Gender identity refers to a person’s fundamental sense of their own gender. For most children, an awareness of core gender identity (one’s sense of inner sexual identity) begins to develop early in life and is usually firmly in place by 3 years of age. For other children, this sense may not develop in the same way or may become clear only with age. Gender identity is distinct from sexual orientation, gender expression, and gender role. Genetic sex is determined at the moment of conception, although, as noted in the following section, even this can be complex. Once they are born, babies start to tell men and women apart from early in life. The term gender identity is used for the child’s personal sense of their own gender, although gender expression does not always mirror this sense. Even when gender expression differs from gender identity, this may not reflect the core gender sense or their gender role. Some of the terms used with reference to gender identity and gender roles are defined in Table 20.1.

Historically, interest in sexual development and its patterns of growth had a major impetus from the work of Freud and his theories of sexuality (1905/1962), particularly when it was repressed and resulting in the pathogenesis of mental illness. Although variations of sexual attraction were widely recognized and accepted in ancient times, more recent Western views, until recently, regarded nontraditional patterns of sexual attraction as a mental disorder and/or as a criminal act. Indeed, until 1973, when it was removed from the DSM, the American Psychiatric Association regarded homosexuality as a disorder (Drescher, 2015). This removal followed the pioneering work of Kinsey and colleagues (2003/1948, 1953) showing more flexibility in sexual roles than previously had been appreciated. Increased activism on the part of the LGBTQ community followed the Stonewall riots (Duberman, 1993) and began the movement toward increased acceptance for a range of sexual expression. Greater acceptance also began to reflect important contributions of scholarship on cultural, sociological, and psychological aspects of sexuality (Foucault, 1979/1976), the growing work on women’s studies (Friedan, 1963), as well as an interesting body of work on patterns of gender identity and role development that are discussed subsequently.

In the late 1940s and early 1950s, the work of Kinsey on the sexual development of males and females began to question the simplistic dichotomy views of sexual orientation. John Money (1988) was a pioneer in the field of gender studies. Money introduced concepts like gender roles, sexual orientation, and gender identity. He studied the development of children born with ambiguous genitalia because of medical or biological factors and the impact of rearing as a male or female in contravention to biological gender. He also was an early investigator of the impact of gender change through surgery. There became a broader awareness of psychological, sociological, and cultural factors in sexual roles and identity; greater acceptance of the range of sexual orientations and identifications began to develop. By the end of 1990s, there was growing interest in the issue of supporting treatments (including hormonal treatments and surgical approaches) for individuals desiring to change their gender identity through medical means. It is important to note that sexual identity and gender identity are distinct concepts. Gender identity refers to the person’s sense of self as female, male, neither, or both whereas sexual identity refers to whom one is sexually attracted, that is, sexual orientation identity.

The conceptualization and classification of gender-related issues have significantly evolved in the various editions of DSM. By the time of the DSM-IV-TR (APA, 2000), the concept of gender dysphoria was explicitly recognized as a disorder; this recognition was, in part, used to justify insurance coverage of medical/surgical treatments for individuals who desired gender change. Criteria for the condition as a disorder (gender identity disorder) in DSM-IV-TR included cross-gender identification that was “strong and persistent” and not simply a result of some presumed advantage to being the other sex. This identification was also associated with continued discomfort about the person’s assigned gender/gender role as well as impairment/distress. The condition was not diagnosed if a physical intersex condition was present. For children with this diagnosis, multiple features of gender identity had to be present (crossdressing, a stated desire, participation in stereotyped activities of other sex, persistent choice of other sex role in play, etc.).

In contrast to the DSM-IV-TR, in DSM-5 (APA, 2013) the condition was renamed gender dysphoria (rather than disorder) and moved to its own category (i.e., out of the sex disorders group). The new name was meant to destigmatize the condition. A separate category for children was included and meant to reflect the presumption of a lesser degree of certainty in developing children about their gender identification. Again, intersex individuals were excluded from this diagnosis, although many would actually receive a diagnosis of unspecified gender dyphoria. Criteria for the category of gender dysphoria in adolescents and adults require at least two of a series of five features, all of which have to do with incongruence between the person’s experience and their sexual characteristics, desire for the sexual characteristics of the other gender, and strong feelings that one’s reactions are more consistent with those of the other gender. It is common for individuals with gender dysphoria to identify as transgender. Some individuals object to this feeling because it tends to be an overly simplistic approach to gender classification. Some groups have suggested the use of terms like gender diversity, nonbinary, or genderqueer to convey the complexity of classification of gender.

