Recorded accounts of men and women displaying cross-gender behavior date back to biblical times. Indeed, the Old Testament expostulated against such displays [Deuteronomy 22:5], and Ovid, a first-century BC poet, referred in verse to the extract of “stuff from a mare in heat,” a reference to conjugated estrogen [4]. Nevertheless, the phenomenon was unknown in the Western world prior to the middle of the twentieth century.

Although accounts of sex-reassignment surgeries were published in Germany as early as 1930, it was not until 1952 when the Danish surgeon Paul Fogh-Andersen employed an innovative surgical technique on Christine Jorgensen, a US citizen, that media reports of “sex-change surgery” captured the public’s attention [5]. Harry Benjamin, an endocrinologist, is credited with identifying the condition and in 1966 published the book The Transsexual Phenomenon, prompting some surgeons to perform the procedure. Thus, a taboo area of human behavior became a medical specialty [6].

Opposition to surgery arose rapidly, in tandem with Ms. Jorgensen’s fame. While some surgeons in Europe, Mexico, South America, and the USA were willing to perform reassignment surgery, most hospitals prohibited the procedure. The “Christine operation” generated legal controversies, religious opposition, and moral outrage. But it was the psychiatric community that was the most vociferous in challenging the legitimacy of transsexualism [6].

The debate centered on the etiology of gender dysphoria. The psychoanalytic model, which prevailed in Europe and the USA, regarded transsexualism as a psychiatric—not a biologic—disorder. The psychiatric community claimed the desire for reassignment surgery was a delusion, an obsession, or a symptom of severe psychopathology. The symptomatology arose from serious object relation disturbances: an inability to separate on the part of both mother and child giving rise to dysregulation of intrapsychic distance for the patient and an attempt to incorporate an alternate persona [7–11]. Some conceptualized this as a psychotic disorder [12]; others viewed it as a form of borderline psychopathology [13]. Lothstein opined that the transsexual was unconsciously motivated to “discard bad and aggressive features” and create in the fabric of the body “a new idealized perfection” [13].

The tenacity of the psychodynamic model, the inflammatory rhetoric of the debate, and the conflation of transsexualism, homosexuality, and transvestism left science and research trailing behind negative public perception. Many transsexual people requesting medical and surgical treatments were committed to mental institutions. The abject failure of the “talking cure” was attributed to resistance on the part of the patient, not to the failure of psychoanalysis [14]. Tragically, electroshock and aversion therapies were too often the default treatments.

Some researchers rejected the psychoanalytic explanation and continued to pursue a biological basis for the condition to “help to replace emotional controversy by rational assessment of facts” [15]. Early attempts to find an organic etiology included roentgenological examination of the skulls of transsexual patients [16], testing for anomalous hormonal milieus [17, 18], cytotoxicity assay inspection of h-y antigen status [19–22], and quantitative frequency EEG analysis. Despite the failure of these attempts to identify an organic marker of transsexualism, some investigators remained convinced that hormonal-dependent structural brain changes, although not documented, were a likely explanation [23].

Interestingly, support for a biological basis came from an unlikely source, namely, clinical psychologists [6]. By utilizing reliable and objective psychological tests, they could substantiate, or fail to substantiate, the claim that gender-dysphoric patients had rampant psychopathology. Several such studies indicated that applicants for sex reassignment showed “a notable absence of psychopathology” [24–27].

By the 1990s, significant numbers of people had sought medical interventions, and thus a “database” of historical and psychological information could be assessed. While all such persons suffered similar distress due to gender incongruity, they had no commonality in regard to biographical indices, childhood abuse, or trauma. There was no evidence to conclude that environment or parenting practices accounted for the development of the condition [28].

Dramatic advances in brain-imaging capacity led to more sophisticated theories of the etiology of atypical gender identity. By the late 1990s, some researchers proffered a model known as “gender transposition” as the underlying principle. Based largely on evidence from animal studies demonstrating a link between steroid hormones, brain structure, and sexual behavior [29], this theory proposed that transsexualism occurred as a result of a switch of hormone-induced cephalic differentiation at a critical gestational point [30–32]. The theory, though appealing, proved too reductionistic. It suffered from the conflation of behavior and identity, its weighty reliance on animal study extrapolation, and the erroneous assertion of a fixed critical period anchoring sexual differentiation in the fetus [33, 34].

In 1995, one study broke new ground. Zhou et al. reported differences in autopsied brains of male-to-female transsexual persons in the bed nucleus of the stria terminalis (BSTc), an area of the hypothalamus central to sexual behavior. The examined male-to-female transsexual brains had a volume of the central sulci of the stria terminalis that was comparable to those of genetic females and unlike both heterosexual and homosexual male brains, which had greater volume [35]. The study generated a great deal of publicity, as it lent support to the conviction held by many: that a structure deep within the brain might hold the answer to complex areas of identity. It also resonated with the layperson’s characterization of the transsexual person as someone who insists they have a female brain in a male body [36].

