March 24, 2024	PSY 340 Brain and Behavior Class 26 Variations in Sexual Behaviors

Remember that PSY 360 Human Sexuality is a semester-long course
which deals more completely with the psychology of sexual behavior

 A.   Evolutionary Interpretations of Mating Behavior

Recall that evolutionary motives lead to behaviors which increase the chance of mating and survival of the resulting offspring. "DNA seeks to pass itself on."

Issue	Males	Females
Interest in Multiple Mates?	M > F: Interest in short-term sexual relationships with multiple partners	Limited number of pregnancies  F have less to gain from multiple mates
What Do Men and Women Seek in Mate?	M = F: healthy, intelligent, honest, physically attractive Younger partner	M who are likely to be good providers M who have an acceptable odor
Jealousy?	M > F are upset over sexual infidelity of mate.	F > M are upset over emotional infidelity of mate.

Caution: Has a behavior evolved or been learned via culture? Note that the survival or reproductive advantage of a behavior and its appearance across cultures is not by itself proof that the behavior is either the product of evolution or learned from cultural influences.

   B.   Sexual/Gender Identity, Gender Dysphoria, & Gender-Differentiated Behaviors [includes material not in the textbook]

The terms "sex/sexual" and "gender" have traditionally been closely related and even considered by many to be identical. However, over the last half-century, the very close identification of sex and gender has been challenged. Without attempting to survey the entire range of issues and concerns, the understanding adopted here in a biopsychological context reflects how the psychological and psychiatric disciplines (and, in particular, the most recent DSM-5-TR [APA, 2022]) approach these terms. Thus, as noted in APA (2022):

• "[S]ex and sexual refer to the biological indicators of male and female (understood in the context of reproductive capacity), such as in sex chromosomes, gonads, sex hormones, and non-ambiguous internal and external genitalia" (p. 511, emphases added)
  
  
• "Disorders of sex development or differences of sex development (DSDs) included the historical terms hermaphroditism and pseudohermaphroditism. DSDs include somatic intersex conditions such as congenital development of ambiguous genitalia (e.g., clitoromegaly, micropenis), congenital disjunction of internal and external sex anatomy (e.g., complete androgen insensitivity syndrome), incomplete development of sex anatomy (e.g., gonadal agenesis), sex chromosome anomalies (e.g., Turner syndrome; Klinefelter syndrome), or disorders of gonadal development (e.g., ovotestes)" (p. 511)"
   
  
• "Gender is used to denote the public, sociocultural (and usually legally recognized) lived role as boy or girl, man or woman, or other gender. Biological factors are seen as contributing, in interaction with social and psychological factors, to gender development. Gender assignment refers to the assignment as male or female. This occurs usually at birth based on phenotypic sex and, thereby, yields the birth-assigned gender, historically referred to as “biological sex” or, more recently, “natal gender." (p. 511)
  
  
• "Transgender refers to the broad spectrum of individuals whose gender identity is different from their birth-assigned gender. Cisgender describes individuals whose gender expression is congruent with their birth-assigned gender (also nontransgender). Transsexual, a historic term, denotes an individual who seeks, is undergoing, or has undergone a social transition from male to female or female to male, which in many, but not all, cases also involves a somatic transition by gender-affirming hormone treatment and genital, breast, or other gender-affirming surgery (historically referred to as sex reassignment surgery)." (pp. 511-512)
  

Gender Dysphoria

• Individuals ranging in age from early childhood through adolescence into adulthood may experience significant incongruence [= a sense of struggle or division] between the gender they were assigned at birth and their currently experienced/expressed gender. Further, this incongruence may lead to significant distress in such individuals. In the current DSM-5-TR, these individuals may be diagnosed or described as having "gender dysphoria" (this term replaced the older term "gender identity disorder".)
   
  
• Note that gender dysphoria is not directly equal to transgender identity.
   
  
• The prevalence of gender dysphoria is very hard to estimate precisely. The DSM-5-TR proposes that “[s]elf-identification as transgender ranges from 0.5% to 0.6%; experiencing oneself as having an incongruent gender identity ranges from 0.6% to 1.1%; feeling that one is a person of a different sex ranges from 2.1% to 2.6%; and the desire to undergo medical treatment ranges from 0.2% to 0.6%.” (p. 515)
  

Transgender: Prevalence & Neurobiological Factors

As we move toward the November 2024 national election, it is clear that the status of transgender individuals, most particularly children under the age of 18 and athletes of any age, has become a major point of political controversy. The material below does not and is not intended to address any of the political arguments. It would be unreasonable, however, not to acknowledge that there are very strong and passionate arguments and beliefs on both sides of the political divide.