Given the various changes in diagnosis and classification, epidemiologic data are rather limited. Often data are derived from work with adults seeking medical treatment. One meta-analysis of this group found rates of 6.8 for transwomen per 100,000 and 2.0 per 100,000 for transmen (Arcelus et al., 2015). One study reported overall rates of about 0.6% of adults who identify as transgender (Turban et al., 2018). In studies of children, this number is larger: between 1% and 2% typically identify as either transgender or unsure (Clark et al., 2014; Shields et al., 2013).

The early focus on psychological and sociocultural factors has shifted over time to a broader focus including neurobiology and other factors. A strong heritability component has been suggested by studies comparing fraternal and identical twins. These suggested a very strong genetic component—usually on the order of 60% to 70% of variance accounted for (Coolidge et al., 2002) and some environmental contribution as well. Differences in brain structure and function relative to behavior have also been shown in animals, and in humans in cisgender males and females (Hines, 2020). Differences are observed in overall brain volumes with males having greater volume and females having more gray matter when this volume difference is taken into context (Giedd et al., 2012; Guillamon et al., 2016; Ruigrok et al., 2014). The sexual differentiation hypothesis has posited a difference in transgender individuals suggesting that their brain structures and function may be more like that of the gender they experience (Swaab & Garcia-Falgueras, 2009). Results of neuroimaging studies have been mixed, although functional neuroimaging has suggested similarities to experienced gender (Turban et al., 2018). Sex hormones have an important role both prenatally and during puberty on development of the physical body as well as of brain structures. In animals, there are very clear effects of prenatal testosterone on behavior (Hines, 2011). In humans, there is some suggestion of higher levels of gender dysphoria and cross-gender identification in relation to specific medical problems (Jürgensen et al., 2013).

Genetic females with congenital adrenal hyperplasia have higher rates of gender dysphoria and cross-gender identification (Pasterski et al., 2015), although most of those reared as females develop a female gender identity (Dessens et al., 2005). Individuals who are XY but have complete androgen insensitivity develop a female gender identity in most cases underscoring the role of testosterone in male gender identity (Mazur, 2005).

Although there is clearly some role for environmental (social and psychological) factors, the literature on this topic has provided mixed and somewhat conflicting results (Zucker & Bradley, 1995), and in any case the topic remains controversial.

Understanding the clinical course of children who are transgender or gender nonconforming is an area of very active research (Olson, 2016). Much of the follow-up data available are based on earlier concepts of gender issues (particularly binary gender identification with almost no research on nonbinary identification) that were used prior to the DSM-5 view, thus complicating the interpretation of available data in light of current diagnostic models. Important questions remain to be answered; for example, data on whether or not those who are gender nonconforming as children persist with this identification are somewhat contradictory with some showing differences between males and females whereas others do not (Turban et al., 2018). Several factors have been related to persistent gender nonconformity (Green, 1987; Steensma et al., 2013; Zucker et al., 2012). These complexities make it tremendously difficult to provide simple generalization for the individual as to whether or not cross-gender identification will persist from childhood into adolescence and beyond (Steensma et al., 2013). The data are reasonably clear that persistence of transgender identification from adolescence into adulthood is usual (Cohen-Kettenis & Pfäfflin, 2003). Many children go on to identify as cisgender and gay, although again it is important to emphasize that gender identification is not the same as sexual orientation. Most of these children will grow up to become adults who have an identity as cisgender persons and will have a same-sex or bisexual orientation (Green, 1987; Wallen & Cohen-Kettenis, 2008). Children growing up with gender incongruence exhibit higher rates of both internalizing and externalizing problems. It is important to note the relevance of the gender minority stress model that underscores the experiences of stressful experiences in the lives of children, youth, and adults who are trans and gender diverse people (Tan et al., 2020). For example, rates of bullying are high and peer problems predict associated behavioral and mental health difficulties (McGuire et al., 2010; Steensma et al., 2014). Anxiety problems appear to be particularly common followed by mood and disruptive disorder (de Vries et al., 2016). Higher rates of suicide and self-harm have also been noted (Olson et al., 2015). There is also some suggestion of increased number of transgender individuals in youth with autism spectrum disorder (ASD)—perhaps as many as 10% to 20% of those with ASD (Jones et al., 2012; Pasterski et al., 2014)—although this has been much debated. Several potential explanations for this have been proposed (Strang et al., 2018; van der Miesen et al., 2016). Clinical management and diagnostic issues can be complicated in the ASD population given social-communication problems and rigidities in thinking and behavior (Parkinson, 2014). Some studies have focused more exclusively on children and youth presenting to specialized gender identity clinical programs and these typically have noted even higher rates of psychiatric comorbidities, particularly mood and anxiety problems, and, to a lesser extent, disruptive behavior disorder (Skagerberg & Carmichael, 2013). Clearly, awareness of potentially associated problems is an important part of clinical management.