But the study raised additional questions. What if the volume differences found in the BSTc were artifacts resulting from contrary hormone use? A subsequent study was designed to address this issue. Krujiver et al. quantified the number of somatostatin (SOM) neurons in the BSTc, rather than volume. Neuron numbers of heterosexual males, homosexual males, heterosexual females, male-to-female transsexuals, male and females with sex hormone disorders, a female-to-male transsexual, and an untreated individual with gender dysphoria were compared. The findings were consistent with earlier results. Regardless of sexual orientation or adult hormone usage, there was a difference in the SOM neuron number in the human BSTc. Not only did the male-to-female postmortem brain tissue have levels corresponding to that of genetic females, but the opposite pattern was displayed in the female-to-male transsexual brain tissue [37].

A proliferation of studies arose in the past decade, due in no small part to technological advances and the expansion of databases. Now, meta-analyses could take place, combining data from several countries. Building on the previous brain structure studies, an assemblage of studies evinced evidence in support of biological theories, none of which were necessarily mutually exclusive, each seemingly contributing a piece of a yet unsolved puzzle.

The previous significant finding of a sexually dimorphic subdivision of the BSTc bolstered support for the hypothesis that gender identity develops as a result of the interaction of a developing brain and sex hormones. However, in 2002, Chung et al. made the surprising discovery that the BSTc volume did not become apparent until adulthood. The late occurrence of sexual dimorphism in the BSTc had yet to be explained. The authors suggested that perhaps long before the difference in BSTc volume became manifest, changes in fetal hormone levels, neuronal activity, or differentiation were paving the way for structural change, which would appear at a later stage of life [38].

Pol et al. used magnetic resonance brain imaging to compare total brain volume before and after hormone treatment. They found that estrogen usage decreased brain volumes in male-to-female transsexual subjects causing them to fall in the range of female proportions. Androgen treatment in female-to-male transsexual subjects increased volume, causing the brain morphology to replicate male proportions. The authors concluded that sex steroid hormones are vital in the maintenance of sexual dimorphic brain organization throughout life [39].

In 2008, a region of the brain was identified that also appeared to be related to transsexualism, namely, the interstitial nucleus of the anterior hypothalamus (INAH3) [40]. In 2012, a study of postmortem human brain tissue determined that the gene-encoding neurokinin B (NKB) in the infundibular nucleus (INF) is sexually dimorphic. In children, both sexes had equivalent levels of NKB immunoreactivity, but adulthood brought dimorphism. As with the BSTc, there was a reversal in transsexual brain tissue, indicating that gonadotropin-releasing harmone (GnRH) secretion is regulated via estrogen feedback and that a mutation in the NKB produces gonadotropin deficiencies. Clearly, the stage was set to implicate hormones, genes, and cephalic structure in the formation of gender identity [41].

The theory that early influences, possibly prenatal, were the precursors for later structural brain change was gaining traction. Dessens et al. reported an elevated incidence of transsexual offspring in women exposed to phenobarbital and diphenylhydantoin [42]. As phenobarbital enhances liver function, it was widely used in many countries as a prophylactic treatment of neonatal hyperbilirubinemia, prior to the use of phototherapy, and it caused a rise in postnatal testosterone. This demonstrated that certain substances could alter steroid hormone levels [43].

Simultaneously, there was rampant evidence that environmental assaults were causing endocrine disruption in wildlife. Disturbing evidence that synthetic chemicals can disrupt sex steroids and the complexity of feedback loops was mounting. Animal researchers exposed the extreme sensitivity of developing mammals to very slight shifts in hormone levels in the womb stating “hormones permanently organize or program cells, organs, the brain, and behavior before birth, in many ways setting the individual’s course for an entire lifetime” [44]. Could this explain the escalating incidence of disorders of sexual development, infertility, hypospadias, cryptorchidism, double uteruses, blind vaginas, and other disorders in the human population [45]?

Diethylstilbestrol (DES), the most studied endocrine disruptor, has been implicated in numerous health problems in female offspring of exposed women [46]. Curiously, few studies have examined the impact on male offspring, the DES sons. An online forum, DES Sons International, conducted a survey of members. Of 500 respondents, 90 members indicated they were transsexual; 48 described themselves as transgender; 17 identified themselves as “gender dysphoric”; and 3 identified themselves as “intersex.” By 2004, more than 130 individuals had joined a forum called “DES Trans” [44]. Clearly, the prevalence of gender dysphoria in persons exposed to DES warrants further study.