Gender identity: what it is and what we know

Gender identity is one of the fundamental aspects of human experience. It is the intimate and deep sense that every person has of their own gender, a subjective experience that may or may not correspond to the sex assigned at birth. Understanding what gender identity is, how it develops, and what science tells us about it is essential for approaching the topics related to transgender and gender non-conforming people with awareness. In this article, we analyze definitions, scientific foundations, diagnostic classifications, and the positions of the major international institutions.

Definition and key concepts

The American Psychological Association (APA) defines gender identity as the inner sense a person has of their own gender, meaning feeling like a man, a woman, a combination of both, neither, or something else [2]. This experience is deeply personal and not necessarily visible from the outside.

To fully understand the meaning of gender identity, it is necessary to distinguish it from other concepts that, while related, describe different dimensions of human experience.

Gender expression

Gender expression refers to the way a person communicates their gender to the outside world through clothing, behavior, voice, hairstyle, and other aspects of appearance and demeanor [2]. Gender expression may be consistent with the social expectations associated with a person’s gender, or it may diverge from them. A person may have a gender expression that does not correspond to traditional social norms without this necessarily implying a gender identity different from the sex assigned at birth.

Sexual orientation

Sexual orientation describes a person’s emotional, romantic, or sexual attraction to other people. It is a dimension completely distinct from gender identity: sexual orientation concerns who one is attracted to, while gender identity concerns who one is [2]. A transgender person can be heterosexual, homosexual, bisexual, asexual, or have any other orientation, exactly like a cisgender person.

Gender role

Gender role includes the set of social and cultural expectations associated with a given gender in a specific society. These roles vary significantly across cultures and historical periods and include expectations regarding behavior, occupations, family responsibilities, and modes of social interaction.

Difference between biological sex and gender

The distinction between biological sex and gender is a central point in understanding gender identity. Although the two terms are often used as synonyms in everyday language, in scientific and medical contexts they describe different realities.

Biological sex

Biological sex refers to a set of physical characteristics that include sex chromosomes (typically XX or XY), levels and types of sex hormones (estrogens, testosterone), internal reproductive anatomy (ovaries, testes, uterus), and external sexual characteristics. These characteristics are assessed at birth to assign a sex, generally male or female.

However, biological sex is not always reducible to a binary classification. Intersex people, who according to some estimates represent about 1.7% of the population under the broadest definition, present natural variations in sexual characteristics that do not fit within the typical categories of male or female. These variations may involve chromosomes (for example, XXY in Klinefelter syndrome), hormonal production, or anatomy.

Gender

Gender is a broader construct that encompasses gender identity, gender expression, and gender roles. As highlighted by the Yale School of Medicine and numerous academic institutions, gender reflects a complex interaction between psychological, environmental, cultural, and biological factors. While biological sex is determined primarily by biological factors, gender differences are the product of this multifactorial interaction.

Neuroscientific research has demonstrated that gender identity has a significant biological component [5]. Twin studies have found a concordance for transgender identity among monozygotic twins significantly higher than among dizygotic twins, with an estimated heritability index between 0.50 and 0.57 in men and between 0.30 and 0.37 in women [13]. Neuroimaging studies have also shown that certain structural and functional brain characteristics of transgender people are more similar to those of cisgender people with the same gender identity than to those of people with the same biological sex assigned at birth [5].

The gender spectrum

The conception of gender identity as a spectrum, rather than as a rigid binary system, is increasingly supported by scientific research and clinical experience. Many people identify outside the traditional categories of man and woman, and their experiences are documented both in contemporary scientific literature and in the cultural history of numerous civilizations.

Non-binary identities

Non-binary (or non-binary) people are those whose gender identity does not fall exclusively within the category of man or woman [12]. This umbrella term encompasses diverse experiences. Recent surveys conducted in the United States suggest that about 1-2% of the adult population identifies as non-binary, with higher percentages among young people.

Genderfluid people experience a gender identity that changes over time. This fluidity does not necessarily follow a predictable pattern and may vary based on the period, context, or other factors. A study found that over 80% of genderfluid youth continued to identify as a gender-diverse person after several years, indicating that it is not a transitory phase.

Agender people do not identify with any gender or feel they do not have a gender identity. Other people may identify as bigender (with two gender identities), pangender (with an identity that encompasses all genders), or with other identity configurations.

Cultural and historical perspectives

The existence of gender identities outside the male-female binary is not a recent or exclusively Western phenomenon. Many cultures around the world have recognized and sometimes revered people with non-conforming gender identities.

