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An Explanation for Homosexuality WILLIAM S. BRADSHAW


FROM TIME TO TIME, ENCIRCLE PROUDLY PRODUCES MATERIALS, LIKE THIS DOCUMENT, PREPARED BY THIRD PARTIES. THIS CONTENT IS THE WORK AND RESEARCH OF THE AUTHOR AND NOT ENCIRCLE.

Copyright © 2019. Encircle: LGBTQ Family & Youth Resource Center. All rights reserved.


About the Author Bill Bradshaw is Professor Emeritus from the Department of Microbiology and Molecular Biology at Brigham Young University where he taught and conducted research for 38 years. At BYU he served as Associate Dean of Honors and General Education. He has an A.B. degree in Biology from Harvard, and a PhD in Biochemistry from the University of Illinois. Bill’s interest in LGBTQ issues stems from three experiences. The first is academic. The question of the origin of homosexuality was occasionally posed by one of his students in an introductory biology, molecular biology, or cell biology course. The second arose during his ecclesiastical service in the LDS Church. With his wife, Marge, he presided over the mission of the church in Hong Kong and South Vietnam from 1971 to 1974. He subsequently served as a Bishop and then a Counselor in a Stake Presidency for married students at BYU. The third,and most important, is because Marge and Bill are parents of a wonderful gay son. Brett and his husband Jeff and their daughter Madeline live in Oakland, CA. Marge and Bill have served as Co-Chairs of LDS Family Fellowship, a support group for parents of LGBTQ children.


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What you will read here is motivated by a desire to be helpful – helpful to as many people as possible who have questions about, or an interest in, the subject of homosexuality. This includes those in the LGBTQ community, their friends and family members, religious and civic leaders, and any others who may wish to become more informed. Consider parents, who upon learning that their young son or daughter does not identify as heterosexual, are likely to ask “What is responsible for this? What did we do wrong? Must we abandon our dreams for grandchildren?” and a host of other concerns about long-cherished beliefs and hopes. We will not attempt to answer all of these important questions. Human sexuality is complex,and many of its important aspects must await consideration at another time. The focus here is on the narrow issue of origins: What is known about why some people are erotically attracted to persons of the opposite sex, and others to persons of the same sex? Many of the details of this orientation pathway remain to be uncovered. However, there is now sufficient knowledge about this question, so that you can be confident in the answers we are providing. We have tried to present this information in a logical fashion, but you may have specific concerns that are not immediately addressed. Please be patient and see us through to the end.

NOTE FROM THE AUTHOR • 5


Table of Contents 9

THE MESSAGE

10

WHY IS THIS IMPORTANT?

13

WHAT IS THE EXPLANATION?

16 HOMOSEXUALITY IS A HUMAN CONDITION 19

DISCREDITED EXPLANATIONS

19

Dysfunctional Parenting Early Same-Sex Experimentation Sexual Abuse

20 21

24 EVIDENCE FOR A BIOLOGICAL ORIGIN 25 Gay People Often Exhibit Atypical Behavior Early in Childhood 26 Gay Men Often Have Multiple Older Brothers 26 Sexually Dimorphic Traits 31 Brain Anatomy 34 Non-Right Handedness 34 Finger Length 35 Steroid Hormones 38 Genetics: Twin Studies 41 Epigenetics

43 CONCLUSION


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The Message “IS AN LGBTQ INDIVIDUAL BORN THAT WAY?” THE ANSWER IS “YES.”

You will find in this booklet the answer to the question “What causes a person to be gay or lesbian?” The most personal way to frame that answer is that no one chooses his or her sexual orientation. You will find here that there is an overwhelming body of research evidence that biological processes operating largely during embryonic development are responsible, and conversely, that alternative explanations based on social experience have not been verified. To reduce this issue to its most simplistic form, “Is an LGBTQ individual born that way?” The answer is “Yes.” This is not to say that the way people live their lives is fixed at birth. We are all influenced, including the expression of our sexuality, by our culture, attitudes to which we are exposed, and our interactions with others. It is also important to acknowledge that not all homosexual persons are the same; the degree of attraction can vary significantly.

