Brief on Genetic Determinism and Sexual Orientation


I.Evolutionary Theories of Homosexuality

II. Biological or Genetic Theories of Homosexuality

A. General
B. Family Studies
C. Twin Studies
D. Molecular Studies: X-Linked
E. Molecular Studies: Other Chromosomes 
F. Molecular Studies: Non-Human Subjects
G. Faternal Birth Order Effect
H. Neurohormonal Influences: Sexual Identity and Prenatal Determination 
I. Brain Differences: General 
J. Brain Difference: Hypothalamus 
K. Brain Differences: Suprachiasmatic Nucleus 
L. Brain Differences: Anterior Commisure 
M. Left-handedness 
N. Dermal Fingerprint Ridges 
O. Finger Length 
P. Penis Length 
Q. Prenatal Effects 
R. Puberty Onset, Weight and Height 
S. Other Possible Biological Causes of Homosexuality: Virus Theory 

III. Cognitive Differences

A. Differences in Spatial Competencies
B. Visuomotor Targeting Tasks
C. General Neuropsychological Findings
D. Color Preferences

IV. Behavioral Differences

A. "Butch" Females 
B. Childhood Sex-Typed Bahvior 

V. Ethical, Legal, and Social Issues

A. General
B. General Legal Issues 
C. Pre-Implantation Genetic Daignosis 

VI. Other Resources

A. Books 
B. Websites

I. Evolutionary Theories of Homosexuality

Bailey, J. M.; S. Gaulin; Y. Agyei; et al., 1994. Effects of gender and sexual orientation on evolutionarily relevant aspects of human mating psychology. Journal of Personality and Social Psychology. 66(6): 1081–1093.

  • This study utilized a sample of self-identified homosexual and heterosexual males and females. The authors utilized a theory of sex selection to analyze gender differences with regards to mating psychology. Their findings indicate that gender has a greater impact on mating psychology than does sexual orientation.

Bobrow, D. and J. M. Bailey, 2001. Is male homosexuality maintained via kin selection?  Evolution and Human Behavior. 22: 361-368.

  • This study found that men who self-identified as homosexual are no more likely to channel resources to their siblings than self-identified heterosexual men.

Dewar, C. S. 2003. An association between male homosexuality and reproductive success. Medical Hypotheses. 60(2): 225-232.

  • The authors propose that male homosexual orientation is adaptive based on characteristics such as improved linguistic skills, enhanced empathy, fine motor skills and impulse control.

De Block, A. and P. Adriaens. 2004. Darwinizing sexual ambivalence: a new evolutionary hypothesis of male homosexuality.Philosophical Psychology. 17(1): 59-76.

  • The authors propose that homosexuality evolved as a means to strengthen social bonds. The evolution of the behavior does not have to do with kin selection but reciprocal altruism.

Diamond, L. M. 2003. What does sexual orientation orient?  A bio-behavioral model distinguishing romantic love and sexual desire. Psychological Review. 110(1): 173-192.

  • The author argues that most individuals experience romantic love for members of both genders.

Gomes, C. A.  2000. True nature; a theory of human sexual evolution. Part 1. Journal of the Gay and Lesbian Medical Association. 4(1): 19-29.

  • This paper discusses how throughout history and across cultures, male homosexuality has been condemned while male bisexuality has been tolerated.

Gomes, C. A.  2000. True nature; a theory of human sexual evolution. Parts 2-4. Journal of the Gay and Lesbian Medical Association. 4(2): 57-83.

  • This paper discusses the hypothesis that a general rise in intelligence has co-occurred with a general decrease in sexual instinct.

Haslam, N.  1997. Evidence that male sexual orientation is a matter of degree. Journal of Personality and Social Psychology. 73(4): 862 –870.

  • The author conducted a study of 1,138 men utilizing the MMPI personality scale to determine if male sexual orientation should be represented by discrete categories versus a continuum rating. The findings suggest that a continuum is the more appropriate scale. This is consistent with Kinsey’s findings. The author further suggests that multiple causes (genetic and environmental) are probably involved in the development of sexual orientation.

Kinnish, K. K.; D. S. Strassberg; and C. Turner. 2005. Sex differences in the flexibility of sexual orientation: a multidimensional retrospective assessment. Archives of Sexual Behavior. 34(2): 173-183.

  • This study tracked the sexual orientation of 762 individuals via cumulative change scores. Data showed differences between the genders in both self-identified homosexual and heterosexual individuals, with females having a greater change in orientation than men. Bisexuals did not differ significantly.

Kirkpatrick, R. C. 2000. The evolution of homosexual behavior. Current Anthropology. 41(3): 385–413.