Two-Spirit people in Indigenous North American cultures represent a significant example. The term, proposed in 1990 at an intertribal conference in Winnipeg, refers to people who in Native tradition occupy a distinct gender role, neither exclusively masculine nor exclusively feminine. Anthropological research has documented over 150 different precolonial North American Native communities that recognized gender identities outside the binary. Two-Spirit people often held important roles in their communities as healers, spiritual guides, and mediators.

Other examples include the hijra in India and the Asian subcontinent, recognized as a third gender for thousands of years, the fa’afafine in Samoan cultures, the muxe in Zapotec culture in Mexico, and the quariwarmi in the pre-Columbian Andean tradition. This cross-cultural recurrence suggests that gender diversity is a constant aspect of human experience, rather than a product of specific contemporary social conditions.

How gender identity develops

The development of gender identity is a process that begins in early childhood and involves biological, psychological, and social factors. Research in this field has expanded significantly in recent decades, although many open questions remain.

Development in childhood

Most children develop an awareness of their gender between ages 2 and 4. At this age, children begin to identify as boys or girls and to manifest preferences for games, clothes, and behaviors associated with the gender they identify with. For most children, this identity corresponds to the sex assigned at birth. However, some children manifest early identification with a gender different from the one assigned.

A systematic review of longitudinal studies on gender identity in children and young people, published in 2021, identified seven longitudinal studies conducted primarily in the Netherlands, North America, and the United Kingdom [11]. The review found that children who show gender non-conforming behaviors in early childhood tend to maintain them over time, suggesting a certain stability of these identity expressions [11].

The TransYouth Project

Among the most relevant studies in this field is the TransYouth Project, launched in 2013 by psychologist Kristina Olson, the first large-scale longitudinal study on transgender children who had undergone a social transition. The study recruited over 300 transgender children aged 3 to 12, with the aim of following them for 20 years.

The results published in 2022, relating to a 5-year follow-up, provided significant data [6]. After an average of 5 years from the initial social transition, 94% of the young people continued to identify as a binary transgender person [6]. Only 2.5% identified as cisgender and 3.5% as non-binary [6]. The study also found that the gender development of socially transitioned transgender children is similar to that of their cisgender peers: transgender girls show preferences for games and clothing statistically indistinguishable from those of other girls, and the same is true for transgender boys compared to other boys [6].

A particularly relevant finding concerns mental health: transgender children supported by their families and living in accordance with their gender identity show levels of depression comparable to those of cisgender peers and only slightly higher levels of anxiety, with good overall self-esteem [6].

Biological factors

Research on the biological bases of gender identity has highlighted the role of multiple factors [5]. Prenatal exposure to sex hormones, particularly testosterone, appears to influence the development of gender identity. Studies on people with variations of sex development (DSD) have provided evidence in this regard, although no single biological factor is sufficient to explain the totality of the observed variance.

A systematic review of the twin literature, published in 2024, confirmed that both genetic and environmental factors contribute to gender diversity [13]. The genetic contribution to gender-related behaviors ranges from 0.10 to 0.77, while the contribution of non-shared environment ranges from 0.15 to 0.75, indicating a complex interaction between nature and environment [13].

Gender dysphoria and gender incongruence

Two of the major diagnostic classification systems worldwide, the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders) and the ICD-11 (International Classification of Diseases), adopt different approaches in describing the experience of people whose gender identity differs from the sex assigned at birth.

Gender dysphoria in the DSM-5

The DSM-5, published by the American Psychiatric Association in 2013 and updated in 2022 with the DSM-5-TR, uses the term gender dysphoria [8]. This term replaced the previous “gender identity disorder” used in the DSM-IV, marking a first step toward depathologization [14]. Gender dysphoria is defined as a marked incongruence between the experienced or expressed gender and the assigned gender, lasting at least 6 months, accompanied by clinically significant distress or impairment of social, occupational, or other important areas of functioning [8].

The DSM-5 maintains gender dysphoria within the classification of mental disorders, albeit with the clarification that gender incongruence itself does not constitute a disorder: it is the associated distress that represents the clinically relevant element [8].

Gender incongruence in the ICD-11

The ICD-11, adopted by the World Health Organization (WHO) in 2019 and entered into force in 2022, represents a more radical change [1]. The term gender incongruence replaced the previous “transsexualism” and the “gender identity disorder” of the ICD-10. The most significant change is that gender incongruence was removed from the chapter on mental and behavioral disorders (Chapter 6) and placed in the new Chapter 17, titled “Conditions related to sexual health” [1].

The ICD-11 defines gender incongruence as a marked and persistent incongruence between the experienced gender and the assigned sex, without requiring the presence of distress or functional impairment as a diagnostic criterion [1]. This choice reflects the WHO’s position that transgender and gender-diverse identities are not conditions of mental illness.