• The prevalence of transgender people (previously labeled, as seen below in the map, "transsexual") varies very widely around the world and, to a large extent, is related to the cultural acceptance and traditions of a particular place. Here are the data drawn from a variety of sources.

• Other analyses conclude that there are significantly larger numbers of transgender individuals than those who have sought medical intervention. For example, Goodman et al. (2019) propose that "[o]n balance, the data indicate that people who self-identify as transgender or gender nonconforming represent a sizable proportion of the general population with realistic estimates ranging from 0.1% to 2%, depending on the inclusion criteria and geographic location” (p. 303, emphasis added). Simillarly, Zucker (2017) finds that "“[r]ecent studies suggest that the prevalence of a self-reported transgender identity in children, adolescents and adults ranges from 0.5 to 1.3%, markedly higher than prevalence rates based on clinic-referred sample” (Abstract, emphasis added)

• As I commented above, the rise of media reports of transgender children and adolescents seeking treatment for their experience of gender dysphoria in recent years has occasioned a great deal of controversy, particularly political, on both liberal and conservative sides. Rather than paying close attention to the scientific literature, multiple commentators have ignored the research and "weaponized" this topic for political advantage. In reading general reports in the media, keep in mind the role of political concerns as motivating the news.

• The complex (and conflicting) views of American adults regarding gender identity and transgender identity can be seen in this June 28, 2022 report by the Pew Research Center.

• In March 2024, the Gallup organization--among the oldest public survey and data analytics companies in the US--reported that the increasing level of LGBT identification among American adults first noted in a 2021 study continued at 7.6% in their 2023 study (Jones, 2024). Compared to 2012, roughly 90% more Americans overrall now identify themselves as LGBTQ+. As seem in the charts below, there is a quite distinctive difference between various generations of U.S. adults with younger adults much more likely to describe themselves as LGBT. Generation Z born between 1997-2005 identifies itself as 22.3% LGBTQ+ compared with 2.3% of Baby Boomers born 1946-1964, or 970% higher!
  

Note, however, that even in this report, the categories of sexual orientation (L-G-B-Other) are still intermixed with the category of Transgender Identity which is NOT about sexual orientation. Since respondents could check more than one category, transgender individuals may or may not have responded as well to the other categories. This report does not show how they responded. As the Mayo clinic notes online: “[b]eing transgender or gender diverse isn't linked to a specific sexual orientation. And sexual orientation can't be assumed based on gender identity or gender expression” (“Transgender facts." 2023).

Data on the variations of sexual orientation and transgender identity are both limited and quite complex (Nieder et al., 2016). Multiple studies describe transgender individuals as having a “fluid” orientation which may change at various stages of adjustment to and acceptance of the new identity. Holmberg, Arver, & Dhejne (2019) offer an in-depth exploration of transgender identity in the course of medical support. For the purposes of this class, this will have to stand as the concluding comment on this specific issue.

• But, what about the issue of the biological role in how people identify their gender? Very frankly, there is little that is currently known about the neurobiological factors that may be involved in transgender identity. Indeed, Kiyan et al. (2023) noted, "thus far, no clear explanation or neurobiological underpinning of being transgender has been identified" (p. 87, emphasis added).
  

• Earlier work in this area did not very clearly differentiate between disorders of sexual development  (DSD) and transgender identity (then usually termed "transsexualism"). Transgender identity was considered to be another "disorder of sexual development" so that sex and gender were considered to be roughly identical. An influential analysis that studied by individuals with DSD as well as persons with transgender identity did not distinguish between them (Bao & Swaab, 2011) and summarized that
  

