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
- 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.
Identity & Gender-Differentiated Behaviors
≠ Transgender or Transexual Identity.
- Transgender people: an identity or gender expression different from assigned sex
- Transsexual: usually applied to transgender people who desire medical assistance to transition from one to another sex
The Intersex Society of North America (ISNA) lists these
Not XX and not XY
1 in 1,666 births
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
1 in 6,000 births
1 in 83,000 births
5 alpha reductase deficiency
Mixed gonadal dysgenesis
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