4
Biology and Sexual Minority Status
William Byne
1 Introduction
The purpose of this chapter is to provide clinicians with an overview of current knowledge pertaining to the biology of sexual minority status. Under the umbrella of sexual minority are included homosexu- als, bisexuals, transgenders and intersexes. The most developed bio- logic theory pertaining to sexual minority status is the prenatal hormonal hypothesis. According to this hypothesis, prenatal hormones act (pri- marily during embryonic and fetal development) to mediate the sexual differentiation not only of the internal and external genitalia but also of the brain. The sexually differentiated state of the brain then influ- ences the subsequent expression of gender identity and sexual orien- tation. Intersexuality results from variation in the normative course of somatic sexual differentiation, and homosexuality and bisexuality have been proposed to reflect variant sexual differentiation of hypothetical neural substrates that mediate sexual orientation. Similarly, transgen- derism has been conjectured to reflect variant differentiation of hypo- thetical neural substrates that mediate gender identity. Some of the same hormones and hormonal receptors mediate the sexual differenti- ation of both the brain and the genitalia. Thus, the brains, as well as the genitalia, of intersexes may exhibit sexual differentiation that is intermediate between that of normatively developed males and females.
The chapter begins with clarification of terminology and then an overview of the genetics and neuroendocrinology of sexual differenti- ation. The prenatal hormonal hypothesis is then elaborated and evalu- ated in light of current evidence. Genetic and other salient biologic evidence is then summarized. Models are examined for considering how biologic factors, in concert with experiential factors, might influ- ence sexual minority status.
2 Terminology
In this chapter, sex refers to the status of biologic variables that can be described as either male-typical or female-typical in normatively developed individuals (e.g., genes, chromosomes, gonads, internal and
external genital structures, hormonal profiles). Particular features of the human brain also appear to be sexually dimorphic, at least in a statis- tical sense, and should perhaps also be considered among the variables of sex (Collaer et al., 2003). Gender refers to social categories (e.g., man or woman, boy or girl) or to factors related to living in the social role of a man or a woman. Gender identity refers to one’s sense of belong- ing to the male or female gender category, whereas gender role refers to behaviors (e.g., mannerisms, style of dress, activities) that convey to others one’s membership in one of those categories (Money &
Ehrhardt, 1972). Sexual orientation refers to one’s pattern of erotic responsiveness and is described here as androphylic (attracted to men), gynephilic (attracted to women), or bisexual (attracted to both).
The course of normative development culminates in full concor- dance among all of the biologic variables of sex (i.e., either all male or all female). In intersexed individuals, however, one or more of those variables is discordant with the others, or its differentiation is inter- mediate between male and female norms. The fact that gender identity and role may be discordant with one or more of the biologic variables of sex underscores the social basis of gender categories.
Intersex has become the preferred term to encompass a variety of syndromes previously classified on the basis of gonadal histology as true hermaphroditism, in which both testicular and ovarian tissue are present in a single individual, and pseudohermaphroditism, in which only one type of gonadal tissue is present. In that system of taxonomy, prece- dence was given to gonadal histology as the arbiter of “true sex” upon which gender assignment should be based. With the advent of kary- otype analysis, chromosomal sex became viewed as the arbiter of “true sex” (Zucker, 1999). When the sexual variables are not fully concordant in a given individual, there is no reason to insist that one variable should hold precedence over the others. Instead, the status of each vari- able must be stated to describe accurately the sex of the individual. In the Diagnostic and Statistical Manual of Mental Disorders (4th edition), the presence of an intersex disorder excludes the diagnosis of a gender identity disorder (American Psychiatric Association, 1994).
3 Overview of Sexual Differentiation
The mammalian embryo is initially sexually bipotential (Collaer et al., 2003; Arnold et al., 2004). During the usual course of male differentia- tion a testis determining gene, SRY, which is normally on the Y chro- mosome, directs the development of testes from the fetal gonadal precursor. Subsequently, testicular secretions orchestrate differentia- tion of the male genitalia and brain. Initially, both male and female embryos possess two sets of primordial internal genital duct systems:
one (the müllerian, or paramesonephric, duct system) is capable of developing into female internal genital structures and another (the woffian, or mesonephric, duct system) is capable of developing into the male internal genitalia. A secretion from the testis, müllerian inhibitory substance, induces regression of the müllerian (i.e., female)
duct system, and the 5α-reduced derivative of testosterone, 5α- dihydrotestosterone (DHT), stimulates the development of male inter- nal genital structures. DHT also stimulates both the growth and dif- ferentiation of the embryonic phallus into a penis and fusion of the labioscrotal folds to form the scrotum into which the testes later descend. In the absence of testes or müllerian inhibitory factor, the internal female genital system fails to regress. In the complete absence of testosterone, the 5α-reductase enzyme that converts it to DHT, or functional androgen receptors, the male internal genital structures fail to develop, the phallic rudiment develops into a clitoris rather than a penis, and the labioscrotal folds develop into labia instead of a scrotum.
Intermediate levels of androgenic exposure result in intermediate development of internal male genital structures and differentiation of the external genital structures that are intermediate between those of normatively developed males and females.
Work in laboratory animals suggests that sexual differentiation of the brain is analogous to sexual differentiation of the internal genitalia, where separate male and female primordia are involved, and thus fully developed male and female structures can theoretically exist in the same individual. Extending this analogy, sexual differentiation can be conceptualized as involving processes of defeminization (i.e., sup- pression of female characteristics—analogous to regression of the müllerian ducts) and masculinization (i.e. the development of male characteristics—analogous to development of the male internal geni- talia). In rats, the most studied aspects of brain defeminization include suppression of the brain’s potential to mediate a stereotypically female mating posture called lordosis, and its ability to orchestrate the neuroendocrine response necessary for normal ovarian function. Both defeminization and masculinization of the rodent brain are brought about by testosterone and its derivatives. Testosterone acts on the brain by two primary pathways: (1) an androgen pathway in which either testosterone or DHT interacts with androgen receptors on target cells and (2) an estrogen pathway in which testosterone is converted to estrogen by aromatase enzymes in the brain. In the latter pathway the brain-derived estrogen interacts with estrogen receptors. In laboratory rodents, the androgen pathway contributes to masculinization of the brain, and the estrogen pathway contributes to both defeminization and masculinization (Goy & McEwen, 1980; Olsen, 1983). In addition to having different hormonal requirements, animal work suggests that the various aspects of somatic and brain sexual differentiation occur during different periods of development in a sequence of temporally overlapping steps (Goy & McEwen, 1980; Byne & Kemether, 2000). In the absence of the cascade set in motion by the testis-determining gene, female development ensues, at least to a first approximation.
