last updated:

April 10, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 26 Variations in Sexual Behaviors


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
  • 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.   Gender Identity & Gender-Differentiated Behaviors


Intersex Chart
Prevalence of Transexualism

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


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
Toy play in CAH
              girls, boys, and unaffectedCAH Girls: Interests & Preferences
[Reimer]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]

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

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.

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

[Concordance Rates for
                Homosexuality in Twins]2. Genetic Factors

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.

4. Prenatal Influences

5. Brain Anatomy

References & Resources

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. doi: 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. doi: 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. doi: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. doi:  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. doi: 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.

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

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

Intersex Society of North America (ISNA).

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.

LeVay, S. (1991), A difference in hypothalamic structure between homosexual and heterosexual men. Science 253:1034-1037.

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.

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

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.

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. DOI: 10.1530/REP-06-0078  (Literature review of current knowledge)

This page was first posted March 22, 2005.