The Message • 9


Why is This Important? A major reason is that the notion that someone would be attracted to an individual of the same sex is initially hard to understand for people in the heterosexual majority; if you can’t imagine that possibility for yourself, how could it be reasonable for someone else? So, understanding the correct explanation for the origin of sexual orientation/partner preference (to whom a person is attracted and wants to be with) is a critical first step in correcting misconceptions and overcoming the negative impressions that so often prevent us from acquiring respect and compassion for people who are different. THIS KNOWLEDGE IS IMPORTANT FOR LGBTQ PEOPLE

in helping them avoid discredited change programs, and proceed to develop self-respect and maintain the dignity that is necessary for a happy and productive life. THIS KNOWLEDGE IS IMPORTANT FOR MOTHERS AND FATHERS, who need to believe the heartfelt expressions

of their LGBTQ children that their non-heterosexuality is an unchosen condition, and who should not harbor any guilt because something they did or failed to do as parents is responsible. THIS KNOWLEDGE IS IMPORTANT IN SOCIETY AT LARGE as we as citizens evaluate the moral and legal

efforts to provide equality for all human beings. So, as we begin an examination of the nature of homosexuality and how it originates, it may be useful to keep in mind what the evidence will show. Consider, then, the following list of false notions that we will want to discard:

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Why is This Important? • 11


HOMOSEXUALITY IS NOT: A DISEASE A DISORDER AN INCLINATION A PROPENSITY A PREDISPOSITION AN ADDICTION A TEMPTATION A TEMPORARY PHASE A LEARNED BEHAVIOR, OR A COMMUNICABLE ILLNESS

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What is the Explanation? Instead, homosexuality is a normal variation of sexual attraction, expressed in a minority of the human population. It has a biological basis, operating through the molecular mechanisms that control the anatomy and function of the developing brain. As you will see, a great deal has been discovered about this biology. But our knowledge of the details is far from complete. A comprehensive overview of the pathway leading from brain structure through neural operations to emotions is not yet possible. But this should not cast doubt on the weight of evidence that leads to this fundamental conclusion – the explanation for homosexuality is biological.

What is the Explanation? • 13


Let’s make one additional important introductory comment. It is common for some people to associate a condition that is “genetic” with an illness, a pathological state that needs to be “cured.” This is not true with respect to sexual orientation. The genes that result in same-sex attractions are no more “bad” than those resulting in red hair or left-handedness.

We strongly reject the notion that because some people come into the world with a biological difference, they are somehow less than fully human. We urge celebration of our diversity, not discrimination. Finally, we acknowledge that the scientific information that follows is somewhat technical, with vocabulary and concepts that are new or may have been forgotten. In order to try to accommodate readers from all backgrounds, we have opted for a format that begins with a simplified “take-home message.” We then cite, as validation, the results of studies from the scientific literature. Each section then ends with a summary conclusion statement in bold print. We will begin with some general statements about the nature of homosexuality. Next we discuss widespread, but now discredited, beliefs about how homosexuality is caused by social experience. We conclude with the evidence that supports a biological origin. Although the primary focus here is on homosexuality, we do include some information about gender identity. Unfortunately there are insufficient research data about persons who are transgender. Nevertheless, the sexual needs of transgender people are important, and they deserve the same measure of sensitivity and kindness as others in the LGBTQ community.

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What is the Explanation? • 15


Homosexuality is a Human Condition. It crosses ethnic and geographical boundaries, and is independent of culture. PREVALENCE—Although estimates vary, there is a general consensus that a minimum of about 3.5-5% of both men and women are homosexual (non-heterosexual). The number may be somewhat higher (perhaps 8-10% overall) if people are included who acknowledge same-sex attractions, desires, or fantasies, but are reluctant to identify themselves as homosexual. There are 700,000 transgender persons in the US (0.3% of the adult population) [1]. HISTORICITY—Historical records document homosexuality

from ancient to modern times throughout the world [2]. CONSISTENCY—The patterns of homosexuality are

consistent from society to society despite their cultural differences [3, 4].

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Discredited Explanations • 17


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Discredited Explanations There are three discredited social explanations about the cause of homosexuality that are based on social experience: DYSFUNCTIONAL PARENTING, EARLY SAME SEX EXPERIMENTATION, AND SEXUAL ABUSE.

These hypotheses have been discredited for lack of evidence.

Dysfunctional Parenting Male homosexuality has been attributed to the actions of an overprotective mother and/or “absent” father, that is, a father who is cold and neglectful or is hostile toward his son during childhood. The notion is that the homosexual male reacts to this paternal indifference by adopting a passive sexual selfimage and seeks a romantic connection with another male. It is true that some gay men report a feeling of distance or even hostility from their fathers, but this is quite reasonably explained as the result of innate homosexuality and consequent atypical childhood behavior, not the cause of it [5, 6]. Some fathers are likely to consciously or unconsciously express disappointment in a young boy who is gender non-conforming, that is, who does not enjoy sports or the other rough-and-tumble activities that the father was expecting to enjoy with his son. The authors of a 1981 study utilizing information obtained earlier from a large survey [7] conclude: “No particular phenomenon of family life can be singled out, on the basis of our findings, as especially consequential for either homosexual or heterosexual development.” And again,“Our findings suggest that HOMOSEXUALITY IS AS DEEPLY INGRAINED AS HETEROSEXUALITY, so that the differences in behaviors or social Discredited Explanations • 19


