Can anyone tell me why I'm gay? What research suggests regarding the origins of sexual orientation.
Chief among the difficulties that such researchers face is agreeing upon a clear definition of what one means by sexual orientation. For the purposes of this manuscript, a definition offered by Gorman (1994) seems to best capture the concept: sexual orientation refers to whether one is primarily aroused by same-sex or opposite-sex stimuli, independent of the sexual behavior engaged in by that individual. Such a definition is preferred because of its focus on arousal and desire, rather than overt behavior which may be influenced by a variety of social, political, and religious pressures.
Based on the research of Kinsey and others, it is estimated that between 3 and 10 percent of the population identifies as exclusively homosexual (Kinsey, Pomeroy, & Martin, 1948; Levay, 1996; Pillard & Bailey, 1995). Many studies use some form of the Kinsey Scale to have research participants self-identify their sexual orientation. The Kinsey Scale is a seven-point scale that represents a continuum of human sexual behavior. The scores range from zero (exclusive heterosexuality) to six (exclusive homosexuality). Although Kinsey argued that there is a continuum of human sexual behavior, researchers have historically used his scale to dichotomize samples into heterosexual and homosexual/nonheterosexual groups. In response to this binary treatment, several researchers have been prompted to develop their own scales to better capture the continuous nature of human sexuality and allow for legitimate discussion of those individuals who identify as bisexual to be considered in their own right (for a more complete discussion see Klein, 1993; Rodriguez Rust, 1999).
Given prevailing social mores, we should approach the scientific literature regarding sexual orientation with some caution for a number of reasons. First, individuals willing to identify themselves as homosexual or bisexual may not be the most representative sample of their respective populations as a whole. Despite his enormous impact on the study of human sexual behavior, Kinsey received vocal criticism of the representativeness of his samples because of their willingness to participate in his studies and because of a presumed overreliance on prisoners and prostitutes (for examples see Cochran, Mosteller, & Tukey, 1954; Maslow & Sakoda, 1952), and some of these concerns continue to resonate today. Second, as previously mentioned, researchers have often imposed a dichotomy upon their samples so that individuals are grouped as either heterosexual or homosexual. This practice may not capture the true continuum of human sexual orientation and behavior and potentially confounds interpretations by treating bisexuals and homosexuals uniformly, despite research that suggests that bisexual individuals are distinct from both hetero- and homosexuals (Klein, 1993; Rodriguez Rust, 1999; Van Wyk & Geist, 1995). Finally, we should be cognizant of the motives for engaging in research focusing on the origins of sexual orientation. In reading this literature, it is clear that personal bias and opinion have colored the presentation of scientific findings and their potential implications (for a more detailed discussion see LeVay, 1996).
Researchers interested in the development of sexual orientation have often taken either a physiological or a psychosocial approach. It would be truly naive to argue that either of these perspectives could operate entirely independently of the other, and neither side is making that argument. Rather, based on their own backgrounds and expertise, scientists are approaching the question of the etiology of sexual orientation from multiple perspectives and levels of analysis in the hope that the truth will emerge from the convergence of their findings. In the following pages, some of the major ideas and hypotheses that are considered include: studies of prenatal development, brain morphology and function, behavioral genetics, environmental factors, early childhood/adolescent experiences, and stability/plasticity of sexual orientation.
Historically, physiological research investigating the development of homosexuality has involved the search for biological markers and focuses on physiological differences between homosexual and heterosexual individuals (DeCecco & Parker, 1995). Often, the investigation of the development of a homosexual orientation stems from the dimorphic nature of the development of males and females. The rationale for such an approach assumes that homosexuals develop as an intermediate to heterosexual males and females. The process of sexual differentiation is beyond the scope of this paper; however, it is important to note that all developing embryos are equally capable of developing along male or female lines. The developmental route taken is generally thought to be determined by a cascade of events set into motion by the presence (or absence) of the Y chromosome, with incomplete overlap in the timelines for the sexual differentiation of internal and external genitalia and neural tissues (for a recent review see Arnold, 2009).