  During the intrauterine period a testosterone surge masculinizes the fetal brain, whereas the absence of such a surge results in a feminine brain. As sexual differentiation of the brain takes place at a much later stage in development than sexual differentiation of the genitals, these two processes can be influenced independently of each other. Sex differences in cognition, gender identity (an individual’s perception of their own sexual identity), sexual orientation (heterosexuality, homosexuality or bisexuality), and the risks of developing neuropsychiatric disorders are programmed into our brain during early development. There is no evidence that one’s postnatal social environment plays a crucial role in gender identity or sexual orientation” (Abstract)

• A recent meta-analysis of the possible biological origins of transgender identity (Levin et al., 2023) concluded that

A number of research studies have investigated biological factors that could potentially contribute to transgender identity, but results often contradict each other. Interpretation of etiological [i.e., origin/cause] studies of transgender identity can be misunderstood and/or misused by media, politicians, and care providers, placing transgender people at risk. We question the utility of etiological studies in clinical care, given that transgender identity is not pathological. When etiological studies are undertaken, we recommend new, inclusive designs for a rigorous and compassionate approach to scientific practice in the service of transgender communities and the providers who serve them” (Abstract)

• Given the relative absence of any firm neurobiological evidence regarding transgender identity, the remainder of this lesson will focus upon intersex conditions and sexual orientation for which we do have much more substantial evidence.
  

Intersexes

Intersex = an individual whose development is intermediate between male and female, i.e., there is ambiguity regarding the sex of new born as male or female.  "Intersex people are born with any of several variations in sex characteristics including chromosomes, gonads, sex hormones, or genitals that, according to the UN Office of the High Commissioner for Human Rights, 'do not fit the typical definitions for male or female bodies.' Such variations may involve genital ambiguity, and combinations of chromosomal genotype and sexual phenotype other than XY-male and XX-female" {Wikipedia}

• 1% of US children have some type of genital ambiguity though generally not profound enough to call the sex into question.
  
• In the US, rates of intersex births range from 1 in 1500 to 1 in 2000.
  
• As noted previously, this does NOT equal either transgender or transexuality in which there is no doubt about the biological sex (i.e., physical genitalia and genetic make-up).

Note that the list of intersex conditions (though very rare) is broad. The Intersex Society of North America (ISNA) lists these (not to memorize, but to realize the breadth of the conditions):

Not XX and not XY	1 in 1,666 births
Klinefelter (XXY)	1 in 1,000 births
Androgen insensitivity syndrome [Note A]	1 in 13,000 births
Partial androgen insensitivity syndrome [Note A]	1 in 130,000 births
Classical congenital adrenal hyperplasia	1 in 13,000 births
Late onset adrenal hyperplasia	1 in 66 individuals
Vaginal agenesis	1 in 6,000 births
Ovotestes	1 in 83,000 births
5-alpha reductase deficiency	no estimate
Mixed gonadal dysgenesis	no estimate
Complete gonadal dysgenesis	1 in 150,000 births
Hypospadias (urethral opening in perineum or along penile shaft)	1 in 2,000 births
Hypospadias (urethral opening between corona and tip of glans penis)	1 in 770 births

Note A: These are higher incidence rates than in my diagram above which uses data from Gottlieb & Trifiro (2017) for the rate of Androgen Insensitivity Syndrome

• Hermaphrodite = an individual whose genitals do not match the normal development for their genetic sex. In a "true" hermaphrodite there is both normal testicular and ovarian tissue. It is the rarest form of an intersex condition. As of 2020 only about 200-500 cases of true hermaphroditism (also called (Ovotesticular Disorder of Sex Development) across the world in the medical literature while the incidence is estimated at no more than 1 in every 20,000 to 100,000 births (Chen et al., 2020; Özdemir et al. 2019).
  
• Where does that word come from? "“In Greek mythology, Hermaphroditus was the son of Hermes (Mercury) and Aphrodite (Venus), who grew together with they nymph Salmacis, while bathing in her fountain, and thus combined male and female characteristics” (Oxford English Dictionary)
  

• Congenital Adrenal Hyperplasia [CAH] Female fetus exposed to elevated androgen levels may be partly masculinized vis-à-vis external anatomy (e.g., the adrenal gland may produce an excess of testosterone via congenital adrenal hyperplasia [CAH] in which adrenal gland does NOT produce enough cortisol to turn off pituitary's production of ACTH. The adrenal gland produces too much testosterone as a result). Hence, the clitoris may have a resemblance to a penis. Note that CAH can appear in males, but has little effect.
  

