Nature’s Choice:
What Science Reveals About the Biological Origins of Sexual Orientation.

Cheryl L. Weill, Ph.D., MSW

Definition: sexual orientation - the direction of sexual feelings or behavior toward the same sex (homosexual), opposite sex (heterosexual), or some combination of the two (bisexual). Alternatively: affectional orientation.

Homosexuality is not a mental disorder. In 1973 the American Psychiatric Association removed homosexuality from its official Diagnostic and Statistical Manual signifying the end to its official classification as a disease.
Homosexuality is not pedophilia. The sexual desire of pedophiles is directed towards children and can have a heterosexual, homosexual, or bisexual orientation.
There is no evidence that childhood sexual abuse alters sexual orientation.

Neurohormonal-Gestational theory: Put forward in “Neurohormonal Functioning and Sexual Orientation: A Theory of Homosexuality-Heterosexuality,” Ellis, L. and Ames, M.A. Psychological Bulletin, 101:233-258 (1987). Sexual orientation in all mammals is primarily determined by the degree to which the nervous system is exposed to testosterone and certain other sex hormones during gestation.
Predictions of theory:
a. Homosexuals should have higher frequencies of other sex-typical behaviors normally associated with the opposite sex.
b. Relationships between parents and homosexual offspring often may be strained and/or assume some cross-sex characteristics.
c. Homosexuality should reflect a significant degree of hereditability.
d. Average neurohormonal differences should exist between homosexuals and heterosexuals in both sexes at comparable ages.
e. Attempts to alter sexual orientation after birth should be minimally effective.
f. Homosexuality should be primarily a male phenomenon.

Sexual development occurs both morphologically and neurologically.
Morphological development: In the absence of any hormonal influence, genetically male and female fetuses will develop morphologically as female. Male sexual development requires testosterone and its derivatives dihydrotestosterone and estrogen. Female sexual development can occur in either the presence or absence of estrogen. The sex hormones that operate during development are produced in the fetus, by either the testes or ovaries and the adrenal glands.
Neurological development: Sexual orientation is associated with the establishment of permanent differences in the hypothalamic and limbic areas of the brain. Sex-typical behavior patterns are associated with changes in diverse areas of the brain and extensively involving the cortex.

Animal studies, primarily of rats or primates, demonstrate that neurological development, as assessed behaviorally, can be inverted by altering the availability of testosterone during a critical period. Elimination of circulating testosterone in males during the critical period results in reduced male-typical behavior and display of female-typical behavior. Similarly, increasing the availability of testosterone to developing females results in reduced female-typical behavior and display of male-typical behavior. The same manipulations carried out after the critical period do not affect the normal development and expression of sex-typical behavior in either males or females.
These and additional observations suggest that testosterone masculinizes selected areas of the brain and de-feminizes other areas to provide for male-typical behavior.
Observations of normal animals demonstrate that sex-typical behavior correlates with the circulating levels of testosterone during gestation.
The human conditions, androgen insensitivity, 5-alpha-reductase deficiency, congenital-adrenal-hyperplasia, and Turner’s syndrome support the conclusion that sex-typical behavior correlates with the circulating levels of testosterone during gestation.
The area of the brain known to be sexually dimorphic from animal and human studies is the hypothalamus.

Anatomical data:
1. Swaab, D.F. and Hofman, M.A. An enlarged suprachiasmatic nucleus in homosexual men. Brain Res. 537: 141-148 (1990).
2. LeVay S. A difference in hypothalamic structure between heterosexual and homosexual men. Science 253: 1034-1037 (1991).
3. Allen, L.S. and Gorski, R.A. Sexual orientation and the size of the anterior commissure in the human brain. Proc. Natl. Acad. Sci. USA 89: 7199-7202 (1992).

Genetic data:
1. Bailey, J.M. and Pillard, R.C. A genetic study of male sexual orientation. Arch. Gen. Psychiatry 48: 1089-1096 (1991).
2. Bailey, J.M., Pillard, R.C., Neale, M.C., and Agyei, Y. Hertitable factors influence sexual orientation in women. Arch. Gen. Psychiatry 50: 217-223 (1993).
3. Hamer, D.H., Hu., S., Magnuson, V.L., Hu, N., and Pattatucci, A.M.L. A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261: 321-327 (1993).

The genetic data do not at this time unequivocally support the idea that there is a gay gene. These data need to be replicated on a 10-fold larger population. It is still unlikely that there is a single gene that codes for a complex behavior like sexual orientation or sex-typical behavior. It seems more likely, that unlike eye color, which is associated with a single gene, sexual orientation results from a subset of traits, associated with a number of genes and gestational environmental factors, that together give rise to a complex behavior.

Conclusion: Homosexuality is naturally determined through normal developmental processes and represents one end of a continuous spectrum of accessible sexual behaviors.

Further reading:
1. Brain Sex, Anne Moir & David Jessel, Dell Publishing, New York, NY, 1991.
2. Discover (magazine) Vol. 3, Number 6, June 1992.
3. The Sexual Brain, Simon LeVay, MIT Press, Cambridge, MA, 1993.
4. The Science of Desire, Dean Hamer & Peter Copeland, Simon and Schuster, NY, 1994.
5. Sex on the Brain, Deborah Blum, Penguin Putnam, Inc., NY, 1997.

Contact: [email protected] or 303-442-8242. See also