The following resources are the result of 3 years of study, many presentations, and with much thanks to my librarian husband for doing most of the leg work. When I started, I had a traditional/conservative point of view regarding the causes of homosexuality, and at that time had rarely ever given thought to gender identity. I was surprised – stunned, actually – to find how much hard science is available. For many years, even as a physician, I simply refused to look at the studies… subconsciously afraid of what I would find. I realize, of course, that is fear rather than faith. And God never, ever needs us to be afraid on his behalf. I’m happy to say that on the other side, these studies now bring me tremendous joy: amazement at the human body and its complexity, as well as the freedom and exhilaration that comes from learning.
There are actually thousands of pertinent studies, and there’s simply no way to present all of them. So I’ve chosen and organized them in a way that makes sense to me and is reflective of the questions I’ve asked and the answers I’ve found. Others might undertake the same journey and come up with a different approach and a different set of articles. Should you question my approach or conclusions, I encourage you to dig in and research it yourself. Good science should reveal the same conclusions regardless of the approach. And good scientists welcome scrutiny and differing perspectives, as it fills in gaps and clears up confusion.
As to my own expertise, you’ll have to decide that for yourselves. I’m a pediatrician with over 20 years of practice experience, so that of course lends a degree of knowledge. But human sexuality bridges a number of different disciplines and true experts are hard to find. My “expertise”, if there is any, began with a desire to understand, which led to stories of real people, and their recurring themes and similarities led me to scientific study. Over time, I’ve had the honor of hearing hundreds of stories and meeting more amazing people than any one human ought to have the privilege to know. Next came membership in the LGBT provisional section of the American Academy of Pediatrics, giving me access to lots of really smart physicians doing all kinds of amazing work. Finally, as my true expertise lies in effectively communicating what I’ve learned, I’ve had the opportunity to speak in a number of different settings … and I always learn way more than I teach.
My goal, in presenting this information, is to educate as accurately as possible… to make us think… to stimulate good conversation… and to help all of us get past our tendency to be afraid of things we don’t understand. Thanks for listening and learning; I hope it’s worth your time.
- Heylens, et al. “Gender identity disorder in twins: a review of the case report literature.” The Journal of Sexual Medicine 9.3 (2012):751-7. This study compared monozygotic (“identical”) twins to dizygotic (“fraternal”) twins. Of the monozygotic twins, 39% were concordant for gender identity disorder, meaning if one twin demonstrated gender identity disorder, the other twin did as well. None of the dizygotic twins, either same-sex or opposite sex, were concordant. The results are statistically significant and point to a strong genetic influence in transgender individuals.
- Bailey, et al. “Genetic and environmental effects on same-sex sexual behavior: a population study of twins in Sweden.” Archives of Sexual Behavior 39.1 (2010): 75-80. This study of 3826 monozygotic (identical) and dizygotic (fraternal) twins indicate genetic concordance in 34-39% of men and 18-19% of women.
- Kirk, K. M., et al. “Measurement models for sexual orientation in a community twin sample.” Behavior Genetics 30.4 (2000): 345-356. This study of 4901 Australian twins indicates heritability between 50-60% in females, and approximately 30% in males.
- Bailey, et al. “Heritable factors influence sexual orientation in women.” Archives of General Psychiatry 50.3 (1993): 217-23. Researchers from Northwestern University examined 71 pairs of identical twins, 37 pairs of fraternal twins, and 35 pairs of adoptive sisters. Concordance for homosexuality was found in 48% of the identical twins, as compared to 16% of the fraternal twins and 6% of the adoptive sisters.
- Bailey, et al. “A genetic study of male sexual orientation.” Archives of General Psychiatry 48.12 (1991): 1089-96. This study compares monozygotic twins, dizygotic twins, non-twin biological brothers, and adoptive brothers, indicating concordance (if one twin is gay the other is as well) in 52% of identical twins, 22% of fraternal twins, 11% of adoptive brothers, and 9% of non-twin biological brothers.