CAH Girls: Interests & Preferences

• Toy interests appear intermediate between stereotypical boy and girl toys
• Highest interest in boys' toys correlated with highest level of testosterone exposure even though their parents did encourage "non-tomboy" behavior
• As adolescents & adults, there is a lower sexual interest in males and a moderate degree of bisexual/homosexual activity. Often difficulties with sexual arousal and absence of orgasm.

• Rearing & Gender Assignment among Intersex Individuals

• In the past genetic males with distinctive female genitalia were surgically altered as children to live as females. But later studies found great dissatisfaction.

• Today, the emphasis is on raising the child as close to their appearance as possible (either as a boy or girl) and doing NO surgery until adulthood.

• Important to be honest with intersex person & family

David Reimer. Response to tragic experience of a Canadian David Reimer (1965-2004) whose penis was accidentally destroyed during circumcision. After sex reassignment surgery as an infant, he was raised as a girl ("Brenda") under the theory that gender identity is mostly the result of environmental factors. His parents were profoundly influenced to do this by a famous Johns Hopkins' psychologist named John Money. However, David realizes by ages 10-12 he was really a boy and began to live as a male. He eventually married at 25, but  committed suicide when he was 39. (See Colapinto, 2004) [Daily Mail UK 2010 story]

• Note that this tragic experience (and the failure of changing the gender identity & sexual orientation of other children in such situations) is also evidence for the major impact of nature for the development of sexual orientation as well. See below.

Androgen Insensitivity Syndrome ("testicular feminization" or, earlier, "pseudohermaphroditism") = genetic males (46XY) with the genital appearance of a female.

• Cause: A genetic defect which causes an inability of androgens to bind to genes in a cell's nucleus. Cells are insensitive to androgens and the external genitals develop similar to those of a female.

5-αlpha-reductase deficiency (5-ARD)= Genetic disorder in males where the penis does not develop until puberty and the child is usually identified as female.

• It comes about because the male lacks the gene for the enzyme 5-alpha-reductase which is necessary to change testosterone into the much more potent androgen "dihydrotestosterone" (DHT). It is dihydrotestosterone (DHT) which actually has the central role in forming the secondary sexual characteristics of the developing boy.
• Incidence is not known although quite rare. It appears in a number of large family groups in the Dominican Republic, Papua New Guinea, Egypt, & Turkey.
  

  C.    Sexual Orientation in Humans: Possible Biological Aspects 

Bottom Line: Most scientific researchers believe that homosexual and heterosexual orientations are the result of interacting genetic and environmental processes which take place early in the development of the child (particularly during gestation). Thus, there is almost certainly no "gay gene" and there is probably no "gay environment" which causes homosexual orientation generally. Sexual orientation is much more likely to be the result of causes from nature rather than social factors.

• Males tend to identify sexual orientation earlier than females.
• Females show a higher proportion of bisexual attraction than males do. Further, "masculine" type play as a child is not particularly predictive of sexual orientation in women.

The American Academy of Pediatrics has stated that "sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences." The American Psychological Association has stated that "there are probably many reasons for a person's sexual orientation and the reasons may be different for different people". It also stated that for most people, sexual orientation is determined at an early age. The American Psychiatric Association has stated that, "to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality. Similarly, no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse."

The most comprehensive scientific overview of sexual orientation and development in psychology is by Bailey et al. (2016).

1. Behavioral and Anatomical Differences

• Heterosexual men 1.5 cm/0.5 in taller on average than homosexual men
• Women & gay men > heterosexual men use/remember landmarks in giving directions (vs. using distances & compass directions NESW)
• There are some subtle anatomical and behavioral differences in gay vs. straight people. But not particularly profound differences.
• BTW, among homosexual men, 80-85% act in clearly "masculine" ways and only 15-20% act in clearly "feminine" ways. Thus, most gay males (4 out of 5) cannot be identified by outward behavioral differences.

2. Genetic Factors

• US data: The probability of male homosexuality is highest in monozygotic twins of the originally identified homosexual person (ca. 50% concordance rate), lower in dizygotic twins (ca. 20%), and lower still in adopted brothers and sisters (ca. 11% vs. expected rate of 5-8% in general population). The figures are slightly lower for female homosexuality. These data come from a skewed sample advertised through gay or lesbian publications.