3.1 Timing of Sexual Differentiation in the Human
Human testes begin to secrete androgens by the seventh or eighth week of gestation (Siiteri & Wilson, 1974), a process initially regulated by human chorionic gonadotropin secreted by the placenta (Moore, 1982).
By the 15th week of gestation the regulation of androgen secretion is taken over by gonadotropin from the fetal pituitary, which is regulated by the fetal hypothalamus. Genital differentiation occurs largely during the period when androgen secretion is regulated by the placenta rather than by the fetal pituitary. Gonadotropin secretion decreases toward the end of gestation presumably due to the development of inhibitory inputs to the hypothalamus in addition to the onset of negative feed- back of androgen on gonadotropin release. Thus, fetal androgen in males is elevated between weeks 8 to 24 of gestation, with peak levels occurring between weeks 14 and 16 (Smail et al., 1981). In males, the level of testosterone increases from birth to a peak at 1 to 3 months and then decreases to prepubertal levels by ages 4 to 6 months (Hrabovsky
& Hutson, 2002). The ovary is believed to be relatively quiescent pre- natally but secretes substantial levels of estradiol during the first 6 to 12 months after birth. A sharp reduction of gonadal activity then occurs in both sexes until 10 to 12 years of age when sex-characteristic adult hormonal profiles emerge and trigger the development of secondary sexual characteristics. Thus, hormonal influences could conceivably influence psychosexual differentiation prenatally (8 to 24 weeks of ges- tation), during the first 6 to 12 months postnatally, and again at puberty.
4 Prenatal Hormonal Hypothesis
4.1 Sexual Orientation
From the turn of the century into the 1970s, a popular hypothesis held that the amount of androgens or estrogens in the bloodstream of adult men and women might influence or determine their sexual orientation.
That hypothesis is no longer viewed favorably because most studies failed to demonstrate a correlation between sexual orientation and adult hormone levels (Meyer-Bahlburg, 1977, 1984). In fact, androgens have been found to increase libido in adults of both sexes but not to alter sexual orientation (Glass & Johnson, 1944; Sherwin, 1991). Simi- larly, alterations in adult hormone levels resulting from gonadal malig- nancies, trauma, or surgical removal do not alter sexual orientation (Gooren, 1990).
Research currently focuses on the potential role of prenatal hormonal influences on the brain. The prenatal hormonal hypothesis posits that:
(1) the brains of heterosexual men and women differ from each other both structurally and functionally; (2) those differences result from early hormonal influences on the developing fetus; and (3) sexual ori- entation is derivative of a hormonally mediated developmental process leading to sexual differentiation of the brain. Consequently, the brains of homosexual individuals are expected to exhibit characteristics that would be considered more typical of the other sex or intermediate between male and female norms (Byne & Parsons, 1993). The expecta- tion that sexual minorities should have brains that are in some ways intermediate between those of normatively developed heterosexual men and women may be referred to as the “intersex hypothesis of homosexuality, bisexuality, and transgenderism.”
The “prenatal hormonal hypothesis” draws upon observations of rodents in which the balance between male and female patterns of mating behaviors is strongly influenced by the amount and timing of early androgen exposure (Meyer-Bahlburg, 1984; Gooren, 1990; Byne &
Parsons, 1993). The period of maximal sensitivity to these organizing effects of androgen varies from one species to the next (Goy & McEwen, 1980). The rat has been employed extensively in such research because the period of brain sexual differentiation extends into the early postnatal period (which corresponds to the midtrimester of human gestation). Thus, the hormonal exposure of the rat’s brain can be experimentally manipulated by perinatal gonadectomy and injection of various hormones.
It is problematic to make assumptions about human sexual psy- chology based on extrapolations from rodent behaviors caused by experimental endocrine manipulations. For example, a neonatally cas- trated male rat that shows lordosis—a receptive posture to permit mounting—when mounted by another male is sometimes considered homosexual, as is the perinatally androgenized female rat that mounts others. The male that mounts another male is sometimes considered heterosexual, as is the female that displays lordosis when mounted by another female. Thus, in this particular laboratory paradigm, sexual orientation is defined in terms of specific behaviors and postures. In contrast, human sexual orientation is defined not by the motor patterns of copulation but by one’s pattern of erotic responsiveness and the gender of one’s preferred sex partner.
Because of the problems when equating rodent mating behavior with human sexual orientation, researchers have begun to employ a variety of strategies to assess partner preference in animals. This is sometimes done by seeing whether a test animal chooses to approach a male or a female stimulus animal placed in opposite arms of a T-maze (Paredes
& Baum, 1995). Although some unaltered laboratory animals sponta- neously direct most of their sexual behaviors toward their own sex (Bagemihl, 1999), animal studies of sexual orientation are usually carried out on animals that have been experimentally manipulated (Hennessey et al., 1986; Paredes and Baum, 1995). For example, a genet- ically male rodent may either be castrated as a neonate, depriving his developing brain of androgens, or particular androgen-responsive regions of his brain may be destroyed. To activate the display of female- typical behaviors and preferences in such male animals, estrogen injec- tions are also required during adulthood. Because adult homosexual men and women have hormonal profiles that are indistinguishable from those of their heterosexual counterparts, it remains unclear how findings based on such hormonally abnormal animals pertain to human sexual orientation.