Male homosexuality has long been hypothesized to result from some sort of prenatal testosterone deficiency. While the cause of such a deficiency is unclear, some have investigated maternal stress during gestation as a possibility. There is evidence from the animal literature to suggest maternal stress during pregnancy demasculinizes and feminizes male sexual behavior and brain morphology (Kerchner & Ward, 1992; Ward, 1972). This demasculinization may be due to a delay in a surge of testosterone necessary for sexual differentiation of the brain (Ward & Weisz, 1980).
In the human literature, a number of retrospective studies have resulted in contradictory findings regarding the role of maternal stress in male homosexuality. When homosexual, bisexual, and heterosexual men were asked to estimate the amount of stress experienced by their mothers while they were pregnant with them, non-heterosexuals estimated higher levels of stress than heterosexuals (Dorner, Schenk, Schmiedel, & Ahrens, 1983). Furthermore, when mothers were asked directly about the amount of stress they had experienced while pregnant with their sons, Ellis (1988) found that mothers of homosexuals were more likely to report stress than mothers of heterosexuals (especially during the second trimester of pregnancy). However, Bailey, Willerman, and Parks (1991) failed to find any difference in the amount of stress experienced by mothers of heterosexuals and non-heterosexuals in a very similar study.
While delays in or a deficiency of androgen activity is a compelling argument for those who view male homosexuals as somehow intermediate to male and female heterosexuals, this idea is in direct opposition to observations that both male and female homosexuality may be associated with hyper-masculinization, perhaps as a function of excess androgen exposure during early development (for examples see Bogaert & Hershberger, 1999; McFadden & Champlin, 2000). There is also some evidence to suggest that females prenatally exposed to diethylstilbestrol (DES), a synthetic estrogen, are more likely to identify as bi- or homosexual compared to those females not exposed to DES (Meyer-Bahlburg et al., 1995), presumably as a function of estrogen's role in the defeminization of the central nervous system.
Some researchers have suggested that a maternal immune response to the H-Y antigen in developing male fetuses may contribute to the development of male homosexuality (Blanchard, 2001; Blanchard, 2004; Blanchard & Klassen, 1997). This hypothesis stems from the observation of what is described as the "fraternal birth order effect." In 1996, Blanchard & Bogaert observed that male homosexuals were much more likely to have older male siblings than their heterosexual counterparts. Furthermore, evidence suggests that this effect only applies to biological, rather than adopted siblings (Bogaert, 2006). While this particular observation is compelling, more recent research suggests that earlier studies conflated issues of gendered-typed behaviors and sexual orientation. Specifically, the birth order effect only seems to apply to those individuals who exhibit high degrees of feminine behavior (Bogaert, 2005).
In rodents, the surge in leutinizing hormone (LH) in response to estrogen seems to be sex specific; however, it is unclear whether this pattern is as absolute in humans. Gladue, Green, and Hellman (1984) found that the pattern of LH secretion in response to an injection of premarin (an estrogen derivative) of homosexual men was intermediate to that of heterosexual men and heterosexual women. In addition, homosexual and heterosexual men differed in terms of the duration of lowered testosterone after the injection. This response was proposed to serve as a biological marker of homosexuality that is indicative of organizational differences in hypothalamic-pituitary-gonadal (HPG) axis between hetero- and homosexual men (Gladue et al., 1984). However, Gooren (1986a) found no difference in the LH secretion pattern between hetero- and homosexual men. Furthermore, Gooren (1986b) determined that LH secretion patterns in the human can be changed as a result of gonadectomy and cross-gender hormone treatment, suggesting this particular response is not due to an organizational difference in the HPG axis. However, in male-oriented rams, it appears that LH secretion pattern is indistinguishable from that of female-oriented rams and that both are defeminized compared to ewes, suggesting that sexual attraction is not related to organizational differences in the hypophyseal portal system (Stormshak, Estill, Resko, & Roselli, 2008).