• Swedish data: Study of all twins between ages 20 and 47 and asked if they had ever had a same-sex partner (Langstrom, 2010). Concordance rates are still monozygotic > dizygotic, but the actual percentages are much lower.

• Note that even the strongest concordance rate still means that almost 50% of twins will be discordant for sexual orientation. If genetic factors completely determined sexual orientation, all pairs of identical twins would have the same sexual orientation. This is a strong argument for other factors involved.

3. Evolutionary Perspective

Why would there be any genetic inheritance of homosexual tendencies since, by definition, there would not be a reproductive outcome? It is very hard to come up with a provable theory.

• Kin Selection Theory: by caring for the children of siblings, homosexuals would actually foster transmission of many of their genes. But, gays don't seem to take care of nieces/nephews to any greater extent than other family members

• Homosexual genes in males may increase the probability of offspring in sisters & female relatives (who do tend to have > average number of children). But, could this be sufficient to compensate for so few children from homosexuals? Unlikely.

• There may be a variety of individual genes that confer advantages to heterosexual males individually, but in some combinations lead to homosexuality.

• Methylization due to environmental factors (adding CH₃ group to gene) may inactivate a gene for heterosexuality and this may be passed on to a new generation.

4. Prenatal Influences

• No hormonal differences among adult heterosexual vs. homosexual individuals. But during a prenatal "sensitive period" there may be some sort of effect. Note, that in (non-human) animal studies, low testosterone levels during pregnancy may cause male offspring to respond sexually to either male or female partners.

• BTW, note that up to 350 animal species show homosexual behavior among pairs (Bagemihl, 1999). For example, note Roy and Silo on the right, a pair of "gay penguins" at New York's Central Park Zoo who were a "couple" for 2 mating seasons  but, then, split. During their time as a couple, they hatched and raised a female penguin (named "Tango").

• Maternal Immune Hypothesis: Mother's immune system may respond react against a protein in a gestating son. This shows itself in the FBOE (see below). According to this hypothesis, “…cells (or cell fragments) from male fetuses enter the maternal circulation during childbirth or perhaps earlier in pregnancy. These cells include substances that occur only on the surfaces of male cells, primarily male brain cells. The mother’s immune system recognizes these male-specific molecules as foreign and produces antibodies to them. When the mother later becomes pregnant with another male fetus, her antibodies cross the placental barrier and enter the fetal brain. Once in the brain, these antibodies bind to male-specific molecules on the surface of neurons. This prevents these neurons from ‘‘wiring-up’’ in the male-typical pattern, so that the individual will later be attracted to men rather than women“ (Blanchard, 2018, p. 10).
  

• Fraternal Birth Order Effect (FBOE): Younger (right-handed) brothers with older biological brothers are more likely to be homosexual. That is, sons of mothers who have previously given birth themselves to boys are somewhat more likely to be gay. (Blanchard, 2008, 2018; Bogaert, 2006).

• Prenatal stress and alcohol in mothers may also alter brain development. Some effects are seen in non-human animals.
  

5. Brain Anatomy

• There are some small, but subtle differences in brain anatomy between homosexual and heterosexual men. For example, the anterior commissure and the suprachiasmatic nucleus are larger in homosexual men. The significance of these differences is not known.

• Simon LeVay studied the interstitial nucleus 3 of the anterior hypothalamus (INAH-3) in gay and straight men and straight women. The INAH-3 of homosexual males was similar in size to that of heterosexual women, but smaller than that of heterosexual men.

• LeVay's (1991) study has been partially replicated by Byne et al. (2001). The latter study found a smaller difference than LeVay (1991) did, but reported additionally that there were differences in cell densities between homosexual and heterosexual males in the INAH-3.

• NOT IN TEXT: The advent of fMRI and PET scanning has suggested that brain areas associated with sexual preferences in human can be identified across subjects of diverse sexual interests. Poeppl et al. (2016) reported that they had identified four areas of the brain, what they termed "a neural circuit encoding sexual preference in humans": the anterior and preoptic area of the hypothalamus, the anterior and mediodorsal thalamus, the septal area, and the perirhinal parahippocampus including the dentate gyrus (see figure below). The researchers note that none of these areas are part of the cortex itself, but are "phylogenetically old, subcortical brain structures"

---

References & Resources

American Psychiatric Association [APA]. (2022). Gender dysphoria. In Diagnostic and statistical manual of mental disorders (5th ed., text rev.). https://doi.org/10.1176/appi.books.9780890425787.x14_Gender_Dysophoria

Bagemihl, B. (1999). Biological exhuberance: Animal homosexuality and natural diversity. New York: St. Martin's Press.

Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., & Epprecht, M. (2016). Sexual orientation, controversy, and science. Psychological Science in the Public Interest, 17(2), 45-101. https://doi.org/10.1177/1529100616637616

Bao, A.-I., & Swaab, D. F. (2011). Sexual differentiation of the human brain: Relation to gender identity, sexual orientation and neuropsychiatric disorders. Frontiers in Neuroendochrinology. https://doi.org/10.1016/j.yfrne.2011.02.007

Blanchard, R. (2008). Review and theory of handedness, birth order, and homosexuality in men. Laterality: Asymetries of Body, Brain, and Cognition, 13(1), 51-70. https://doi.org/10.1080/13576500701710432

Blanchard, R. (2018) Fraternal birth order, family size, and male homosexuality: Meta-analysis of studies spanning 25 years. Archives of Sexual Behavior, 41(1), 1-15. https://doi.org/10.1007/s10508-017-1007-4.

Bogaert, A. F. (2006, June 28). Biological versus nonbiological older brothers and men's sexual orientation. PNAS, 103(28), 10771-10774. https://10.1073/pnas.0511152103

Byne, W., Tobet, S., Mattiace, L. A., Lasco, M. S., Kemether, E., Edgar, M. A., Morgello, S., Buchsbaum, M. S., & Jones, L. B. (2001). 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. https://doi.org/10.1006/hbeh.2001.1680

Chen, C-Q., Liu, Z., Lu, Y.-S., Pan, M. & Huang, H. (2020). True hermaphroditism with dysgerminoma: A case report. Medicine, 99(22), e20472. https://10.1097/MD.0000000000020472

Colapinto, J. (2004, June 3). Gender gap: What were the real reasons behind David Reimer's suicide? Slate. Retrieved March 28, 2010 from http://www.slate.com/id/2101678/

Clutton-Brock, T. H. (1990). The evolution of parental care. Princeton, NJ: Princeton University Press.

Endotext.org (2003). Online endocrinology textbook.

Genetics Home Reference (2004, April). Androgen insensitivity syndrome. National Institutes of Health: National Library of Medicine. Retrieved March 21, 2005 from the Web site: http://ghr.nlm.nih.gov/condition=androgeninsensitivitysyndrome

Goodman, M., Adams, N., Corneil, T., Kreukels, B., Motmans, J., & Colman, E. (2019) Size and distribution of transgender and gender nonconforming populations: A narrative review. Endocrinology and  Metabolism Clinics of North America, 48(2), 303-321. https://doi.org/10.1016/j.ecl.2019.01.001

Gottlieb, B., Beitel, L. K., & Trifiro, M. A. (2004, April). Androgen insensitivity syndrome. GeneTests: Medical Genetics Information Resource (database online). University of Washington, Seattle. Retrieved March 21, 2005 from the Web site: http://www.geneclinics.org/profiles/androgen/details.html

Gottlieb, B., & Trifiro, M. A. (2017, May 11) Androgen Insensitivity Syndrome. GeneTests: Medical Genetics Information Resource (database online). University of Washington, Seattle.  https://www.ncbi.nlm.nih.gov/books/NBK1429/

Herek, G. M. (2005). Sexual orientation: Science, education, and policy. University of California at Davis: Psychology Department. Retrieved March 21, 2005 from the Web site: http://psychology.ucdavis.edu/rainbow/index.html

Holmberg, M., Arver, S., & Dhejne, C. (2019). Supporting sexuality and improving sexual function in transgender persons. Nature Reviews Urology, 16, 121-139. https://doi.org/10.1038/s41585-018-0108-8

Intersex Society of North America (ISNA; original site). Updated site: https://interactadvocates.org

Jones, J. M. (2024, March 13). LGBTQ+ identification in U.S. now at 7.6%. GALLUP. https://news.gallup.com/poll/611864/lgbtq-identification.aspx

