I. Savic (Ed.) Progress in Brain Research, Vol. 186 ISSN: 0079-6123 Copyright © 2010 Elsevier B.V. All rights reserved. CHAPTER 4 Sexual differentiation of the human brain in relation to gender identity and sexual orientation † ‡ ‡ Ivanka Savic , Alicia Garcia-Falgueras ,§ and Dick F. Swaab ,* † Department of Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden ‡ Netherlands Institute for Neuroscience, an Institute of the Royal Netherlands Academy of Arts and Sciences, Amsterdam, The Netherlands §Medical Psychology Unit, Institute of Neuroscience, Autonomous University of Barcelona, Bellaterra, Barcelona, Spain Abstract: It is believed that during the intrauterine period the fetal brain develops in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. According to this concept, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation should be programmed into our brain structures when we are still in the womb. However, since sexual differentiation of the genitals takes place in the first two months of pregnancy and sexual differentiation of the brain starts in the second half of pregnancy, these two processes can be influenced independently, which may result in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain. There is no proof that social environment after birth has an effect on gender identity or sexual orientation. Data on genetic and hormone independent influence on gender identity are presently divergent and do not provide convincing information about the underlying etiology. To what extent fetal programming may determine sexual orientation is also a matter of discussion. A number of studies show patterns of sex atypical cerebral dimorphism in homosexual subjects. Although the crucial question, namely how such complex functions as sexual orientation and identity are processed in the brain remains unanswered, emerging data point at a key role of specific neuronal circuits involving the hypothalamus. Keywords: Gender identity; Homosexuality; Human brain; Sexual orientation; Sexual differentiation; Transsexuality * Corresponding author. Tel.: þ31 20 5665500; Fax: þ31 20 5666121; E-mail: [email protected] DOI: 10.1016/B978-0-444-53630-3.00004-X 41 42 General concepts Sexual organization and activation of the human brain Gender identity and sexual orientation represent two fundamental functions in human neurobiol­ The process of sexual differentiation of the ogy. These functions have hitherto mainly been brain brings about permanent changes in brain discussed in relation to the specific signs of sexual structures and functions via interactions of the dimorphism in the brain and the potential developing neurons with the environment, mechanisms thereof. By mapping differences understood in its widest sense. The environment between men and women in cerebral anatomy, of a developing neuron is formed by the sur­ function, and neurochemistry, neuroscientists are rounding nerve cells and the child’s circulating trying to identify sex typical and sex atypical hormones, as well as the hormones, nutrients, actors in transsexual and homosexual individuals. medication, and other chemical substances This has been done in postmortem analyses from the mother and the environment that of the brain, and investigations of neuronal enter the fetal circulation via the placenta. anatomy, connectivity, and function by means of Along with the genetic code, all these factors positron emission tomography (PET) and mag­ may have a lasting effect on the sexual differen­ netic resonance imaging (MRI). The extracted tiation of the brain. networks are then mapped onto those known to The testicles and ovaries develop in the sixth be related to sexual behavior in animals to week of pregnancy. This occurs under the influ­ formulate biological underpinnings of homo- ence of a cascade of genes, starting with the sex- and transsexuality in humans. This widely used determining gene on the Y chromosome (SRY). approach has several difficulties with this The production of testosterone by a boy’s testes is approach: (1) gender identity cannot be investi­ necessary for sexual differentiation of the sexual gated in animals; (2) sexual behavior in animals is organs between weeks 6 and 12 of pregnancy. The reflex-like and cannot simply be translated to peripheral conversion of testosterone into dihy­ sexual orientation and attraction in humans; drotestosterone is essential for the formation of a (3) reliable sex differences in the human brain boy’s penis, prostate, and scrotum. Instead, the require investigations of large populations and development of the female sexual organs in the have only recently been demonstrated reliably; womb is based primarily on the absence of andro­ (4) the majority of studies on sex differences do gens (Swaab et al., 2003). not account for sexual orientation of the investi­ Once the differentiation of the sexual organs gated participants; (5) studies of homo- and into male or female is settled, the next thing that transsexual persons are very limited, and only is differentiated is the brain, under the influence, few comparisons have hitherto been presented mainly, of sex hormones on the developing brain between homo- and transsexual subjects. cells. The changes (permanent) brought about in An alternative and parallel approach is pin­ this stage have organizing effects; later, during pointing the specific neuronal networks related puberty, the brain circuits that developed in the to gender identity and sexual orientation, ana­ womb are activated by sex hormones. This para­ lyzing the factors programming these networks digm of sexual differentiation of the brain was and possible differences between control, homo-, coined by Phoenix et al. (1959) and has dominated and transsexual subjects. Emerging fMRI and the view on cerebral sex dimorphism during the PET studies suggest that sexual arousal is last decades. mediated by specific core neuronal networks, The fetal brain is protected against the effect of which may be also involved in sexual circulating estrogens from the mother by the pro­ orientation. tein α-fetoprotein, which is produced by the fetus 43 and binds strongly to estrogens but not to testos­ end of pregnancy, when the α-fetoprotein level terone (Bakker et al., 2006, 2008). However, not declines, the fetus is more exposed to estrogens only estrogens reach the brain via circulation, but from the placenta, this exposure inhibiting the brain itself is capable of producing estrogens. the hypothalamus–hypophyseal–gonadal axis of In human beings testosterone may thus not only the developing child. Loss of this inhibition once have a direct effect on a masculine brain, but, once the child is born causes a peak in testosterone in converted into estrogens by aromatase, may also boys and a peak in estrogens in girls (Quigley, act on developing neurons. In addition, there are 2002). The testosterone level in boys at this time sex differences in brain steroid receptor distribu­ is as high as it will be in adulthood, although a tion not only in adulthood (Ishunina and Swaab, large part of the hormone circulates bound. Also 2008; Kruijver and Swaab, 2002; Kruijver et al., at this time the testosterone level is higher in boys 2001; Swaab et al., 2001) but also during develop­ than in girls. During these two periods, therefore, ment (Chung, 2003), which may be genetically girls do not show high levels of testosterone. determined. In addition, in rat hormone receptor These fetal and neonatal peaks of testosterone, genes a sex difference in methylation pattern together with the functional steroid receptor activ­ occurs during development (Schwarz et al., ity, are, according to the current dogma, thought 2010). In rats, the formation of estradiol in the to fix the development of structures and circuits in brain by aromatization of circulating testosterone the brain for the rest of a boy’s life (producing is the most important mechanism for virilization of “programming” or “organizing” effects). Later, the brain (Gorski, 1984), but, as seen below, it the rising hormone levels that occur during pub­ does not determine human gender identity or sex­ erty “activate” circuits and behavioral patterns ual orientation. that were built during development, in a masculi­ There may also be direct genetic effects that nized and de-feminized direction for male brains affect the sexual differentiation of the brain with­ or in a feminized and de-masculinized direction out involving the sex hormone receptors. for female brains. The brain structure differences that result from the interaction between hormones and Sex hormones and human brain development developing brain cells are thought to be the major basis of sex differences in a wide spectrum During fetal development, the brain is influenced of behaviors, such as gender role (behaving as a by sex hormones such as testosterone, estrogens, man or a woman in society), gender identity (the and progesterone (Swaab, 2004). From the earliest conviction of belonging to the male or female stages of fetal brain development, many neurons gender), sexual orientation (heterosexuality, throughout the entire nervous system already homosexuality, or bisexuality), and sex differ­ have receptors for these hormones (Chung, ences regarding