4.2 Gender Identity
Early hormonal exposure is widely believed to influence subsequent gender identity (Zhou et al., 1995). This possibility is potentially rele- vant not only to the etiology of transgenderism but also to early gender
assignment in intersexes. Among male-to-female transgenders who elect sex-change surgery, most are sexually attracted primarily to men;
but a substantial minority (perhaps 40%) are attracted either exclu- sively to women or to both women and men. Similarly, most but not all female-to-male transgenders are sexually attracted to women (Zucker, 1995). Because sexual orientation and gender identity can vary independently of one another, in keeping with the prenatal hormonal hypothesis one must propose either that gender identity and sexual orientation would be sensitive to the organizing effects of androgens during different periods of development or that they would be influ- enced by different androgens or their metabolites. Similarly, because testosterone and its metabolites are responsible for masculinization of the external genitalia, the absence of genital anomalies in most homo- sexuals and transgenders suggests that genital differentiation occurs during a different developmental period or is sensitive to different metabolites compared to the neural substrates that mediate sexual ori- entation and gender identity.
5 Testing the Prenatal Hormonal Hypothesis
Tests of the prenatal hormonal hypothesis have been reviewed exten- sively elsewhere with respect to both sexual orientation (Byne &
Parsons, 1993) and gender identity (Gooren, 1986a,b, 1990). Here we review the search for correlates of sexual orientation and gender identity and then focus on outcome studies of individuals with known endocrine anomalies and/or gender reassignment during childhood.
5.1 Neuroendocrinologic Studies
One of the most-studied sex differences in the rat brain pertains to its role in regulating the secretion of luteinizing hormone (LH) from the pituitary gland. In brief, the brain of a normal female rat responds to an injection of a large amount of estrogen by signaling the pituitary gland to secrete large amounts of LH, a phenomenon referred to as the positive feedback effect of estrogen on LH release. This positive feed- back effect is a measure of the brain’s ability to support normal cyclic ovarian function in females. In contrast, the male brain, in response to the same injection that produced positive feedback in the female, signals the pituitary gland to decrease its secretion of LH. This is because in the course of normative male development testosterone defeminizes the feedback mechanism.
Many textbooks and popular accounts suggest that defeminization of the positive feedback mechanism also occurs during human male development; however, several lines of evidence suggest that it does not. In fact, laboratory work carried out on nonhuman primates sug- gests that defeminization of the positive feedback mechanism may not occur in any primate. Prolonged developmental exposure to testosterone does not defeminize the feedback mechanism in genetic female monkeys (Resko & Phoenix, 1972) or in human females with
congenital virilizing adrenal hyperplasia (Wilkins, 1952). Moreover, ovarian tissue continues its cyclic pattern of hormonal secretion when transplanted into male monkeys that were castrated as adults (Norman
& Spies, 1986). Developmental studies suggest that the positive feed- back system matures during puberty in boys as well as in girls (Kulin
& Reiter, 1976).
Despite the lack of evidence for sexual differentiation of the feedback mechanism in humans, there has been considerable speculation that male homosexuals should exhibit feminized feedback responses to estrogen. As a direct test of that hypothesis, Gooren and collaborators (Gooren, 1986a, 1986b) examined the positive feedback response in normal men and women and in transgendered individuals (homo- sexual and heterosexual) both before and after surgical and hormonal reassignment. Those studies suggested that in humans, in contrast to laboratory rats, the neuroendocrine response in question depends on the hormonal status of the individual at the time of the estrogen challenge and that it is independent of sex, gender identity, or sexual orientation.
5.2 Neuroanatomic Studies
Over the past two decades, sex differences have been confirmed by the size of several brain structures in a variety of laboratory animals. These findings have generated speculation concerning the existence of paral- lel differences in the human brain associated not only with sex but also with gender identity and sexual orientation (Swaab & Fliers, 1985;
Allen et al., 1989; Levay, 1991; Byne et al., 2001). Several of the struc- tural sex differences identified in animals involve specific cell groups in a broad region of the rodent hypothalamus that participates in reg- ulating a variety of functions including sexually dimorphic copulatory behaviors. Several structur sex differences in the rodent brain have been demonstrated to develop in response to sex differences in early androgen exposure (Collaer et al., 2003; Arnold et al., 2004). The best- studied anatomic sex difference in the rodent brain involves a cell group straddling the medial preoptic and anterior regions of the hypothalamus—the sexually dimorphic nucleus of the preoptic area (SDN-POA). In the rat this structure is five to eight times larger in males than in females. Damage to the preoptic region decreases mounting behavior in laboratory animals, whereas electrical stimulation of the region elicits mounting behavior. These observations and the finding that the size of the SDN-POA correlates positively with the frequency of mounting behavior displayed by male rats have established the belief that the SDN-POA participates in regulating male sex behavior.
However, electrolytic lesions in the region of the SDN-POA of male rats do not disrupt mounting behavior. Instead, they allow male rats (cas- trated as adults and therefore having fully defeminized and masculin- ized brains) to exhibit lordosis if they are given injections of estrogen and progesterone (Hennessey et al., 1986). Thus, rather than regulating male behavior in rats, the SDN-POA may act to inhibit the display of female mating behavior.
The belief that the SDN-POA participates in regulating sex behavior in rats has led to the search for a comparable nucleus in humans. The human third interstitial nucleus of the anterior hypothalamus (INAH3) has been identified as the most promising candidate (Byne et al., 2001).