Kiyar, M., Collet, S., T’Sjoen, G., & Mueller, S. C. (2020). Neuroscience in transgender people: an update. Neuroforum, 26(2), 85-92. https://doi.org/10.1515/nf-2020-0007

Langstrom, N., et al. (2010). Genetic and environmental effects on same-sex sexual behavior: A population study of twins in Sweden. Archives of Sexual Behavior, 39, 75-80. https://doi.org/10.1007/s10508-008-9386-1

Levin, R. N., Erickson-Schroth, >, Mak, K., & Edmiston, E. K. (2023). Biological studies of transgender identity: A critical review. Journal of Gay & Lesbian Mental Health, 27(3). https://doi.org/10.1080/19359705.2022.2127042

LeVay, S. (1991), A difference in hypothalamic structure between homosexual and heterosexual men. Science, 253, 1034-1037. https://www.jstor.org/stable/2878803

LeVay, S. (2009). The biology of sexual orientation. Retrieved March 31, 2011 from the author's Web site: http://www.simonlevay.com/the-biology-of-sexual-orientation

Meier, S. C., & Labuski, C. M. (2013). The demographics of the transgender population. In A. K. Baumle (Ed.), International handbook on the demography of sexuality (pp. 289-327). Springer, Dordrecht, The Netherlands: Springer. https://doi.org/10.1007/978-94-007-5512-3_16

NIDA Research Report: Steroid Abuse and Addiction: NIH Publication No. 00-3721, Printed 1991, Reprinted 1994, 1996. Revised April, 2000. Retrieved March 17, 2005 from the NIDA Web site: http://www.nida.nih.gov/ResearchReports/Steroids/AnabolicSteroids.html

Nieder, T. O., Elaut, E., Richards, C., & Dekker, A. (2016). Sexual orientation of trans adults is not linked to outcome of transition-related health care, but worth asking, International Review of Psychiatry, 28(1). https://doi.org/10.3109/09540261.2015.1102127

Özdemir, M., Kavak, R. P. et al (2019) Ovotesticular disorder of sex development: An unusual oresentation. Journal of Clinical Imaging Science, 9. https://dx.doi.org/10.25259/JCIS_45_2019

Pasterski, V. L., Geffner, M. E., Brain, C., Hindmarsh, P., Brook, C., & Hines, M. (2005). Prenatal hormones and postnatal socialization by parents as determinants of male-typical toy play in girls with congenital adrenal hyperplasia. Child Development, 76(1), 264-278. https://doi.org/10.1111/j.1467-8624.2005.00843.x

Poeppl, T. B., Langguth, B., Rupprecht, R., Laird, A. R., & Eickhoff, S. B. (2016) A neural circuit encoding sexual preference in humans. Neuroscience and Biobehavioral Reviews. http://dx.doi.org/10.1016/j.neubiorev.2016.06.025

SEER Program. Endocrine System in the Anatomy & Physiology Guide. SEER's Training Web Site. Atlanta SEER Cancer Registry, Emory University, Atlanta, GA. Funded by the U.S. National Cancer Institute, Surveillance, Epidemiology and End Results (SEER) Program.

Smith, D. (2004, February 7). Cental Park Zoo's gay penguins ignite debate. San Francisco Chronicle. Retrieved March 22, 2005 from the SFGate.com Web site: http://sfgate.com/cgi-bin/article.cgi?file=/c/a/2004/02/07/MNG3N4RAV41.DTL&type=printable

Wikipedia. (2005, February 11). Androgen insensitivity syndome. Retrieved March 21, 2005 from the Wikpedia web site: http://en.wikipedia.org/wiki/Androgen_insensitivity_syndrome

Wikipedia (2005, March 13). Intersexual. Retrieved March 21, 2005 from the Wikipedia web site: http://en.wikipedia.org/wiki/Intersex

Wilson, C. A., & Davies, D. C. (2007). The control of sexual differentiation of the reproductive system and brain. Reproduction, 133, 331-359. https://doi.org.10.1530/REP-06-0078  (Literature review of current knowledge)

Zucker, K. J. (2017). Epidemiology of gender dysphoria and transgender identity. Sexual Health, 14(5), 404-411. https://doi.org/10.1071/SH17067

  This page was first posted March 22, 2005.