experiences of prehomosexual boys and girls and their preheterosexual counterparts reflect or express, rather than cause, their eventual homosexual preference.” A very recent meta-analysis of 77 published studies conducted by Columbia University Law School examined the influence of the sexual orientation of parents on their children. The conclusion was that there were differences in attitudes and behaviors among children raised by lesbians or gay men, but there was no evidence for differences in sexual orientation compared to children raised in heterosexual households [8].

Homosexuality is not learned; parents’ attitudes or rearing styles cannot cause their children to be gay.

Early Same-Sex Experimentation It has been shown that children who attend boarding schools with only those of their same gender, where homosexual experimentation is more common than in other settings, are not more likely to emerge as homosexual adults [9]. Studies have been conducted among the Sambia, a tribe of people in New Guinea in which boys are required to engage in homosexual practices throughout adolescence in order to ensure male health and vitality. As adults, these men marry and behave heterosexually [10]. Research fails to support the thesis that childhood sex play leads to homosexual orientation. The majority of participants

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in a study of gay male siblings (gay men with gay brothers) did not engage in such sex play. They reported awareness of homosexual feelings at about age 11, but were not aware of their brothers’ homosexual orientation until about age twenty-one [11].

Homosexuality does not result from early sexual experience. Moreover, LGBTQ persons cannot be recruited.

Sexual Abuse A research investigation found that 21% of nearly 2000 adult lesbian women reported being sexually abused or raped as children. This is certainly an unfortunate statistic. However, the authors indicate that “the results of the current study indicate that the rate of incest among lesbians (18.7% overall) is quite similar to that among the general female population (16%) [12]. Another investigation of childhood sexual experience in the lives of 54 adult gay men and 29 lesbians showed the events occurred outside of the nuclear family, with cousins, and were perceived as positive, especially the men. This suggests that the relationship represented sexual experimentation/curiosity, not abuse [13]. A study was conducted of gay men who as adolescents between the ages of 12 and 17 had sexual relations with male adults. Almost all of the subjects reported being aware of their homosexual orientation on average 4 years before such encounters took place [14]. In another related study with a non-clinical population of adults, researchers found elevated levels of childhood and Discredited Explanations • 21


adolescent molestation compared to heterosexuals [15]. A majority (68% of men and 62% of women), however, identified themselves as homosexual before the molestation took place. These authors also are cautious in interpreting the data: “[Molestation] may not, however, be a causal factor in either gender. Perhaps children or adolescents with a higher potential for homosexual behavior are more likely to enter a situation that leads to same-sex molestation.” Innocent LGBTQ children and youth have been the unwitting victims of sexual assault by predatory adults. Other authors of research into the consequences of molestation offer this observation [16]: “Once self-labeled as homosexual, the boy may later place himself in situations that leave him open to homosexual activity. It should be emphasized that the vast majority of homosexuals do not report childhood sexual experiences and also that the vast majority of male pedophiles do not regard themselves as homosexual.”

Sexual abuse, though highly regrettable, does not cause homosexuality.

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Discredited Explanations • 23


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Evidence for a Biological Origin The evidence does not point to social causes for homosexuality. We present here evidence from many fields of research showing that homosexuality has a biological origin.

Gay People Often Exhibit Atypical Behavior Early in Childhood There is a high correlation between Childhood Gender Non-conformity and non-heterosexuality in adulthood. A review of over 40 retrospective studies found a high correlation between atypical behavior during childhood and the sexual orientation of adult homosexual men and woman. As children, gay men engaged in less rough-and-tumble play and less aggressive behavior (less competitive athletic sports, for example) compared to heterosexual men as children. They more often wore girl’s clothes and were identified as “sissies”. Lesbians as children were more physically active (engaged in rough-and-tumble play and sports), had more boy playmates, more often wore boys’ clothing than heterosexual woman as children, and were identified as “tomboys” [18].

Evidence for a Biological Origin • 25


Gay Men Often Have Multiple Older Brothers In men, there is a correlation between homosexual orientation and the number of older brothers (but not younger brothers or sisters, or older sisters): the “older brother effect.” This observation has been replicated many times [19]. Recent data demonstrate that this relationship holds for biological older brothers reared apart, but not for non-biological older brothers in the same household [20], providing strong evidence that this effect has a prenatal origin. The following immunological explanation has been proposed for the older brother effect. During pregnancy, an expectant mother may generate antibodies against those proteins in her male fetus that are programmed by genes on his Y chromosome. She may retain an immunological memory in subsequent pregnancies, and the gender-related development of younger male children who have been exposed to an increasing quantity of those antibodies is affected.