This nucleus is much larger (Allen et al., 1989; Levay, 1991) and con- tains substantially more neurons in presumed heterosexual men than in women (Byne et al., 2001). By extrapolation from animal work, this human sex difference is widely believed to reflect sex differences in early hormone exposure. The acquired immunodeficiency syndrome (AIDS) epidemic has made it possible to study this nucleus in indi- viduals whose medical records indicated homosexual behavior as the risk factor for contracting AIDS. (Unless someone dies from complica- tions of AIDS there is usually no documentation of sexual orientation in the medical records available for autopsy studies. To date, therefore, postmortem studies on the brains of gay men without AIDS and les- bians have not been possible.) These studies suggest that the volume of INAH3 may be smaller in homosexual men than in heterosexual men but that the number of neurons in the nucleus does not vary with sexual orientation (Levay, 1991; Byne et al., 2001). The suggestion of volume reduction must be viewed skeptically for a variety of technical reasons, including the confounding of sexual orientation and AIDS in the execution of the research (i.e., all of the brains of gay men were from AIDS victims). In addition, tissue shrinks in the process of fixation for histologic analysis. This shrinkage influences measures of size but not measures of cell number. Thus, the finding of equal numbers of neurons in homosexual and heterosexual men may be a more reliable finding than the suggestion of a difference in the volume of the nucleus between the two groups. Alternatively, a difference in the volume of the nucleus between groups could reflect a difference in the volume of neuropil, the brain substance surrounding neuronal cell bodies, which includes neuronal fibers and synapses. A similarly appearing nucleus has been identified in sheep and has been reported to be larger in those male sheep that preferentially mount females than in those that pref- erentially mount other males (Roselli et al., 2004). Although that study is intriguing, it has yet to be subjected to independent replication attempts.
Another putatively sexually dimorphic hypothalamic nucleus, the central part of the bed nucleus of the stria terminalis (BSTc), has been investigated in a small number of postmortem brains for variation with both gender identity and sexual orientation (Zhou et al., 1995). That study measured the BSTc in postmortem tissue from 6 male-to-female transssexuals (2 of whom were exclusively androphilic, 3 of whom were gynephilic, and 1 bisexual), 12 presumed heterosexual men, 11 presumed heterosexual women, and 9 homosexual men. The BSTc was found to be statistically significantly larger in both homosexual and heterosexual nontransgendered men than in women; however, among the male-to-female transgenders, the nucleus was the same size as in the heterosexual women. In the transgendered group, the size of the nucleus did not appear to vary with sexual orientation. The suggestion that this nucleus varies with gender identity (but not with sexual
orientation) in men must be viewed cautiously given the absence of replication studies and the small sample size. Moreover, five of the six transgenders had been orchiectomized, raising the possibility that the observed difference in this androgen-responsive nucleus reflected hor- monal status rather than gender identity.
In addition to the hypothalamus, researchers have sought to identify variation with sex and sexual orientation in the brain commissures, the fiber bundles that connect the left and right hemispheres of the brain (Lasco et al., 2002). These studies have produced conflicting results regarding variation with both sex and sexual orientation. Thus, to date there is no compelling evidence of sexual atypicality in the cerebral commissures of homosexuals. The commissures have not been investi- gated with regard to gender identity.
5.3 Anthropometric Characteristics
Several anthropometric characteristics have been explored in relation to sexual orientation. Most of these studies have been executed with the expectation that homosexual individuals would exhibit character- istics intermediate between those of heterosexual men and women or more typical of heterosexuals of the other sex. Such measures have included not only height and weight but also the amount and distri- bution of facial hair, the ratio of shoulder width to hip width, the size of the genitalia, and more recently dermatoglyphic (fingerprint) char- acteristics, and finger length ratios (ratio of the length of the index finger to that of the ring finger). Most of these studies have been flawed in one or more ways, making their findings difficult to interpret. Some of these flaws include reliance on self-reports of small self-selected samples or on measures obtained by raters who were not blind to the sexual orientation of the subjects. The finger length ratio appears to be a sexually dimorphic phenomenon, but whether it truly varies with sexual orientation remains to be established by further investigation (Williams et al., 2000). It also remains to be demonstrated that the sex difference in the finger length ratio is a function of early androgen exposure. Recent research suggests that cell autonomous mechanisms (e.g., sex differences in gene dosing due to incomplete inactivation of one X chromosome in female cells) contributes more to the establish- ment of sexual dimorphisms than previously appreciated (Arnold et al., 2004). Finally, one laboratory (Loehlin and McFadden, 2003) has reported that lesbian women exhibit masculinized otoaccoustic emis- sions (an echo-like waveform emitted by the inner ear response to brief sounds). Replication by independent laboratories is required to sub- stantiate those results.
5.4 Outcome Studies Following Childhood Gender Reassignment Beginning in the 1950s, John Money and colleagues (Money et al., 1957) observed that because intersexes are neither completely male nor com- pletely female they “are likely to grow up with contradictions existing between the sex of assignment and rearing, on the one hand, and various physical sexual variables, singly or in combination, on the
other.” They therefore collected data on the psychosexual development of children born with various intersex conditions to determine whether their gender role and identity are more likely to be concordant with the sex of assignment and rearing or with one or another of the physical variables of sex. Of 105 intersexes studied, they reported that only 5 had a gender role or identity that was “ambiguous and deviant from the sex of assignment and rearing.” Thus, they concluded, as had Ellis a dozen years previously (Ellis, 1945), that the sex of assignment and rearing is a much better predictor of gender role and identity than the biologic variables of sex. The brain was not counted among those variables.
Evidence for the early malleability of gender identity instigated a shift away from prior attempts to assign gender to intersexes on the basis of their “true sex.” Instead, as detailed by Zucker (1999) and Meyer-Bahlburg (1994), an “optimal gender policy for psychosocial and medical management” was developed by Money and his collabo- rators. This policy aimed to optimize the prognosis with respect to six variables: reproductive potential, sexual function, minimization of medical procedures, gender-appropriate appearance, stable gender identity and psychological well-being. Widespread implementation of this optimal gender policy eventuated in the assignment of most inter- sexed infants to the female gender because, compared to phalloplasty, vaginoplasty produces superior cosmetic and functional results (Fausto-Sterling, 2000) and because it has been believed that a small or absent penis would be a tremendous psychosocial burden for a boy (Zucker, 1999). The surgical policy was premised on the untested assumption that gender-appropriate genitals are necessary not only to convince the affected child that he/she is truly a member of the assigned gender but also to convince the parents whose job it is to unambiguously rear the child in a manner consistent with that gender assignment (Zucker, 1999). Prompt surgical attention to the genital anomalies of intersexed infants was viewed as necessary to establish the dominance of social influences over biologic predispositions on gender identity. Under the optimal gender policy, many 46XY infants with normally functioning testes but with a stretched phallus length less than 2 cm have been surgically reassigned to the female gender (Zucker, 1999; Fausto-Sterling, 2000), as have some normal male infants who suffered traumatic loss of the penis (Diamond & Sigmundson, 1997; Bradley et al., 1998; Zucker, 1999). The indications for early feminizing genitoplasty have been called into question on a variety of grounds (Schober, 1999), including the possibility that gender identity has been substantially organized by hormonal influences prior to birth (Reiner, 1997).