- Bouchard, Thomas J., et al. “Sources of human psychological differences: The Minnesota study of twins reared apart.” Science 250.4978 (1990): 223-228. This longitudinal study of 100 twins reared apart measures a wide range of physical, emotional and mental characteristics and helps demonstrate the overall importance of twin studies in understanding genetic influence.
- Summary of Twin Studies: Twin studies indicate a pure genetic influence on sexual orientation of around 10-60% depending on the specifics of the study, with an average of 30-40%. Around 30 such studies are available with similar findings, each differing slightly in population studied and specific findings. To help put this in perspective, IQ (intelligence) has a genetic influence of around 60%. Twin studies are used across science to sort out presence and strength of pure genetic influence not impacted by other embryonic factors. The absence of 100% concordance does not indicate lack of genetics; it simply sorts out the steps in the inherited process.
- Sanders AR, et al. “Genome-wide scan demonstrates significant linkage for male sexual orientation.” Psychological Medicine 45.7 (2015): 1379-88. This study conducted a genome-wide linkage scan of 409 pairs of gay brothers (908 total individuals studied), and found that genes on chromosome 8 and Xq28 influence homosexual orientation. This is the largest study to date, and replicates earlier studies conducted on smaller numbers or using less precise methods of study.
- Mustanski BS, Dupree MG, Nievergelt CM, Bocklandt S, Schork NJ, Hamer DH (2005). A genomewide scan of male sexual orientation. Human Genetics 116, 272–278. The first report of a full genome scan of sexual orientation in men demonstrated linkage with homosexual orientation on chromosome 7, chromosome 8, and chromosome Xq28. This study overlapped previous findings using less precise methods of study.
- Hamer DH, Hu S, Magnuson VL, Hu N, Pattatucci AM (1993). A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261, 321–327. This small study was the first to link chromosome Xq28 with male homosexual orientation. Later studies were mixed, with the most recent large study replicating these findings.
- Summary of Genome Studies: Genome-wide studies conducted over two decades with increasing size and sophistication implicate the pericentromeric area of chromosome 8, and chromosome Xq28 as impacting male homosexuality. Other genes have been implicated in other studies, but these two areas are the most consistently replicated.
- Bailey, J. Michael, et al. “A family history study of male sexual orientation using three independent samples.” Behavior genetics 29.2 (1999): 79-86. Using three different samples, this study found that the siblings of homosexual males and females are more likely to be homosexual than the prevalence found in the general population.
- Hamer, et al. “A linkage between DNA markers on the X chromosome and male sexual orientation.” Science 261.5119 (1993): 321-327. The role of genetics in male sexual orientation was studied using pedigree analysis and chromosome linkage analysis. Increased rates of same-sex orientation in maternal uncles and cousins were present, but not paternal relatives, indicating an x-linked chromosomal effect.
- Pillard, Richard C., and James D. Weinrich. “Evidence of familial nature of male homosexuality.” Archives of General Psychiatry 43.8 (1986): 808-812. This family study of homosexual and heterosexual men indicates that gay men are four times more likely than their straight counterparts to have gay brothers.
- Summary of Pedigree Studies: While it is often assumed that something “inherited” is found only in the chromosomes, that isn’t entirely true. In the case of sexuality, the chromosomes do not exert their effect directly, but rather through different hormones each requiring many steps. Therefore, sexuality has many aspects that are likely inherited and occur prior to birth, but cannot be explained only by chromosomes. These studies explore the familial nature of homosexuality without attempting to pinpoint the cause.
- Iemmola, Francesca, and Andrea Camperio Ciani. “New evidence of genetic factors influencing sexual orientation in men: Female fecundity increase in the maternal line.” Archives of sexual behavior 38.3 (2009): 393-399. This study is philosophically fascinating. It has long been questioned why the gay population doesn’t die out because of inability to reproduce in same-sex relationships. However, this study demonstrates that women in the maternal line of homosexual individuals (sisters, mothers, aunts, grandmothers, great-aunts, etc) are more fertile and have more children. This gives some explanation to the steady prevalence noted in homosexuality despite the decreased likelihood of bearing children.
