition of transgender people and the elimination of barriers to accessing legal gender recognition. In numerous European countries and around the world, trans women can obtain documents consistent with their gender identity through procedures that respect self-determination.

Why the question is reductive

Posing the question “are trans women women?” as if the answer depended on a personal opinion means ignoring the scientific evidence accumulated over decades of research. But it also means asking a question that is never directed at cisgender women.

No cisgender woman is asked to prove her femininity through chromosomes, reproductive capacity, or anatomical conformation. Women who have had a hysterectomy, menopausal women, women with Turner syndrome (who have only one X chromosome), and women with CAIS (who have XY chromosomes) are not questioned in their female identity. Femininity has never been defined by a single biological marker.

Gender identity is a complex biological characteristic, influenced by prenatal brain differentiation, genetic and hormonal factors, and integrated into every person’s deepest sense of self [7][10]. Trans women, like cisgender women, have a female gender identity. This identity is real, is consistent with the structure of their brains [1][2][5][9], and is recognized by the international scientific and medical community [11][12][13].

Beyond the debate: real-world consequences

The question about the identity of trans women is not an intellectual exercise. It has concrete consequences on people’s lives.

When the gender identity of trans women is denied, it legitimizes discrimination in access to healthcare, employment, housing, and education. It fuels the social stigma that contributes to the high rates of depression, anxiety, and suicidal ideation documented in the transgender population. It creates an environment where violence against trans women is normalized or minimized.

Conversely, the recognition of gender identity is associated with better mental and physical health outcomes [12]. Studies show that transgender people who live in accepting environments and have access to appropriate care exhibit levels of well-being comparable to those of the general population.

Science is clear. Medical organizations are unanimous. International law is moving in the same direction. Trans women are women. Not because an ideology says so, but because the biological, neurological, and medical evidence we have available points to it.