Various intersex and related syndromes have been reviewed exten- sively with regard to gender identity and sexual orientation (e.g., Zucker, 1999; Byne & Sekaer, 2004). Below is a selective review of those syndromes most frequently cited with reference to gender minority status.
5.4.1 Ablatio Penis
Zucker (1999) reviewed six cases of normal males who suffered accidental or traumatic loss of the penis during infancy and were
reassigned to the female gender prior to 2 years of age. The brains of such female-assigned individuals would have been exposed to the full complement of defeminizing and masculinizing hormones prenatally.
Although they are not intersexes, such cases are informative with respect to the question of gender neutrality at birth. These cases would provide a stringent test of the hypothesis of gender neutrality at birth if the female reassignments had been made at birth. Contrary to popular accounts of some of these cases (e.g., Diamond and Sigmund- son, 1997), however, gender reassignment has not been made at birth.
Of the six cases reviewed by Zucker (1999), at least two (orchiec- tomized at 6 and 21 months, respectively) had switched to a male gender identity by or during puberty, whereas two (orchiectomized at 2 and 6 months, respectively) had retained a female identity at last follow-up (one at age 17 and the other in her mid-twenties); no reliable information was available concerning the gender development of the other two. As described below, detailed information is available for only two cases. Although these two detailed cases differed with respect to ultimate gender identity, both exhibited tomboyism during child- hood and described predominant or exclusive gynephilia as adults.
In the first detailed case, Money and Ehrhardt (1972) reported on a case of ablatio penis that continues to receive widespread attention.
The case involved a pair of normal monozygotic 46XY twins, one of whom suffered accidental penile ablation at the age of 7 months. After much debate, the decision for gender reassignment was made at 17 months, with orchiectomy and preliminary vaginoplasty occurring at 21 months (Diamond & Sigmundson, 1997; Zucker, 1999). Follow-up when the twins were 7 years old suggested that the patient had accepted the female gender identity and that the twin brother was a normal male (Money & Ehrhardt, 1972). Two years later, the patient was described as having many tomboyish traits but that “Her activity is so normally that of an active little girl. . . .” (Money, 1975). Thus, it was concluded that “gender identity is sufficiently incompletely dif- ferentiated at birth as to permit successful assignment of a genetic male as a girl . . . and differentiates in keeping with the experiences of rearing.” This case was lost to follow-up for many years, and for approximately two decades that conclusion was cited in innumerable medical review articles and textbooks, forming the crux of theories con- cerning the malleability of gender and gender reassignment in inter- sexed individuals (Reiner, 1997). Follow-up when the patient was in his early thirties, however, suggested that he had rejected the female identity, had resisted feminizing estrogen therapy, and had begun to live as a male by the age of 14 (Diamond & Sigmundson, 1997). At the age of 14, he underwent a mastectomy and began testosterone replace- ment therapy and surgical procedures for phallus reconstruction. At the age of 25, he married a woman and adopted her children. At last follow-up, prior to his suicide in 2004, he reported a history of exclu- sive gynephilic orientation.
A second detailed case of ablatio penis was reported by Bradley et al. (1998). This normal 46XY patient’s penis was destroyed during an electrocautery circumcision at 2 months. At 7 months the patient was admitted to the hospital for orchiectomy and initial feminizing surgery,
but the decision for gender reassignment had been made previously.
Feminizing hormone therapy was initiated at 11 years. At age 16 she reported being a tomboy but denied uncertainty about her female gender identity. At age 26 she remained confident of her female gender.
Although at that time she was sexually active with a man, she reported primarily gynephilic fantasies and described her sexual orientation as bisexual.
5.4.2 Congenital Virilizing Adrenal Hyperplasia
With congenital virilizing adrenal hyperplasia an enzymatic abnor- mality in cortisol synthesis results in an overproduction of androgens beginning during the fetal period (New and Levine, 1981). In genetic males, no genital abnormality ensues; however, in genetic females varying degrees of external genital masculinization can occur ranging from mild clitoral enlargement to complete fusion of the labioscrotal folds with a phallic urethra. Consequently, there is sometimes uncer- tainty regarding gender assignment at birth. Cortisol replacement therapy can minimize further virilization after birth and allow normal ovarian function and fertility to emerge with puberty. In one large cohort, 9% of genetic females were assigned and reared as males without reported complications (Mulaikal et al., 1987). It is likely that as early detection and diagnosis improve the proportion assigned male will decrease in keeping with the optimal gender policy that places emphasis on female reproductive potential.
Affected individuals with this condition who were reared as females have been studied extensively with regard to cognitive profiles (Beren- baum, 2001), childhood gender conformity (Reinisch & Sanders, 1984), gender identity (Hines, 1998), and sexual orientation (Money, 2002).
Most affected individuals are believed to retain their female gender identity into adulthood although with a statistically increased inci- dence of gender nonconformity (Reinisch & Sanders, 1984; Hines, 1998), gender dysphoria (Slijper et al., 1998) or ambivalence about gender (Ehrhardt et al., 1968) during childhood and gynephilia during adulthood (Hines, 1998; Money, 2002). The retention of female gender identity appears to be the rule even when treatment is delayed result- ing in heavy postnatal virilization and lack of feminine secondary sexual characteristics. Meyer-Bahlburg et al. (1996), however, described four cases in which a male identity emerged gradually between late adolescence and adulthood despite having been assigned female within a few weeks of birth. Gender dysphoric subjects appear to be less willing to participate in follow-up studies than are subjects without gender dysphoria (Zucker, 1999), making the proportion of affected individuals who change from female to male gender identity difficult to know with any degree of certainty. In one study that figure was approximately 1 of 50, statistically significantly higher than the rate (approximately 1 per 34,000) of transgenderism among nonintersex females (Zucker, 1999).