BRAIN MORPHOLOGY AND FUNCTION
In 1978, Gorski, Gordon, Shryne, and Southam described a sexually dimorphic area of the medial preoptic nucleus of rats. Specifically, this area was found to be larger in males than in females and this difference arises as a function of perinatal hormonal milieu. In the case of the medial preoptic area, it is clear that this structure is critical for the normal display of sexual behavior in the male rat (De Jonge, Louwese, Ooms, Evers, & De Poll, 1989; Everitt, 1990; Hull & Dominguez, 2006; Hull & Dominguez, 2007).
Structures that display sexual dimorphism in the human brain have received attention as sites of interest among scientists interested in finding morphological correlates of sexual orientation as well. Swaab and Fliers (1985) identified an area of the human hypothalamus that appears to be analogous to the sexually dimorphic nucleus of the rat medial preoptic area described by Gorski and colleagues. Allen, Hines, Shryne and Gorski (1989) were unable to replicate the sexual dimorphism of this area which they referred to as the interstitial nucleus of the anterior hypothalamus-1 (INAH-1); however, they did identify two other sexually dimorphic nuclei in the preoptic-anterior hypothalamus which they labeled INAH-2 and INAH-3. Levay (1991) conducted a study investigating how and if the anterior preoptic hypothalamus differed between heterosexual and homosexual men. Levay conducted a postmortem analysis of tissue obtained from women, heterosexual men, and homosexual men. While he observed no differences between men and women in the size of INAH-1 or 2, Levay did observe that INAH-3 was sexually dimorphic (larger in men than in women) and that this area was significantly larger in heterosexual men than in homosexual men. More recent research suggests that this size difference is due to cell density rather than cell number (Byne et al., 2001).
Levay's findings have recently received corroboration from researchers investigating the brains of a naturally occurring population of male-oriented rams. An area of the hypothalamus dubbed the "ovine sexually dimorphic nucleus" is larger in male rams than in female sheep, and it is larger in female-oriented rams than in male-oriented rams (Roselli, Larkin, Resko, Stellflug, & Stormshak, 2004). These converging lines of evidence definitely point to sexually dimorphic hypothalamic areas under the influence of hormones as sites of great importance in an individual's sexual orientation, and this particular animal model is especially intriguing because these rams represent a naturally occurring population that displays not only same-sex behavior but also what seems to be a true preference for same-sex interactions (Roselli, Larkin, Schrunk, & Stormshak, 2004; Roselli & Stormshak, 2009a,b).
Differences in brain morphology and function are not limited to sexually dimorphic areas, however. The suprachiasmatic nucleus is larger in homosexual men than in heterosexual men, but this area does not appear to be sexually dimorphic (Swaab & Hofman, 1990). The anterior commissure also varies in size as a function of sexual orientation (Allen & Gorski, 1992) without differing between women and men.
Global, morphological differences in brain structure as a function of sexual orientation are related to differences in the ways in which homosexual and heterosexual brains function. Male and female homosexuals differ from their heterosexual counterparts in their performance on a variety of tasks that assess cognitive functions including: spatial memory, mental rotation, verbal fluency, and recognition of facial expressions of emotion (Rahman, Abrahams, & Wilson, 2003; Rahman & Wilson, 2003a,b; Rahman, Wilson, & Abrahams, 2003; Rahman, Wilson, & Abrahams, 2004a,b).
Furthermore, different neural circuits are activated in the visually evoked sexual arousal of heterosexual and homosexual men. While a number of structures are similarly activated in hetero- and homosexual participants, activation of the left caudate, left angular gyrus, and right pallidum is exclusive to homosexual subjects and activation in the left and right lingual gyri and right hippocampus and parahippocampal gyrus is exclusive to heterosexual subjects. (Hu, et al., 2008). Furthermore, sex-partner preference specific activation of certain regions of the reward circuitry of the brain (the ventral striatum and centromedian thalamus in this case) lead some to propose that there is a "functional endophenotype" indicative of the individual's sexual orientation (Ponseti et al., 2006), and recently, it has been demonstrated that this endophenotype has excellent predictive validity for an individual's sexual orientation (Ponseti et al., 2009).