- Bogaert, Anthony F., and Malvina Skorska. “Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review.” Frontiers in Neuroendocrinology 32.2 (2011): 247-254. This review of available studies discusses findings, replicated numerous times, that gay men tend to have more older brothers than straight men. The cause most consistently implicated is a prenatal maternal immune response that increases with each male pregnancy, so that each male child has a progressively increased likelihood of being gay as a result of an antibody effect on the sexuality of the brain.
- Blanchard, Ray. “Review and theory of handedness, birth order, and homosexuality in men.” Laterality 13.1 (2008): 51-70. This review article examines and discusses the evidence that older brothers increase the odds of homosexuality, but more so in right-handed males than in non-right-handed males (such as left-handed or ambidextrous.) Possible explanations for these associations are that non-right-handed male fetuses are resistant to the maternal anti-male antibodies, or that mothers of non-right-handed male infants do not produce anti-male antibodies.
- Blanchard, Ray. “Fraternal birth order and the maternal immune hypothesis of male homosexuality.” Hormones and Behavior 40.2 (2001): 105-114. One of the earlier studies, replicated many times in the US, Germany and England, indicating that increasing numbers of older brothers increase the likelihood of homosexuality in the younger/later-born brothers.
- Summary of Birth Order Studies: An increased number of older brothers increases the odds of a male child being gay. This is demonstrated in right-handed males more than non-right-handed males. A maternal immune response is the theorized cause, with increasing male pregnancies resulting in production of antibodies that prenatally impact the sexuality of the male brain.
- Berenbaum, SA, et al. “Sexual differentiation of human behavior: effects of prenatal and pubertal organizational hormones.” Frontiers of Neuroendocrinology 32.2 (2011): 183-200. This paper reviews evidence that prenatal sex hormones produce permanent changes to behavior. There is good evidence that exposure to high levels of androgens (such as testosterone) during prenatal development results in masculinization of activity, occupational interests, sexual orientation, spatial effects and gender identity.
- Meyer-Bahlburg, et al. “Sexual orientation in women with classical or non-classical congenital adrenal hyperplasia as a function of degree of prenatal androgen excess.” Archives of Sexual Behavior 37.1 (2008): 85-99. This study discusses findings that women with congenital adrenal hyperplasia, which due to an enzyme defect results in higher prenatal exposure to androgens, have variable degrees of masculinization of body and behaviors, increased bisexuality and increased homosexuality.
- Cohen-Bendahan, Celina CC, et al. “Is there an effect of prenatal testosterone on aggression and other behavioral traits? A study comparing same-sex and opposite-sex twin girls.” Hormones and Behavior 47.2 (2005): 230-237. This study demonstrates an increase of “male-type behaviors” in girls with a twin brother, as compared to girls with a twin sister. The likely cause is a prenatal testosterone surge that occurs during development of the male fetus, with an indirect exposure to the female twin sharing the prenatal environment.
- Cohen-Bendahan, Celina CC, et al. “Prenatal exposure to testosterone and functional cerebral lateralization: a study in same-sex and opposite-sex twin girls.” Psychoneuroendocrinology 29.7 (2004): 911-916. Girls with a twin brother demonstrate brain findings that are more masculine than girls with a twin sister, indicating a prenatal hormonal impact of the testosterone surge that male fetuses experience during fetal development.
- Summary of Prenatal Hormones: The exposure of an unborn fetus (or lack of exposure) to prenatal hormones has an important and permanent effect on the infant brain. There is no evidence that hormones or other environmental factors after birth impact sexual orientation or gender identity. Hormonal effects likely account for much of human sexuality that is not accounted for by genetics.