5.4.3 Complete Androgen Insensitivity
With complete androgen insensitivity, 46XY individuals develop nor- mally functioning testis but lack functional androgen receptors. Thus,
their tissues are unable to respond to androgens, and they develop normal female external genitalia. Because they are capable of respond- ing to müllerian inhibitory substance, however, internal female genital structures regress. Untreated, they develop breasts and female-typical fat distribution at puberty in response to estrogens derived from testos- terone synthesized by their testes. Historically, these individuals were assumed to be normal females at birth and did not come to medical attention until testes descended into the labia, or until they failed to menstruate or conceive children. According to Meyer-Bahlburg (1998), the literature does not contain any reports of affected individuals changing to a male gender identity. Thus, in the absence of functional androgen receptors, female gender identity appears to be the rule in individuals with an XY karyotype and normally functioning testes.
Although it has been suggested that in the absence of functioning androgen receptors these individuals would have female-typical brain differentiation (Collaer & Hines, 1995), in laboratory rodents androgens appear to orchestrate differentiation of the male brain primarily by interaction with estrogen receptors after conversion to estrogen by aro- matase enzymes in the brain (Goy & McEwen, 1980). For example, mice with complete androgen insensitivity appear to be female physically but exhibit defeminized and masculinized behavior, including mating behavior (Olsen, 1983). It has therefore been suggested that humans, in contrast to rodents, require functional androgen receptors for male brain development (Goy & McEwen, 1980). From a psychosocial stand- point, however, one might suggest that gender outcome in humans with complete androgen insensitivity is due to the fact that they were assigned unequivocally to the female gender at birth and were sub- jected to the same gender socialization as unaffected girls.
5.4.4 Partial Androgen Insensitivity
Partial androgen insensitivity refers to disorders in which there is only partial resistance to androgens at the cellular or receptor level. The testes are believed to function normally, and there is no deficiency of 5α-reductase enzymes; however, the partial insensitivity to androgens results in external genitalia that are only partially masculinized. The degree of masculinization of the external genitalia varies according to the degree of androgen resistance. Affected individuals have been assigned and reared as males or females depending in part on the degree of external genital virilization. Zucker (1999) reviewed six cases in which individuals with partial androgen insensitivity were reared as girls (although one was initially assigned male until 13 days after birth). Among them, five retained a female gender identity into adult- hood (ages 19 to 30 at follow-up), even though one remained gonadally intact into adulthood and experienced marked postnatal virilization.
The sixth patient was reassigned female 5 days after birth and was reared as a girl. At age 30 he requested sex reassignment following a long history of masculine gender role interests and gynephilia.
Data are available on an additional 18 subjects from two group studies. The first of these studies involved 10 patients of whom 8 were reared as boys, 1 as a “hermaphroditic girl,” and 1 as a girl (Chase,
1998). At follow-up between 13 and 39 years it was concluded that gender identity differentiated in accordance with the gender of rearing.
The second study included eight patients, seven of whom were assigned female at birth (Slijper et al., 1998). Details on gender identity were not provided at follow-up (at ages 6 to 23 years); however, the authors concluded that the female assignment had been wrong on the basis of the patients’ “boyish behavior. . . . In particular, the wild, rough play . . . [which was] difficult for their parents to regulate.”
5.4.5 5a-Reductase Deficiency
Deficiency of the enzyme 5α-reductase affects 46XY individuals.
During fetal development the gonads differentiate into normal testes and secrete appropriate amounts of testosterone; however, because of the deficiency of 5α-reductase, affected individuals are unable to convert testosterone to dihydrotestosterone in amounts sufficient for the external genitalia to masculinize normally. Consequently, the newborn may have a phallus that more closely resembles a clitoris than a penis and unfused labioscrotal folds resembling labia majora. In the absence of sophisticated diagnostic testing, affected individuals have often been assumed to be females at birth and have been reared accord- ingly (Imperato et al., 1974; Imperato-McGinley et al., 1979). At puberty, however, testosterone, not dihydrotestosterone, is the essential andro- gen for growth of the male external genitalia and the emergence of male secondary sex characteristics (Wilson, 2001). Thus, masculinizing puberty ensues: The phallus markedly enlarges, the testes descend into the bifid labioscrotal folds, the beard grows, the voice deepens, and a masculine habitus develops (Imperato et al., 1974; Imperato-McGinley et al., 1979).
This condition began to receive much attention a quarter of a century ago with a report on a cohort of affected individuals in a region of the Dominican Republic (Imperato et al., 1974; Imperato-McGinley et al., 1979) where the prevalence of the condition is unusually high due to consanguineous marriages. Of 18 individuals who reportedly had been assigned and reared as females from birth, 17 changed to a male gender identity and 16 to a male gender role at puberty. The authors concluded that male gender identity and gynephilia “appear to be testosterone and not dihydrotestosterone related . . . and that sex of rearing as females . . . appears to have a lesser role in the presence of two mas- culinizing events—testosterone exposure in utero and again at puberty with the development of a male phenotype.”
Because the studied individuals came from interrelated families living in the same village, questions were raised about the initial gender assignments as females. Specifically, it was wondered if, despite being declared female and issued a female birth certificate, would parents “rear their child as one of ambiguous sex, not knowing what to expect at puberty” (Money, 1976). Imperato-McGinley et al. (1979) deny that this was the case for their subjects, although they state that now that the villagers are familiar with the condition, they “raise the subjects as boys from birth, rear them as boys as soon as the problem is recognized in childhood or raise them ambiguously as girls.”