Sexually Dimorphic Traits Investigations of sexually dimorphic traits show that gay, lesbian, and bisexual persons are different from their straight counterparts. Some traits, including anatomical and physiological characteristics, are significantly different in heterosexual men and heterosexual women. These are termed sexually dimorphic, and are known to be established prenatally. The research question is “How are gay men and lesbians compared to their heterosexual counterparts with respect to these traits?” Let’s first examine traits reflecting brain function.

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Evidence for a Biological Origin • 27


Homosexual and heterosexual persons differ in certain cognitive abilities, several aspects of hearing, and the way in which the brain responds to odors.

COGNITIVE ABILITIES (WAYS OF THINKING) In certain verbal, quantitative, and visual-spatial tests, heterosexual men and women perform differently (examples include letter fluency, synonym fluency, object location memory, mental rotation, and judgment of line orientation).

Homosexual men and women are atypical for their genders in tests of these cognitive abilities; gay men perform like women, lesbians perform like men [21]. HEARING In homosexual and bisexual women the strength of otoacoustic emissions (OAEs, tiny “echoes” produced by the inner ear), is intermediate between heterosexual females and heterosexual males [22]. These responses are influenced by steroid hormones: women with male cotwins have OAEs more like males than other females (an effect mediated from the testosterone received by their fetal brother through the intra-uterine circulation). Certain measures of auditory evoked potentials (AEPs) in homosexual males are hyper-masculinized [22]. An AEP is an electrical brain response to click stimuli that is detected and recorded by electrodes placed on the scalp.

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Men and women differ in the degree to which they are startled by a loud sound stimulus if they are exposed earlier to a weaker sound. This non-learned behavior is called “prepulse inhibition” (PPI) and can be experimentally measured via eye blink. PPI is lower in women than it is in men, but is significantly masculinized in homosexual women. There is no PPI difference between heterosexual and homosexual men [23].

PROCESSING OF ODORS Investigators in Sweden used PET (positron emission tomography) scans to monitor brain activity of people in response to the odors produced by chemicals derived from testosterone (that might target women) or estrogen (that might target men). The scans map the regions of the brain that are active during the test. They found that the brains (hypothalamus) of homosexual men, like heterosexual women, respond to the metabolic derivative of testosterone (called AND), and respond atypically (unlike heterosexual men) to a metabolic derivative of estrogen (called EST) [24]. The same research group subsequently determined that lesbian women process AND in different areas of the brain than heterosexual women. In addition, these women respond to EST by activating the hypothalamus in a shared pattern with heterosexual males.

The authors conclude “the data support the notion of a coupling between hypothalamic neuronal circuits and sexual preferences and encourage further evaluation of the possible neurobiology of homosexuality and human sexuality in general” [25].

Evidence for a Biological Origin • 29


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Brain Anatomy Some brain structures differ between homosexuals or transsexuals and heterosexuals. HYPOTHALAMUS (REGULATES SEXUAL BEHAVIOR) Work from 3 different laboratories shows that a structure in the human hypothalamus (the third interstitial nucleus, INAH3) is sexually dimorphic (of significantly larger volume – more neurons –in men than in women). Two of these studies also demonstrate that this measure for homosexual males is intermediate between that of heterosexual men and women (due to a greater density, but not number of neurons) [26, 27]. The INAH3 volume and number of neurons of male-tofemale transsexual people is similar to that of control females [28].

Evidence for a Biological Origin • 31


The analogous brain structure in rats is known to regulate sexual behavior. At sexual maturity, the male offspring of mothers who were placed under stress (confinement and heat) during pregnancy exhibit female reproductive behavior (they adopt the female mating posture and allow mounting) in the presence of aggressive males. The hypothalamic structure in question is feminized in these males [29]. Eight percent of rams are naturally male-oriented (will not engage in reproductive behaviors with ewes, but will with males). The oSDN (ovine sexually dimorphic nucleus, which is analogous to the structure in humans and rats described above), is significantly larger in female-oriented rams than in ewes, and is significantly reduced in male-oriented rams compared to female-oriented males. In sheep, the enzyme aromatase converts the steroid hormone testosterone into estrogen, which then acts to program the differentiation of the male brain during embryonic development. Expression of the gene that encodes that aromatase enzyme is significantly reduced in the oSDN of male-oriented rams [30].