Pillard and Weinrich (1986) found that homosexual men had about four times more homosexual brothers than would be expected given prevalence rates of male homosexuality, and Bailey and Pillard (1991) found that concordance rates for male homosexuality were 52% among monozygotic twins, 22% among dizygotic twins, and 11% for adopted brothers. After reviewing a number of studies like those mentioned above, Rodriguez-Larralde & Paradisi (2009) concluded the heritability of male homosexuality was somewhere in the range of 0.27 to 0.76. In 1993, a study was published that demonstrated a linkage between male homosexuality and a portion of the X chromosome (Hamer, Hu, Magnuson, Hu, & Pattatuci, 1993), however, subsequent studies have been unable to replicate this result (Mustanski et al., 2005; Rice, Anderson, Risch, & Ebers, 1999).
More recently, Hamer's group has conducted a genome-wide scan of male sexual orientation which identified several markers that are associated with genes important for organization and sexual differentiation of neural tissues and the HPG axis (Mustanski et al., 2005). It has been suggested that the candidate genes may confer increased fecundity upon the relatives of homosexuals (Iemmola & Camperio, 2009; Rahman et al., 2008) or that these genes may be associated with increased altruism and empathy that could be directed towards close relatives most likely to reproduce (for example see Salais & Fischer, 1995). So, even if one accepted the flawed (Dickemann, 1995) assumption that male homosexuals are less likely to successfully reproduce than heterosexuals, there are a number of potential explanations for the maintenance of the gene(s) that might contribute to homosexuality.
There is evidence that differential patterns of gene expression and/or the resultant in utero hormonal milieu that mediate developmental pathways are different between heterosexuals and homosexuals. For instance, Rahman & Wilson (2003a) demonstrated differences in finger length ratios between heterosexuals and homosexuals. Differences in handedness (for example Lippa, 2003), dermal ridge patterns (Hall & Kimura, 1994), hair whorl (Klar, 2004), and functional and structural aspects of the auditory system between heterosexuals have all been observed (Loehlin & McFadden, 2003; McFadden & Champlin, 2000; Mcfadden & Pasanen, 1998), although replicating some of these effects consistently has proven to be problematic (as discussed in Schwartz, Kim, Kolundzija, Rieger, & Sanders, 2010).
A behavioral pattern and identity as complex as sexual orientation, is subject to an interaction of the individual's physiology with his or her environment, and researchers have proposed sundry environmental factors that may contribute to the development of sexual orientation. However, the results of these efforts have been somewhat equivocal. For instance, Bieber and colleagues (1962a) claimed that male homosexuality resulted more commonly among families with a domineering mother and a weak father. Research investigating this possibility, however, has not provided consistent evidence to support this hypothesis. While Milic and Crowne (1986) report differences in terms of lovingness and rejection by parents of homosexual versus heterosexual men, others have found no difference in parenting styles experienced by heterosexual and homosexual men (Bell, Weinberg, & Hammersmith, 1981; Ross & Arrindell, 1988).
Increasing numbers of homosexual individuals are having children either through adoption, marriage, or artificial inseminations; and many wonder whether their children are more likely to develop a homosexual orientation. Imitating the homosexual parent, being socialized by the homosexual parent to adopt a homosexual orientation, or viewing homosexuality as an acceptable alternative to heterosexuality--an option that might not be as obvious to children of strictly heterosexual parents, could all play a role in the development of a homosexual orientation. However, the vast majority of children of homosexual parents are heterosexual (Bailey, Bobrow, Wolfe & Mikach, 1995; Golombok & Tasker, 1995). It should be pointed out, however, that a concordance rate of 9% for non-heterosexual behavior of the sons of homosexual fathers has been observed, which is slightly larger than would be expected given overall prevalence rates of male homosexuality (Bailey et al., 1995). In this particular study, however, genetic influences could not be controlled for and there was no correlation between the sexual orientation of these children and the amount of time spent with their fathers.
EARLY CHILDHOOD AND ADOLESCENCE
One of the most consistent, robust findings among homosexuals has been that they exhibited behaviors during childhood that are considered gender nonconforming (Bell et al., 1981; Bieber et al., 1962b), and recently, this observation has been replicated through the analysis of childhood videos obtained from male and female homosexual and heterosexual research volunteers. Pre-homosexual children, regardless of gender, displayed more gender non-conformity in the videos than their pre-heterosexual counterparts, and this pattern of behavior continued through adulthood (Rieger, Linsenmeier, Gygax, & Bailey, 2008).