Miller, E. M. 2000. Homosexuality, birth order, and evolution: toward an equilibrium reproductive economics of homosexuality. Archives of Sexual Behavior. 29(1): 1-34.

  • The authors hypothesize that birth order is a by-product of a biological mechanism that shifts the personalities of later-born sons towards prototypically feminine behaviors. This results in reducing the probability of these sons engaging in unproductive competition with their older brothers.

Murray, S. O.  2000. Homosexualities. Chicago, IL: University of Chicago Press.

  • Presents a history of homosexuality. Discusses the possible biological basis of homosexuality.

Muscarella,F.; B. Fink; K. Grammer; et al.  2001. Homosexual orientation in males: evolutionary and ethological aspects. Neuroendocrinology Letters. 22:  393-400.

  • The authors examine the combined effects of evolutionary factors and neuro-hormonal processes on the development of homosexual orientation.

Scasta, D. 1998. Historical perspectives on homosexuality. Journal of Gay and Lesbian Psychotherapy. 2(4): 3-17.

  • Provides a perspective on how historical views of homosexuality influence current thought.


II. Biological and Genetic Theories of Homosexuality

A. General

Bem, D. J. 1996. Exotic becomes erotic: a developmental theory of sexual orientation. Psychological Review. 103: 320–335.

  • Proposes that genes and other biological factors do not code for sexual orientation, but cause children to engage in gender-atypical behavior. The result of this behavior is estrangement from same-gender peers, whom they come to view as exotic.

Burr, Chandler. 1995. A separate creation: how biology makes us gay. London, UK: Bantam Press.

Burr, Chandler. 1996. A separate creation: the search for the biological origins of sexual orientation. New York, NY: Hyperion Books.

Burr, Chandler. 1999. A separate creation: the search for the biological origins of sexual orientation. Collingdale, PA: Diane Publishing.

Byne, W. 1997. Why we cannot conclude that sexual orientation is primarily a biological phenomenon. Journal of Homosexuality, 34(1): 73-80.

D’Alessio, V. 1996. Born to be gay. New Scientist. 151(2049): 32-36.

Ellis, Lee and Linda Ebertz. 1997. Sexual orientation: toward biological understanding. West Port, CT:  Praeger Press.

Landen, M. and S. Innala. 2002. The effect of a biological explanation on attitudes towards homosexual persons:  a Swedish national sample study. Nordic Journal Psychiatry. 56(3): 181-186.

  • This survey of 992 Swedish adults was conducted to assess views of homosexuality. The authors found evidence of greater tolerance compared to previous studies. This may be due to recent anti-discrimination legislation, increased visibility of homosexuals, and/or an increasing belief that homosexuality has a biological cause. Men surveyed were more likely to attribute homosexuality to disease than women.  Women and younger individuals indicated a greater support for gay marriage than men and those from older age groups.  Those who stated a belief in biological causes evinced a greater tolerance for homosexuality.

Mubarak, D. The Advocate. 2001. Why are we gay? July 17.

  • This article provides a review of research on the genetic basis of homosexuality.

Mustanski, B. S.; M. L. Chivers; and J. M. Bailey. 2002. A critical review of recent biological research on human sexual orientation. Annual Review of Sex Research. 13: 89-140.

Rahman, Q. and G. D. Wilson. 2003. Born gay? the psychobiology of human sexual orientation. Personality and Individual Differences. 34: 1337-1382.

Wilson, Glenn D. and Qazi Rahman. 2005. Born gay: the psychobiology of sex orientation. London, UK: Peter Owen Publishers.

  • This book provides a broad review of research into the biological origins of sexual orientation.


B. Family Studies

Bailey, J. M. and R. C. Pillard. 1991. A genetic study of male sexual orientation. Archives of General Psychiatry. 48: 1089-1096.

Bailey, J. M. and A. P. Bell. 1993. Familiality of female and male homosexuality. Behavior Genetics. 23: 313-322.

Bailey, J. M. and R. C. Pillard. 1995. Genetics of human sexual orientation. Annual Review of Sex Research. 6: 126-150.

Bailey, J. M.; R. C., Pillard; K. Dawood; et al. 1999. A familyhistory study of male sexual orientation using three independent samples. Behavior Genetics. 29(2): 79-86.

  • The authors conducted a survey of 262 self-identified gay males from three independent samples regarding their family history of sexual orientation. They found a higher rate of homosexuality in families than in the general population for both male and female and maternal and paternal relatives. Homosexuality in brothers of these men was measured at 7-11% compared with a population prevalence of 2%.