The depathologization process

The shift from considering transgender identities as mental disorders to recognizing them as natural variants of human experience has been a gradual process [14]. Already in 2016, a study published in World Psychiatry provided the rationale for the reclassification in the ICD-11, observing that gender incongruence does not meet one of the fundamental requirements for the definition of a mental disorder, namely the presence of distress or dysfunction caused by the condition itself [7]. The distress experienced by many transgender people, the researchers argue, is largely the product of social stigma, discrimination, and lack of access to appropriate care, rather than an intrinsic characteristic of gender incongruence [7].

The maintenance of a diagnostic category in the classification systems, despite the removal from the mental disorders section, is motivated by the need to ensure access to healthcare services [1]. Without a recognized diagnostic code, transgender people would risk being denied health coverage for treatments such as hormone therapy or gender-affirming surgeries.

What major institutions say

The major international scientific and health institutions have taken increasingly clear and convergent positions on gender identity in recent years.

World Health Organization (WHO)

The WHO, with the reclassification of gender incongruence in the ICD-11, took a significant step toward the recognition that transgender identities are not mental disorders [1]. The WHO has also stated that maintaining a diagnostic category is necessary to recognize the links between gender identity, sexual behavior, exposure to violence, and sexually transmitted infections, while ensuring access to healthcare services [1].

American Psychological Association (APA)

The APA adopted in 2021 a historic resolution on gender identity change efforts (GICE), approved with the support of over 95% of representatives [3]. The resolution states that transgender or non-binary identities are “normal variations of human gender expression” and opposes the dissemination of inaccurate information about gender identity, including the claim that it can be modified through treatments [3].

In 2024, the APA further strengthened its position with a policy statement supporting unhindered access to evidence-based healthcare for transgender, gender diverse, and non-binary individuals of all ages, stating that such care is supported by a robust body of scientific evidence [4].

Endocrine Society

The Endocrine Society published in 2017 the clinical guidelines for the endocrine treatment of people with gender dysphoria or gender incongruence, supported by over 260 research studies [9]. The guidelines recommend a conservative approach, with no medical interventions before puberty. With the onset of puberty, the guidelines provide for the possibility of using puberty blockers, considered generally reversible, followed by hormone therapy in later stages, when the adolescent is able to provide informed consent [9].

In 2024, the Endocrine Society reaffirmed its support for gender-affirming care with an official statement, emphasizing that therapeutic decisions should be made on a case-by-case basis within a doctor-patient relationship, based on the best available scientific evidence [10]. An update to the 2017 guidelines is currently underway.

American Academy of Pediatrics (AAP)

The American Academy of Pediatrics has supported an affirmative approach toward transgender youth, highlighting the importance of family support and a welcoming environment for the psychological well-being of minors with a gender identity different from the sex assigned at birth.

Current state

The understanding of gender identity is continuously evolving. The last decades have seen significant progress both in scientific research and in the social and legal recognition of transgender and gender-diverse people. However, open questions remain and areas require further investigation.

Advances in scientific understanding

Research has established that gender identity has biological foundations, while being the result of a complex interaction between genetic, hormonal, neurological, and environmental factors [5][13]. No single factor has been identified as the exclusive determinant, which is consistent with the complexity of many other human characteristics. The depathologization of transgender identities in the major diagnostic classification systems represents a paradigm shift, based on scientific evidence that these identities are not inherently associated with psychological distress or functional impairment [1][7].

Open questions

Despite the progress, several aspects of gender identity remain subjects of research and scientific debate. The specific biological mechanisms contributing to the development of gender identity are still only partially understood. The interactions between genetic, epigenetic, prenatal hormonal, and environmental factors require further study. Longitudinal research, while growing, is still limited by relatively small samples and significant dropout rates.

The debate on care pathways for minors with gender incongruence is particularly active. While the major scientific societies support an individualized and evidence-based approach [9][10], several countries are revising their guidelines, and the scientific community recognizes the need for further long-term studies on the outcomes of different therapeutic approaches.

Another area of development concerns the understanding of non-binary identities, a relatively young field of research that is producing increasingly consistent data but still needs large-scale and long-term studies [12].

Toward a more inclusive understanding

The evolution in the understanding of gender identity reflects a broader process of integration between biological, psychological, and sociocultural evidence. The convergence of the major scientific and health institutions toward the recognition of gender diversity as a natural part of human experience represents an important reference point, while acknowledging that research in this field is constantly developing and that new evidence may enrich and refine the current understanding.