Homosexual individuals have reported that they were aware from a very early age of their gender nonconformity and erotic attraction towards the same sex (Bell et al., 1981; Rieger et al., 2008). There is some evidence to suggest that masturbation and fantasy may play a role in integrating early feelings into the development of their sexual orientation (Money & Tucker, 1975; Van Wyk & Geist, 1984). It is unclear, however, whether these behaviors are involved in the development of a homosexual orientation or whether they are products of an already established homosexual orientation.
Theories of the development of a homosexual orientation often concentrate on adolescence as a critical period in this process because of the changes in physiology, psychological functioning, and social conditions occurring during this time (Savin-Williams, 1988). Frequently, adolescents engage in homosexual behavior at a higher rate than the rest of the population; however, this is often viewed as a natural phase on the way to developing a heterosexual orientation. However, for some, these activities are a part of a developing homosexual identity (Savin-Williams, 1990). Troiden (1988) describes adolescence as a time when homosexuals first become aware of their same sex attraction and begin to act on these attractions, and he proposed that male homosexuals generally begin to accept the fact that they are homosexual between the ages of 19 and 21. In this framework, adolescence may reflect the acceptance and recognition of an individual's homosexual orientation, rather than the development of it, per se.
The rate of physical maturation and first sexual experiences are other aspects of the individual's life that have received attention from researchers investigating the development of a homosexual orientation. Because puberty is associated with increased sexual urges, it has been hypothesized that homosexuality results when an individual reaches puberty at an early age when there is no access to heterosexual outlets. Instead, these early "bloomers" are surrounded by same-sex stimuli with which they might engage in sex play, thereby associating sexual pleasure with homosexual experience. Masturbation could then be used to reinforce these homosexual feelings in a time when sexual urges are quite intense but heterosexual experience is rare. In addition to the physical component, the individual may have close friendship ties with these same-sex peers that could create an emotional preference for members of the same sex (Savin-Williams, 1988). This writer is unaware of any research suggesting that homosexuals enter puberty earlier than heterosexuals; however, there is at least one study that suggests that homosexual males report becoming sexually active earlier in life than their heterosexual counterparts (Manosevitz, 1970). It is important to reiterate that most homosexuals are aware of their same-sex attraction long before puberty (Bell et al., 1981). In addition, in cultures in which early homosexual experiences are the norm, there is no apparent increased (in fact, there is a smaller) rate of adult male homosexuality (Baldwin & Baldwin, 1989), and other researchers have actually posited theories of the development of sexual orientation which generally state that the gender group that the individual does not have access to will actually become the object of sexual desire. In other words, the "exotic," not the familiar, becomes "erotic" (Bem, 1996).
STABILITY/PLASTICITY OF SEXUAL ORIENTATION
Some individuals have claimed that they led an exclusively heterosexual life until they had a given homosexual experience after which point they were exclusively homosexual (as described in Meijer, 1993). Certainly, many homosexuals have tried to change their sexual orientation through various "reparative" therapies. In fact, there are reports suggesting that such programs are successful (for example, see Spitzer, 2003), though the rigor of the science assessing these therapeutic techniques described in such studies is often called into question (Ford, 2001; Haldeman, 2002). Aside from any legitimate evidence of the efficacy of the reparative therapies, the ethical grounds for providing such treatment have also been called into question, given that homosexuality is not an illness (Halpert, 2000).
While reparative therapies are largely eschewed by the scientific community, there is some evidence of malleability of sexual orientation because both homosexual men and women may have at one time identified as either heterosexual or bisexual. However, it is very uncommon to see individuals that once identified as homosexual to identify as heterosexual later in life. Thus, this plasticity may in fact reflect the "coming out" process undertaken by many gay men and lesbians (for a more detailed description see Levay, 1996). It should be noted, however, that with regards to "erotic plasticity" women are generally thought to be more malleable than men; and there are compelling arguments made about women "switching" sexual orientations at various points in their lives (as discussed in Baumeister, 2004). There have also been numerous reports of individuals who experience changes in sexual interests, sexual orientation, and even perception of the gender of themselves and others as a function of brain injury in the temporal lobes (Cheasty, Condren, & Cooney, 2002; Kasper, Kerling, Graf, Stefan, & Pauli, 2009; Miller, Cummings, McIntyre, Ebers, & Grode, 1986).