ISTHMUS OF THE CORPUS CALLOSUM (CONNECTS THE TWO BRAIN HEMISPHERES A study using magnetic resonance imaging (MRI) [31] corroborates earlier work [32] showing that the anatomy of another structure in the brain, the corpus callosum (specifically the structure known as the isthmus), is different between homosexual and heterosexual men.

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The size of the corpus callosum is under genetic control (94% heritability), and this brain structure, which connects the two hemispheres, may well underlie certain genderspecific cognitive differences in which homosexual persons differ from heterosexuals.

CEREBRUM AND AMYGDALA (ENABLES CONSCIOUS THOUGHT & EMOTIONS) MRI analysis was also used to measure the volumes of the left and right hemispheres of the cerebrum of the brain. The results demonstrate sexual dimorphism (heterosexual men and women differ), and that homosexual individuals are atypical for their gender. Heterosexual males and homosexual women showed a rightward cerebral asymmetry; the values for homosexual men and heterosexual women were symmetrical. In the same study, PET scans were used to measure cerebral blood flow as an index of functional connections from the left and right amygdalae. The amygdala functions in the processing of emotions. In homosexual men and heterosexual women, the connections were more widespread from the left amygdala; in homosexual women and heterosexual men, the connections were more widespread from the right amygdala. Importantly, these effects could not be ascribed to learning (perception or behavior) [33].

Brain anatomy of homosexual or transsexual people is different.

Evidence for a Biological Origin • 33


Non-Right Handedness Homosexual women, in contrast to heterosexual women, are more often left handed or ambidextrous [34]. In the population of men with no older brothers, homosexuals are more likely to be non-right handed [35]. Handedness is believed to be influenced by prenatal exposure to androgens (testosterone).

Finger Length The ratio of the length of the index finger to the 4th finger (2D:4D) is established before birth, is stable by age two, and develops as a function of exposure to testosterone. This anatomical index of growth is greater in women (a ratio close to 1) than it is in men (a ratio less than 1). The 2D:4D ratio of homosexual individuals is atypical for their gender, an observation that has been replicated in multiple studies.

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Thus, this digit length measure is smaller, therefore masculinized, in lesbians than it is in heterosexual women. In studies with males, comparisons of the 2D:4D ratio between heterosexual and homosexual men vary with the geographical location from which the subjects were sampled. In some studies, the values for homosexual men are reduced, thus supermasculinized; in other samples the values for homosexual men are greater, thus feminized. In each case, however, the raw data for homosexuals are constant; it is the measures for the heterosexual control groups that are variable [36]. Male to female transsexuals have a higher digit ratio than control males, but one comparable to control females [37].

Steroid Hormones Sexual differentiation in mammals includes development of the external genitalia, the internal reproductive organs (gonads and accessary structures), and the brain. The normal result is structural and functional differences between males and females [38]. At an early stage the gonads of both males (possessing an X,Y chromosome pair) and females (with 2 X chromosomes) are “indifferent, “ that is unspecified, and may develop into either ovaries or testes depending on the intervention of steroid hormones. The intrinsic default program is female; in the absence of testosterone the gonads become ovaries and both the internal accessory structures and external sex organs are female. In contrast, differentiation in males is under the control of the SRY gene (Sex-determining Region Y) located on the Y chromosome. The protein programmed by this gene binds to DNA and regulates the expression of other genes which then function to program the gonads as testes. Testosterone produced by specific testicular cells then directs

Evidence for a Biological Origin • 35


the formation of the accessory ducts and the male genitalia. In males, defeminizing events are also required. A second regulatory molecule, Mullerian inhibitory factor/hormone/ substance (MIF, also programmed by a Y chromosome gene) causes regression of the potentially female duct system. That sexual differentiation and sexual orientation are under hormonal control is dramatically exemplified by two examples in humans of genetic mutations that produce intersex individuals, whose sexual anatomy is inconsistent with his or her sex chromosomes. In the first, called congenital adrenal hyperplasia (CAH), a recessive genetic lesion alters the biochemical pathways that synthesize steroid hormones in the adrenal glands. The result is that cortisol (the stress hormone) is reduced, and instead, there are excessively high levels of testosteronerelated steroids. The genital anatomy of CAH males is normal. However, genetic females with this condition exhibit masculinization of the genitalia, and highly elevated rates of non-heterosexuality compared to non-CAH adults [39]. In another human intersexual condition called androgen insensitivity syndrome (AIS), the gene that encodes a steroid receptor protein is mutant. This receptor is the molecule that normally escorts testosterone to its destination in the nucleus of a target cell. Thus, hormonal signally by testosterone is impossible in this mutation. Males with this condition are convincingly female in anatomy. These persons are nearly always raised as women, and are no different in psychological well-being compared to control women. They uniformly exhibit sexual attraction to men [40]. In instances when infant boys are surgically altered, their sex reassigned, and socially raised as girls, they remain erotically attracted to females as adults [41]. The small number of documented cases of this kind are the result of surgical accidents in infancy or a naturally occurring penile malformation called cloacal exstrophy.