- D. F. Swaab and Garcia-Falgueras, Alicia. “Sexual differentiation of the human brain in relation to gender identity and sexual orientation.” Functional neurology 24.1 (2009): 17-28. During the prenatal period, the infant brain develops in the male direction under the influence of testosterone, and in the female direction in the absence of testosterone. Gender identity and sexual orientation are programmed into our brain structures by these hormones (or lack thereof) prior to birth. However, because the outer sexual parts are formed in the first 2 months of pregnancy, and sexual differentiation of the brain occurs much later, the hormonal influence can be different for the brain than for the external sexual characteristics, resulting in same-sex attraction or transgender identity (the brain has a different gender identity than the outside sexual parts.)
- I. Savic and P. Lindstrom. “PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects.” Proceedings of the National Academy of Science (2008). This study of 90 people (25 heterosexual men and women, and 20 homosexual men and women) used PET scans and functional MRI to study the structural and functional differences in their brains. The findings demonstrated that the brains of heterosexual men and homosexual women were more alike, and the brains of heterosexual women and homosexual men were more alike. The results cannot be explained by learned behaviors, and suggest a linkage to neurobiological causes.
- Allen, LS et al. “Sexual orientation and the size of the anterior commisure in the human brain.” Proc Natl Acad Sci 89.15 (1992): 7199-202. This early study was one of the first to demonstrate a difference in brain structure based on human sexuality. The anterior commisure is an area of the brain known to demonstrate differences in size based on sexuality (male versus female). In this study, this area of the brain was 34% larger in homosexual males than heterosexual males.. Heterosexual women had an anterior commisure that was 28% larger than heterosexual males. Homosexual males and heterosexual women has similar-sized anterior commisures, with the homosexual males 6% larger than the heterosexual women.
- LeVay, S “A difference in hypothalamic structure between heterosexual and homosexual men.” Science 253 (1991): 1034-7. The hypothalamus is an area of the brain with a known relationship to sexual behavior in males. This double-blind study, since cited by nearly every expert in the field, demonstrated that the hypothalamus of heterosexual men is nearly twice as large as that of homosexual men. The hypothalamus of homosexual men and heterosexual women were similar in size.
- Summary of Brain Sexuality: Based on imaging studies and sophisticated functional studies, the literature continues to support marked differences in the brain structure and function based on sexual orientation as well as gender identity. Brain structure is formed in the second and third trimester of prenatal development, with no evidence that postnatal environment has any effect. While these studies do not pinpoint a cause for the differences, they are very compelling in supporting the involuntary nature of sexual orientation and gender identity.
There are no peer-reviewed published scientific studies supporting the hypotheses that life experience causes homosexuality, that sexual orientation or gender identity are learned, that there is a psychological cause of homosexuality, that sexual orientation or gender identity are chosen, or that sexual orientation can be changed.
Evidence-based study of sexual and gender nonconformity has a number of challenges: the prevalence is relatively low, leading to small studies; the cultural stigma causes reluctance of people to subject themselves to study; methods of identifying patients can introduce bias; and early studies relied on self-report of sexuality which is often unreliable. However, despite these challenges, the field of study is robust, and the results are compelling. Researchers have replicated smaller studies to add validity, have worked diligently to address and remove bias, and have developed increasingly sophisticated and precise methods to measure human sexuality objectively and reliably.
Based on available scientific evidence, the jury for me is no longer out: sexual orientation is programmed into the brain of an infant prior to birth. The evidence includes studies of genetics, prenatal hormonal impact, maternal immunity, and other prenatal factors. Conversely, no peer-reviewed studies have identified any postnatal factors, such as parenting techniques, abuse, neglect, or spiritual beliefs, that can prevent or alter homosexuality or gender identity variations. The methodology for such study is readily available; therefore, any sound theory of postnatal factors should be easily demonstrable through the same fields of study. And yet, these studies do not exist. This absence speaks loudly, and should be strongly considered by thoughtful people prior to touting such theories as causing gender or sexual variations.
Our LGBTQ community has consistently told us that sexual orientation and gender identity are not chosen, and cannot be changed. Based on the consistency of their stories, the available scientific literature, and the complete lack of evidence in any opposing viewpoints, it is time that we believe them.