I have attempted to provide a brief overview of some of the types of research that have been done and continue to be done in order to better understand the development of a homosexual orientation. This area of research is dynamic and researchers have attempted to address a variety of perspectives in their studies and for a variety of reasons. Despite decades of research, the ultimate answer to the question posed in the title of this paper, "Can anyone tell me why I'm gay?" is a resounding "Not yet."
While there is some evidence to suggest physiological correlates ranging from genetic markers to structure and function of the brain, one can feel quickly overwhelmed by the sheer amount of contradictory findings on all of these fronts, and the news is no better, if not worse, for research concerning the role that familial and environmental factors play in the development of sexual orientation.
In reviewing this literature, several concerns emerge for this writer: 1) sample representativeness is problematic when researching issues as socially charged as sexual orientation, 2) the ethical considerations that must be made in human research often result in correlative data which makes causative inference problematic, 3) conceiving of individuals' sexual orientations in binary terms may be overly simplistic and potentially misleading, 4) there is a paucity of information regarding bisexuality and female homosexuality, and 5) there seems to be an increasing focus on physiological mechanisms of origin for sexual orientation without regard for the role that familial/social/cultural environments may play in this process.
Despite these limitations, there are some developments that this writer feels have tremendous potential as tools for future research including the advent of a "functional endophenotype" for sexual orientation which may provide a more objective measure of sexual orientation (for example Ponseti et al., 2006). As neural imaging techniques become more sophisticated, our ability to use them in further delineating the differences in functions of the brains among individuals of various sexual orientations will be invaluable. The utilization of naturally occurring animal models of sexual orientation is also a promising approach, especially when conducted in conjunction with continued human studies. Aside from the male-oriented rams described earlier (Roselli & Stormshak, 2009), the animal kingdom is replete with homosexual, bisexual, and heterosexual interactions (for a wonderful discussion of the plethora of examples please see Bagemihl, 1999) that scientists might take advantage of in trying to understand how a given individual develops his or her sexual orientation. Furthermore, advances in our understanding of the human genome and its interaction with environmental factors will continue to elucidate potential mechanisms by which an individual's sexual orientation develops. Finally, as social mores around the world evolve, cross-cultural studies may also provide important insight into the roles that sociocultural factors play in shaping one's sexual orientation.
Few areas of research can match the social implications associated with investigating the etiology of sexual orientation. From "Don't Ask, Don't Tell" to the ongoing debate about legalization of gay marriage in the United States, this is an issue that is politically and socially divisive and that generates social stigmas that can be personally devastating. Nearly 6,000 hate crimes motivated by sexual orientation were reported to have occurred in the United States between 2005 and 2008 (Johnson, 2008), and perhaps as a function of stigmatization and discrimination, homosexual/bisexual individuals are at higher risk for attempted suicide, suicidal ideation, substance abuse, and several mental disorders (King et al., 2008).
It is not inconceivable that we may one day fully understand the origins of sexual orientation and be faced with the implications that such an understanding engenders. Despite compelling arguments to the contrary (Greenberg & Bailey, 2001), this author believes we have a moral imperative to use that information in a socially responsible and positive way. It is this writer's opinion that this information should be used to facilitate the development of a social and political environment in which diversity is fostered, valued, and celebrated for the richness it adds to the human experience, rather than being seen as a means to "treat", "cure", or "fix" what some may view as socially deviant and/or morally reprehensible behavior.
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William J. Jenkins
Author info: Correspondence should be sent to: Dr. William J. Jenkins, Dept. of Psychology, Mercer University, 1400 Coleman Ave., Macon, GA 31207.
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|Author:||Jenkins, William J.|
|Publication:||North American Journal of Psychology|
|Date:||Jun 1, 2010|
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