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Evidence for a Biological Origin • 37


Genetics: Twin Studies Genetics governs the biological mechanisms responsible for sex differences, including partner preference. Sexual behavior is clearly under genetic control in animals. A single gene, Fruitless, controls male and female sexual behavior in fruit flies. When the female route of expression of the gene is experimentally induced in genetic males, they do not exhibit male courtship movements and sounds. When the male route of expression of the gene is experimentally induced in genetic females, they behave sexually like males [42]. It is not valid to dismiss evidence obtained from nonhumans with the complaint that, “But, of course, people are not fruit flies.” Evidence continues to mount that the biochemical mechanisms that operate during embryonic life are remarkably similar, in general outline, among animals, and a large number of the genes that control development in fruit flies and people (and worms, and frogs, and mice) are the same [43]. Data from random samples show that gay men are at least twice as likely to have gay brothers as are heterosexual men. Lesbians tend to have a much higher incidence of lesbian sisters compared to those in the general population) [44]. Studies of twins have established that both homosexuality and transsexuality have a genetic basis.

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In these studies concordance (the degree to which both members of a pair of twins has the same sexual orientation) is greatest among identical (monozygotic) twins, next highest in fraternal (dizygotic) twins, and least in nontwin siblings. The more closely related the siblings are biologically, the higher the probability that they will exhibit the same traits. The data were obtained in the United States from populations of gay men [45] and lesbians [46], from a U.S. national sample of twin and non-twin sibling pairs [47], from a national registry of twins in Australia [48], and most recently, from a national twin registry in Sweden [49]. Identical twins are more often both transgender than fraternal twins (20% compared to 2.6%) [50]. Nevertheless, the genetic story behind homosexuality is not a simple one. The concordance among identical twins is not 100%. Some set of factors in addition to the presence of a set of genes must also be responsible. Those additional factors are sometimes referred to as “environmental,” but that does not mean they are influences such as social experience acting outside the body of a person. The professional view is:

“Environmental effects’ does not refer to hypotheses about the causal role of dominant mothers and distant fathers or sexual abuse in homosexuality that have long been discredited scientifically, but continue to retain their cachet among some circles” [51].

Evidence for a Biological Origin • 39


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Epigenetics These factors are, instead, internal biological mechanisms acting on existing genes. They are epi (on top of) genetic. How might we understand this? The fertilized egg divides, producing two new cells with identical chromosomes. Each subsequent division brings a new generation of cells, and they are all genetically equivalent. But at a certain point equivalent sets of genes produce different kinds of cells, each with a unique structure and function. This is possible because there are regulatory genes that determine when, where, and with what intensity the other genes for liver, heart, and brain, for example, are to be expressed. Turn on the brain genes in what will become brain cells, and keep the liver and heart genes off in those cells. Subtle differences in epigenetic control, then, could explain why one of a pair of identical brother is gay, while the other twin is straight. Homosexuality may not be the result of mutations, but subtle differences in the pattern of expression of non-mutant genes [52]. The biochemistry behind epigenetics is now quite well understood. One major goal in the process is to facilitate the conversion from DNA in its highly condensed state that exists in a chromosome, into the open, accessible state in which it can be activated. Nevertheless, the intricate experiments currently being performed to confirm the epigenetic model are complex and challenging [53].

Homosexuality has a complex genetic and epigenetic basis.

Evidence for a Biological Origin • 41


Love and respect for LGBTQ persons is called for no matter what the cause of their nonheterosexuality. But the knowledge that no one chooses his or her sexual orientation should help us to more quickly become allies in the quest for social equality.