Similar accounts of gender change from female to male have been made in cohorts from Mexico, Papua New Guinea, and Brazil (Herdt, 1990; Zucker, 1999; Wilson, 2001). Herdt (1990) questioned the lack of ambiguity in the female gender assignments in the New Guinea cohort.
Al-Attia (1997) reported on a cohort of six affected Omani Arabs who had been assigned as females at birth, 4 of whom (ages 16 to 28 years) had reached puberty prior to the last assessment. Among them, only one individual (Allen et al., 1989) was unequivocally male in identity and role. One (age 28) was unequivocally female in identity and role.
The other two individuals were ambivalent in identity and role: One (age 16) expressed erotic interest in females and requested gender reas- signment but refused to declare as a male publicly and “continued the role of a conservative female.” The final subject “engaged in sexual activity as a male” but would dress as a male only when away from his home community.
5.4.6 Summary
Review of the clinical data leads to a conclusion similar to that sug- gested by Money et al. (1957) nearly half a century ago. By and large, individuals with functional androgen receptors and prenatal exposure to elevated levels of androgens appear to have the capacity at birth to develop either a male or a female gender identity in response to gender assignment and rearing. When assigned female, these individuals nevertheless have an increased likelihood of exhibiting masculinized play preferences, gender dysphoria, and gynephilia; and a small number ultimately reject the female identity and role. The proportion that ultimately rejects the female assignment is unknown, as it can occur quite late in life, and relatively few studies have followed and adequately assessed affected individuals during adulthood. The data on individuals with 5α-reductase suggest that the probability of switching to a male gender identity and role after female assignment is increased in androgen-sensitive individuals whose testes are left in place until puberty. The data do not justify the conclusion that prena- tal androgen exposure produces a brain that is hardwired for male gender identity at birth. A more conservative interpretation of the data is that prenatal androgens may bias particular behavioral propensities in a manner that facilitates acquisition of a male identity. This effect of prenatal androgens might be reinforced by the elevated androgen secretion that occurs during the neonatal period and again at puberty.
At puberty, the psychological impact of somatic virilization in response to elevated androgens must be considered in addition to the possibil- ity of physiologic effects on the brain. The data suggest that among pre- natally virilized individuals who were assigned female in infancy the proportion who subsequently exhibit gynephilia is greater than the proportion who reject the female gender assignment. Thus, any effect of prenatal hormones may be greater for sexual orientation than for gender identity.
Data from the studies of intersex disorders also suggest which path- ways of androgen action may contribute to psychosexual differentia- tion of the brain. Although individuals with complete androgen
insensitivity have normal testes and all the hormones and metabolic machinery necessary to masculinize the rodent brain (i.e., testosterone, aromatase enzyme, estrogen receptors), all reported cases have been reared unambiguously as females, have retained that identity into adulthood, and are described as having stereotypically feminine inter- ests and behaviors as children (Zucker, 1999). Thus, if prenatal hor- mones exert an organizing influence on the human brain with respect to gender, masculinization of the brain in this regard must be mediated primarily via androgen receptors. The evolution of masculine behav- ior and male identity among individuals with 5α-reductase deficiency suggests that those androgen receptors may be activated by testos- terone in the absence of 5α-reduction. Moreover, as reviewed by Byne and Sekaer (2004) the gender and sexual orientation outcomes among individuals with very little testosterone production in utero suggest that very little testosterone is required to bias psychosexual develop- ment in the male direction. As discussed below, however, the mecha- nism through which such a bias might be exerted is far from clear. The variability of gender outcomes even among related intersexed indi- viduals known to share identical genetic mutations suggests the impor- tance of psychological, social, and cultural factors as co-mediators of gender development (Wilson, 2001).
6 Genetic Studies
As in neuroendocrinologic research, some genetic studies of sexual ori- entation have been premised on the intersex hypothesis. To date there have been no such studies of transgenderism perhaps because of its rel- atively lower prevalence. Genetic studies include (1) attempts to show that homosexuals have opposite-sex chromosomal material in their cells (Money & Ehrhardt, 1972) and (2) studies seeking to link homo- sexuality with genetically controlled aberrations in the process of sexual differentiation (Macke et al., 1993). None of those studies has met with success. More recent genetic studies (discussed below) are not necessarily based on the intersex assumption and are compatible with a variety of more diverse and complex pathways.
6.1 Heritability Studies
Although studies have suggested that homosexuality runs in families (Pillard & Weinrich, 1986; Bailey & Pillard, 1991), such studies are not helpful for distinguishing between genetic and environmental influ- ences because most related individuals share environmental influences as well as genes. Disentangling genetic and environmental influences often involves comparisons between identical and fraternal twins.
The most thorough study of this sort was conducted by Bailey and Pillard (1991). Their study assessed sexual orientation in identical and fraternal twins, nontwin biologic brothers, and unrelated adopted brothers of gay men. The concordance rate for identical twins (52%) in that study was much higher than the rate for the fraternal twins (22%).
The higher concordance rate for the identical twins is consistent with
a genetic effect because identical twins share all of their genes whereas fraternal twins, on average, share only half of their genes. These studies assume that environmental influences would be the same for all brothers.
It would be a mistake, however, to attribute the increased concor- dance rate in identical twins to increased gene sharing alone. If there were no environmental effect on sexual orientation, the rate of homo- sexuality among the adopted brothers should have equaled the rate of homosexuality in the general population. Recent studies place the rate of homosexuality in men between 2% and 5% (Hamer et al., 1993;
Bailey et al., 1994). The fact that the concordance rate in adopted broth- ers was eleven percent (two to five times higher than in the general population) suggests a major environmental contribution. The rate for homosexuality among non twin biologic brothers was only 9%, a figure statistically indistinguishable from the 11% recorded for adopted brothers. If the concordance rate for homosexuality among nontwin brothers is the same regardless of whether the brothers are genetically related, the concordance rate cannot be explained exclusively by genetics.