Dawood, K.; R. C. Pillard; C.  Horvath; et al., 2000. Familial aspects of male homosexuality. Archives of Sexual Behavior. 29(2): 155-163.

  • Utilizing a sample of 29 self-identified homosexual brother pairs, the authors investigated familial factors that are thought to influence male sexual orientation. 


C.  Twin Studies

Bailey, J. M. and R. C. Pillard. 1991. A genetic study of male sexual orientation. Archives of General Psychiatry. 48: 1089-1096.

Bailey, J. M. and A. P. Bell. 1993. Familiality of female and male homosexuality. Behavior Genetics. 23: 313-322.

Bailey, J. M.; R. C. Pillard; M. Neale; et al. 1993. Heritable factors influence sexual orientation in women.  Archives of General Psychiatry. 50(3): 217-223.

  • This study utilized twin sister pairs recruited through advertisements in gay magazines. Of their sample, 71 female identical twins showed a 48% concordance rate for homosexual orientation, 37 female fraternal twins showed a 16% concordance rate for homosexual orientation, and 35 adoptive sisters showed a 6% concordance rate for homosexual orientation. 

Bailey, J. M.; M. P. Dunne; and N. G. Martin. 2000. Genetic and environmental influences on sexual orientation andits correlates in an Australian twin sample. Journal of Personality and Social Psychology. 78(3): 524–536.

  • This Australian twin study utilized a sample size of 4901. The sample included 312 male identical twins, 182 male fraternal twins, 668 female identical twins, 376 female fraternal twins and 353 fraternal male and female twins.  Childhood gender nonconformity appeared to be heritable in both men and women. There was a lower concordance found compared to prior studies for self-identified sexual orientation. The males in this sample scored higher than the females on the Kinsey scale. The authors suggested that sexual orientation should be analyzed separately for males versus females.

Kirk, K. M. and J. M. Bailey. 2000. Etiology of male sexual orientation in an Australian twin sample. Psychology, Evolution & Gender. 2(3): 301-311.

  • This Australian study focused on the sexual orientation of male twins. The authors reduced volunteer bias by enrolling via a twin registry and using anonymous questionnaires. Several variables were found to be significantly heritable including sexual feelings, sexual fantasies, attraction to men, and number of sexual partners.

Kirk, K. M.; J. M. Bailey; M. P. Dunne; et al. 2000. Measurement models for sexual orientation in a community twin sample. Behavior Genetics. 30(4): 345-356.

  • This study utilized a sample of 4901 Australian twins. Sexual orientation was assessed utilizing a self-report method. The data suggested a heritability of between .50 and .60 in females and about .30 in males for homosexual orientation.

Kendler, K.; L. M. Thornton; S. E. Gilman; et al., 2000. Sexual orientation in a U.S. national sample oftwin and non-twin sibling pairs. American Journal of Psychiatry. 157(11): 1843-1846.

  • 1930 non-twin siblings and 794 twin pairs were studied.  3.1% of males self-reported as homosexual while 2.5% of females reported as homosexual.  Concordance for homosexuality in MZ twins was 31.6%, 44.4% for female MZ, and 20% for male MZ twins. This indicated that familial influences may differ between the sexes. Evidence did not indicate that similar environmental experiences in MZ versus DZ twins contributed to the greater concordance for sexual orientation in MZ pairs.


D.  Molecular Studies:  X-Linked

Bailey, J. M. and R. C. Pillard. 1995. Genetics of human sexual orientation. Annual Review of Sex Research. 6: 126-150.

Hamer, D. H.; S. Hu; V. L. Magnuson; et al. 1993. A linkage between DNA markers on the X chromosome and male sexual orientation. Science. 261(5119): 321–327.

  • 40 male, self-identified homosexual sibling pairs were studied. The pairs were recruited through advertisements in magazines. The authors found that 33 of the pairs had concurrent markers on the Xp28 loci.  No linkage was found outside the Xp28 region.

Hamer, D. H. 1999. Genetics and male sexual orientation. Science. 285: 803.

  • Comments in Science made in response to a 1999 study conducted by Rice and colleagues.

Hu, S.; A. M. Pattatucci; C. Patterson; et al. 1995. Linkage between sexual orientation and chromosome Xp28 in males but not in females. Nature Genetics. 11:  248-256.

  • The authors studied 33 self-identified homosexual brother pairs. A linkage was found for the Xp28 region in males but not in females.

Mustanski, B. S.; M. G. Dupree; C. M. Nievergelt; et al. 2005. A Genome-wide scan of male sexual orientation. Human Genetics. 116:  272-278.