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Conclusion The data presented here (much of which have only been compiled in the last decade) are compelling and justify the conclusion that the factors responsible for sexual orientation are biological. In contrast, empirical evidence for the alternative psychological and social explanations that have been proposed is weak at best. The best model consistent with the current information is that there are causal genetic and biochemical mechanisms operating in a highly complex fashion during prenatal and early postnatal development that contribute to make some persons romantically oriented to others of the same sex. The evidence that homosexuality is innate is much stronger than the suggestion that it is not. Obviously, the data are incomplete. In comparison to other well established physiological scenarios, such as the biochemical pathways that regulate the breakdown and utilization of sugar or the biosynthesis of cholesterol or DNA, for example, the “sexual orientation pathway” remains to be defined. There may be a diversity of mechanisms. Thus, the biochemical route to homosexuality in males may well prove to be different than it is in females. Likewise, the etiology may not be identical among all homosexual men or among all homosexual women. It is also important to acknowledge that bisexual individuals, who have some degree of attraction to both men and women, may have more flexibility in life choices than those who are exclusively homosexual [55]. In addition, it is known that some persons experience “fluidity,” an unprogrammed modification of their sexual feelings over the life span [56]. THE FOLLOWING STATEMENT SUMMARIZES THE CURRENT PROFESSIONAL OPINION: “Sexual differentiation is clearly the result of

an interaction between endocrine, genetic and epigenetic mechanisms, and this conclusion largely applies to the differentiation of sexual orientation in animals and humans. Human sexual orientation and in particular its less common form homosexuality, is thus not mainly the result of postnatal education, but is, to a large extent, determined before birth by multiple biological mechanisms that leave little to no space for personal choice or effects of social interactions” [54].

CONCLUSION • 43


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paper.) 16. Finkelhor, D. Child Sexual Abuse: New Theory and Research. New York: Free Press, 1984. 17. Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., and Epprecht, M. Sexual Orientation, Controversy, and Science. Psychological Science in the Public Interest, 17 (2), 45-101 (2016). 18. Bailey, J.M., and Zucker, K.J. Childhood sex-typed behavior and sexual orientation: a conceptual analysis and quantitative review. Developmental Psychology, 31, 43-55 (1995). 19. Blanchard, R. Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology, 230, 173187 (2004). 20. Bogaert, A.F. Biological versus nonbiological older brothers and men’s sexual orientation. Proc. Nat. Acad. Sci., 103, 10771-10774 (2006). 21. Halpern, D.F. and Crothers, M. Sex, sexual orientation, and cognition. In Sexual Orientation: Toward Biological Understanding. L. Ellis and L. Ebertz, Eds. Praeger, Westport, CT. London, 1997. 22. McFadden, D. Masculinization effects in the auditory system, Archives Of Sexual Behavior, 31, 98-111 (2002). 23. Rahman, Q., Kumari, V, and Wilson, G.D. Sexual orientation related differences in pre-pulse inhibition of the human startle response. Behavioral Neuroscience, 117, 1096-1102 (2003). 24. Savic, I., Berglund, H., and Lindstrom, P. Brain response to putative pheromones in homosexual men. Proc. Nat. Acad. Sci., 102, 7356-7361 (2005). 25. Berglund, H., Lindstrom, P., and Savic, I.. Brain response to putative pheromones in lesbian women. Proc. Nat. Acad. Sci., 103, 8269-8274 (2006). 26. LeVay, S. A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253, 1034-1037 (1991). 27. Byne, W., et al. The Interstitial Nuclei of the Human Anterior Hypothalamus: An Investigation of Variation with Sex, Sexual Orientation, and HIV Status. Hormones and Behavior, 40, 86-92 (2001). 28. Garcia-Falqueras, A, and Swaab, D.F. A sex difference in the hypothalamic uncinate nucleus: relationship to gender identity. Brain, 131, 3132-3146 (2008). 29. Lephart, E.D., Call, S.B., Rhees, R.W., Jacobson, N.A., Weber, K.S., Bledsoe, J. and Teuscher, C. Neuroendocrine regulation of sexually dimorphic brain structure and associated sexual behavior in male rats is genetically controlled, Biology Of Reproduction, 64, 571-578 (2001). 30. C.E. Roselli, K. Larkin, J.A. Resko, J.N. Stellflug, and F. Stormshak. The volume of a sexually dimorphic nucleus in the ovine medial preoptic area/anterior hypothalamus varies with sexual partner preference. Endocrinology, 145, 478-483, 2004. 31. Witelson, S.F., Kigar, D. L., Scamvougeras, A., Kedeckel, D.M., Buck, B., Stancheve, P.L., Bronskill, M., and Black, S. Corpus callosum anatomy in right-