When considered together, the data from the twins and the adopted brothers suggest that the increased concordance in the identical twins may be due to the combination of both genetic and environmental influences. Perhaps the most interesting finding to emerge from twin studies is that approximately 50% of identical twins are discordant for sexual orientation even when they are reared together. This finding, which has been consistent across studies, underscores just how little we actually know about the origins of sexual orientation.
6.2 Linkage Studies
In 1993 a highly publicized study (Hamer et al., 1993) presented sta- tistical evidence that genes influencing sexual orientation reside on a portion of the X chromosome known as the q28 region. Contrary to some media reports and popular belief, that study did not claim to dis- cover any particular gene or sequence of DNA associated with homo- sexuality. That is not the aim of linkage studies. The aim of such studies is merely to identify chromosomal regions in which such genes might reside. It is important to understand that the statistical significance of genetic linkage studies depends on assumptions about the rate of homosexuality in the population (Risch et al., 1993). Problems involved in calculating this rate have been reviewed elsewhere (McGuire, 1995).
Hamer’s conclusions rest on the assumption that the rate of male homosexuality in the population at large is 2%; however, if the base rate is actually 4% or higher, the results that he reported are not statis- tically significant. One leading geneticist argues that Hamer’s own data support a 4% estimate (Risch et al., 1993). A Canadian team has been unable to duplicate the Xq28 finding in men using a comparable experimental design (Rice et al., 1999), and Hamer’s team found no evidence that Xq28 is linked to sexual orientation in women (Hu et al., 1995).
6.3 “Gay Genes”
It cannot be overemphasized that “gay genes” are not required for homosexuality to run in families or for researchers to determine that it is “heritable.” This is because, to geneticists, heritability has a precise technical meaning. It is defined as the ratio between genotypic varia- tion (genetic variation) and phenotypic variation (observable expressed variation in a trait). Thus, heritability reflects only the degree to which a given trait is associated with genetic factors. It says nothing about the specific genetic factors involved or about the mechanisms through which they exert their influence. Furthermore, heritability gives no information about how a particular trait might change under different environmental conditions. Therefore, as described in the next section, homosexuality could be highly heritable even if genes influenced sexual orientation entirely through indirect pathways.
7 Models for Conceptualizing the Role of Biology
Most efforts to explain the development of gender identity and sexual orientation have focused exclusively on either biologic or psychosocial factors. Three models for integrating biologic and psychosocial contri- butions are considered here. The first model is the permissive effects model in which biology primarily provides the neural substrate on which gender identity and sexual orientation are inscribed by forma- tive experience (i.e., at birth the brain would be viewed as a blank slate). In this model, genes or other biologic factors could also delimit the period during which the relevant formative experience(s) must occur. By analogy, some song birds can only learn their species’ song by hearing it sung during a relatively restricted period of early devel- opment. If they hear the song of another species during that time, they may learn it instead (Nottenbohm, 1972). Once a song has been learned, that is the bird’s song for life. The bird can neither unlearn that song nor learn another. Whereas the song is clearly acquired through expe- rience, biology determines when during development that experience must occur.
In the second, direct effects, model, biologic factors exert their influence through the organization of hypothetical brain circuits that mediate gender identity and sexual orientation. The fact that sexual orientation and gender identity can vary independently suggests different, though perhaps overlapping, circuits. This model is called “direct” because the arrows of causation point directly from discrete biologic factors such as genes or hormones to gender identity and sexual orientation. This model allows for the possibility that direct biologic effects could be subsequently modified by experience.
For example, some have speculated that most women who were exposed as fetuses to masculinizing hormones become heterosexual because “social factors override their biological predisposition toward lesbianism” (Money & Ehrhardt, 1972). Thus, direct model effects could be either determinative or predisposing. They could also be graded in magnitude such that exposure to graded amounts of
androgen would produce correspondingly graded degrees of brain masculinization.
In the third, indirect effects, model, the arrow of causation does not lead from biologic factors directly to gender identity or sexual orienta- tion. Instead, biologic factors would directly influence other personal- ity traits or temperamental characteristics that would then influence not only how the environment is experienced internally but how one interacts with and modifies the environment in shaping the relation- ships and experiences that influence the development of gender iden- tity and sexual orientation. This model is similar to the permissive effects model but goes beyond that model by including the possibility that the relevant formative experiences may themselves be strongly affected by hormonally or genetically influenced personality variables.
7.1 Gender Identity
The direct model with respect to gender identity is best exemplified by the work of Swaab and collaborators who suggested that gender iden- tity is difficult to change because it is fixed in brain structure as a con- sequence of gender differences in developmental androgen exposure (Zhou et al., 1995). As discussed above, they identified the BSTc as a component of the hypothetical gender identity circuit. Exactly how such a hypothetical brain circuit could act, independent of experience, to cause the child to feel or say “I am a boy” or “I am a girl” is unknown. Moreover, it remains to be explained how such a circuit could be sex-reversed in transsexuals who have no demonstrable genetic or endocrine abnormality.
Irene Fast’s (1984) work exemplifies the permissive effects model.
She suggests that gender identity in both sexes begins by building self- representations by identification with others. At first these self- representations are over-inclusive such that the toddler is not aware that all sex and gender characteristics are not open to him (e.g., he does not realize that he cannot grow up to be both a mommy and a daddy).
Upon recognizing sex differences, the toddler moves from this assump- tion to a recognition of limits imposed by the reality of his body structure and function. Relinquishing attributes of the over-inclusive self-representation does not involve suppression of biologically based gender constructs, as in the direct model, but the abandonment of self-representations acquired by identification. For example, the child realizes he is a boy and therefore cannot become a mommy. Under normative circumstances the formative experiences that shape gender identification would be consistent with one’s gender assignment, which in turn would be in accord with all biologic variables of sex. The psychic pain (narcissistic injury) in response to relinquishing valued but cross-gendered aspects of the over-inclusive self-representation would be mitigated in environments in which the assigned gender is valued.
Conflicts might arise in environments in which the child perceives that the gender to which he has been assigned is not valued as highly as the other or where being one gender is perceived as unsafe. In such
