Gender identity disorder (GID), recently renamed gender dysphoria (GD), is a rare condition characterized by an incongruity between gender identity and biological sex. Clinical evidence suggests that schizophrenia occurs in patients with GID at rates higher than in the general population and that patients with GID may have schizophrenia-like personality traits. Conversely, patients with schizophrenia may experience alterations in gender identity and gender role perception. Neurobiological research, including brain imaging and studies of finger length ratio and handedness, suggests that both these disorders are associated with altered cerebral sexual dimorphism and changes in cerebral lateralization. Various mechanisms, such as Toxoplasma infection, reduced levels of brain-derived neurotrophic factor (BDNF), early childhood adversity, and links with autism spectrum disorders, may account for some of this overlap. The implications of this association for further research are discussed. 1. Introduction Gender identity disorder (GID) is a rare condition characterized by an incongruity between gender identity and biological sex. Individuals with this condition experience distress related to their biological sex and frequently express a desire to change it by hormonal or surgical means; in simple terms, they identify themselves as belonging to the opposite gender, and behave accordingly. GID is distinct from disorders of sexual development, such as chromosomal abnormalities or congenital adrenal hyperplasia, in that there is no anatomical or physiological ambiguity regarding the individual’s sex; rather, there is a subjective dissatisfaction with it [1, 2]. The central role played by this dissatisfaction and distress has led to the condition being renamed “gender dysphoria” (GD) in the DSM-5, though the term “gender incongruence” (GI) has also been proposed by some authors [1]. Various theories have been proposed to account for GID/GD and related conditions, known as gender identity variants (GIVs) [3]. Abnormalities in genes related to gonadal hormone synthesis and action have been found in these individuals, suggesting that GID/GD is a disorder of sexual brain differentiation caused by exposure to altered hormone levels during a sensitive period [2, 3]; however, evidence for such a viewpoint has not been consistently found, and it fails to account for patients with late-onset GID who have few or no symptoms during childhood [4]. Alternately, GID has been conceptualized as a disorder of cortical representation of sex-specific bodily features, particularly
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