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

A.   Evolutionary Interpretations of Mating Behavior

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? Survival or reproductive advantage of a behavior and its appearance across cultures is not by itself proof that the behavior is evolved or learned.

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

Definitions [from DSM-5-TR, 2022]

• Sex & sexual = the biological indicators of male and female (understood in the context of reproductive capacity)
  
  
• Disorders of sex development or differences of sex development (DSDs) = somatic intersex conditions
  
  
• Gender is used to denote the public, sociocultural (and usually legally recognized) lived role as boy or girl, man or woman, or other gender
  
  
• 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)

Gender Dysphoria = Individuals ranging in age from early childhood through adolescence into adulthood may experience significant incongruence between the gender they were assigned at birth and their currently experienced/expressed gender

• The prevalence of gender dysphoria is very hard to estimate precisely

Transgender: Prevalence & Neurobiological Factors

In the November 2024 national election, 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.  Since the beginning of the new administration on January 20, 2025, there have been many initiatives at the federal level to erase or otherwise alter programs and policies which are intended to support transgender individuals.

• The prevalence of transgender people (previously labeled, as below, "transsexual") varies very widely around the world and, to a large extent, is related to the cultural acceptance and traditions of a particular place. Note that "transsexual" was often applied to those who sought medical intervention to change their physical sexual characteristics.
  

• There are significantly larger numbers of transgender individuals than those who have sought medical intervention. Estimates of transgender identity overall run from 0.1% to 2% in one study and 0.5% to 1.3% in a second study (however, see below, there are major generational differences)
  

• As noted above, transgender issues have become very politically controversial and have often been used for political advantage without paying attention to the scientific research literature

• Significant changes in American adult self-identification across the LGBTQ+ spectrum between 2012 and 2024 (Jones, 2025) as found by the Gallup Organization which has conducted studies on this matter since 2012. In their latest survey of 14,000 US adults they found that there has been roughly a 266% increase in these 12 years.
  

• With significant differences in LGBTQ+ identification across different US adult generations (Jones, 2025). The difference between Generation Z (born 1997-2006) at 23.1% and Baby Boomers (born 1946-1964) at 3.0% is a difference of roughly 770%.
  

• How do American adults in general identify themselves as LGBTQ+ vs.LGBTQ+ Americans in 2024?

Among all American adults in 2024, 5.2% describe themselves as bisexual, 2.0% as gay, 1.4% as lesbian, and 1.3% as transgender.

• Notice that even this survey continues to link sexual orientation with gender identity though they are separate issues. There are limited data on how gender identity and sexual orientation are linked though multiple sources suggest "fluidity" in sexual orientation at various stages in the journey of transgender individuals (childhood to adulthood; across differing levels of medical support).
  

• Relatively little is known about the neurobiological basis of transgender identity.

• Earlier work (pre-2015 or so) did not distinguish or differentiate between disorders of sexual development (DSD) and transgender identity (which was called "transsexualism").
  

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]

The Intersex Society of North America (ISNA) lists these

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

Rearing & Gender Assignment among Intersex Individuals

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.

C.    Sexual Orientation in Humans: Possible Biological Aspects 

Bottom Line: No "gay gene," no "gay environment," interaction of genetics x environment early in development, nature probably > 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.

1. Behavioral and Anatomical 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

3. Evolutionary Perspective

• Kin Selection Theory
• Homosexual genes in males may increase the probability of offspring in sisters & female relatives
  

• Genes that confer advantages to heterosexual males individually, but in some combinations lead to homosexuality.

• Methylization due to environmental factors
  

4. Prenatal Influences

• Maternal Immune Hypothesis: Mother's immune system may respond react against a protein in a gestating son. This shows itself in the 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 slightly more likely to be gay.

5. Brain Anatomy

• Neural circuit for human sexual preference? (Poeppl et al., 2016). None of these areas are part of the cortex itself, but are "phylogenetically old, subcortical brain structures."