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handed homosexual and heterosexual men. Archives Of Sexual 32. Scamvougeras, A., Kigar, D. L., Jones, D., Weinberger, D.R., and Witelson, S.F. Size of the human corpus callosum anatomy is genetically determined: An MRI study in mono and dizygotic twins. Neuroscience Letters, 338, 91-94 (2003). 33. Savic, I., and Lindstrom, P. PET and MRI show differences in cerebral asymmetry and functional connectivity between homo– and heterosexual subjects. Proc. Nat. Acad. Sci., 105, 9403-9408 (2008). Www.pnas.org/cgi/doi/10.1073/ pnas.0801566105. 34. Lalumiere, M.L., Blanchard, R. and Zucker, K.J. Sexual orientation and handedness in men and women: a meta-analysis. Psychological Bulletin, 126, 575592 (2000) 35. Blanchard, R., Cantor, J.M., Bogaert, A.F., Breedlove, S.M., and Ellis, L. Interaction of fraternal birth order and handedness in the development of male homosexuality. Hormones and Behavior, 49, 405 (2006). 36. McFadden, D. et al., A reanalysis of five studies of sexual orientation and the relative length of the 2nd and 4th fingers (the 2D:4D ratio). Archives Of Sexual Behavior, 34, 341-356, 2005. 37. Schneider, H., Piackel, J, and Stalia, G. Typical female 2nd-4th nger length (2D:4D) ratios in male-to-female transsexuals – possible implications for prenatal androgen exposure Psychoneuroendocrinology, 31, 265-269 (2006). 38. Money, J. Sin, sickness, or status? Homosexual gender identity and psychoneuro-endocrinology. American Psychologist, April, 384-398 (1987). 39. Zucker, K.J., Bradley, S.J., Oliver, G., Blake, J., Fleming, S., and Hood, J. Psychosexual development of women with congenital adrenal hyperplasia, Hormones and Behavior, 30, 300-318 (1996). 40. Hines, M., Ahmed, S.F., and Hughes, I.A. Psychological outcomes and genderrelated development in complete androgen insensitivity syndrome, Archives Of Sexual Behavior, 32, 93-101 (2003). 41. Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., and Epprecht, M. Sexual orientation, controversy, and Science. Psychological Science in the Public Interest, 17, 45-101 (2016). 42. Dulac, C. Sex and the single splice. Cell, 121, 664 (2005). 43. Carroll, S.B. Endless Forms Most Beautiful. W.W. Norton and Co., New York, N.Y. 2005. 44. Pillard, R.C. The search for a genetic influence on sexual orientation. In Science and Homosexualities, Rosario, V.A. Ed. Routledge, New York, 1997, pp. 226-241. 45. Bailey, J.M., Pillard, R.C. A genetic study of male sexual orientation. Arch. Gen. Psychiatry, 48, 1089-1096 (1991). 46. Bailey, J.M., Pillard, R.C., Neale, M.C., and Agyei, Y. Heritable factors influence sexual orientation in women. Arch. Gen. Psychiatry, 50, 217-223 (1993). 47. Kendler, K.S., Thornton, L.M., Gilman, S.E., and Kessler, R.C. Sexual orientation in a U.S. national sample of twin and non-twin sibling pairs. Am. J. Psychiatry, 157, 1843-1846 (2000).

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49. Langstrom, N., Rahman, Q., Carlstrom, E., and Lictenstein, P. Genetic and environmental effects on same-sex sexual behavior: A population study of twins in Sweden. Archives Of Sexual Behavior, DOI 10.1007/s10508-008-0386-1 (2008). 50. Diamond, M. Transsexuality among twins: Identity concordance, transition, rearing, and orientation. International Journal of Transgenderism, 14, 24-38 (2013). 51. Ngun, T.C., and Vilain, E. The biological basis of human sexual orientation: Is there a role for epigenetics? Advances in Genetics, 86, 167-184 (2014). 52. Rice, W. R., Friberg, U., and Gavrilets, S. Homosexuality as a consequence of epigenetically canalized sexual development. The Quarterly review of Biology, 87, 344-368 (2012). 53. Rice, W. R., Friberg, U., and Gavrilets, S. Homosexuality via canalized sexual development: A testing protocol for a new epigenetic model. Bioessays 35, 764770 (2013). 54. Balthazart, J. Sex differences in partner preferences in humans and animals. Philosophical Transactions Royal Society B. 371, 20150118 (2016). 55. Bradshaw, W.S., Heaton, T. B., Decoo, E., Dehlin, J. P., Galliher, R. V. and Crowell, K. A. 2015. Religious Experience of GBTQ Mormon Males. Journal for the Scientific Study of Religion. 54 (2), 311-329 (2015). 56. Diamond, L.M. Sexual Fluidity: Understanding Women’s Love and Desire. Harvard University Press, Cambridge, MA. 2008.

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OUR COSTS Encircle continues to produce publications that affirm and support LGBTQ+ folks and their families. Donations are greatly appreciated as we strive to continue to make the most relevant and meaningful content available. Information and donation opportunities are available at: www.encircletogether.org

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