Rice, G.; C. Anderson; N. Risch; et al. 1999.  Male homosexuality: absence of linkage to microsatellite markers at Xq28. Science. 284:  665–667.

  • This study utilized 52 self-identified homosexual brother pairs recruited from advertisements in gay magazines in Canada.  The authors were unable to replicate the finding for the Xp28 region.

Turner, W. J. 1995. Homosexuality, type: Xp28 phenomenon. Archives of Sexual Behavior. 24(2): 109-134.

  • The author studied the families of male and female self-identified homosexuals. He hypothesizes that the “gay gene” (Xp28) is transmitted maternally.


E.  Molecular Studies: Other Chromosomes

DuPree, M. G.; B. S. Mustanski; S. Bocklandt; et al. 2004. A candidate gene study of CYP19 (aromatase) and male sexual orientation. Behavior Genetics. 34(3): 243-250.

  • The authors studied Aromatase-CYP19 (codes for cytochrome P450 of the aromatase enzyme comple) and found no linkage to male homosexuality.

Green, R. and E. B. Keverne. 2000. The disparate maternal aunt- uncle ratio in male transsexuals: an explanation invoking genomic imprinting. Journal of Theoretical Biology.202: 55-63.

  • This authors in this paper discount the maternal immune hypothesis.  Study looking at transsexuals, 417 male to female and 96 female to male.  Males had an excess of maternal aunts v. uncles, but not for females.  Proposes Theory for a woman who have Xp with paternal imprints.  She would have some XpY- and XmY offspring. The XpY would not survive because they have the Xist gene that would silence the gene.  Females with XpXp- having sons then with XpXp- inheriting the feminizing Xp imprinted genes from the mother feminizing them causing either homosexuality or transsexuality.  X-ist escapes X inactivation are imprinted and enhance feminine social traits.


F. Molecular Studies: Non-Human Subjects

Demir, E. and B. J. Dickson. 2005.  Fruitless splicing specifies male courtship behavior in drosophilaCell. 21:  785-794.

  • The authors found that splicing a single neuronal gene in drosophila caused male courtship behaviors in females.


G. Fraternal Birth Order Effect

Blanchard, R.; K. J. Zucker; S. J. Bradley; et al. 1995. Birth order and sibling sex ratio in homosexualmale adolescents and probably pre-homosexual feminine boys. Developmental Psychology. 31(1): 22-30.

  • The authors studied two groups of 56 males recruited from a psychiatric unit. One group was labeled pre-homosexual and one group was being treated for psychiatric disorders unrelated to sexual orientation. The groups were matched based on number of siblings and age.  Findings indicated that the group labeled pre-homosexual had a greater than average number of older male siblings. 

Blanchard, R. and A. F. Bogaert. 1996. Homosexuality in men and number of older brothers. American Journal ofPsychiatry. 153(1): 27–31.

  • This study utilized a sample of 302 self-identified homosexual males and 302 heterosexual males matched by date of birth. Findings indicated that homosexual males had a higher ratio of older brothers than the heterosexuals. Each additional brother increased chances of homosexuality by 33%.  Sibling sex ratio in the two groups was not different than in the general population. The authors suggest a possible maternal immune response or presence of the H-Y antigen as possible causes.

Blanchard, R. 1997. Birth order and sibling sex ratio in homosexual versus heterosexual males and females. AnnualReview of Sex Research. 8:  27-67.

  • This review of research discusses the different psychological and biological theories of the birth order effect.  The author concludes that there is no such effect for females.

Blanchard, R. and A. F. Bogaert. 1997. Additive effects of older brothers and homosexual brothers in the prediction ofmarriage and cohabitation. Behavior Genetics. 27(1): 45-54.

Blanchard, R. and P. Klassen. 1997. H-Y antigen and homosexuality in men. Journal of Theoretical Biolog.y. 185:  373-378.

  • The authors discuss how the H-Y antigen theory of homosexuality is consistent with the birth order effect.

Blanchard, R. and L. Ellis. 2001. Birth weight, sexual orientation and the sex of preceding siblings. Journal of Biosocial Science. Vol. 33: 451-467.

  • This study of 3229 adult men and women found that males with older brothers weigh less than males without older brothers, and self-identified homosexual males weighed less than heterosexual males. No correlations were found for women and birth weight.

Blanchard, R.  2001. Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior. 40: 105-114.

Blanchard, R.; K. J. Zucker; A. Cavacas; et al. 2002. Fraternal birth order and birth weight in probably pre-homosexual feminine boys. Hormones and Behavior. 41: 321-327.

Blanchard, R. 2004. Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology. 230: 173-187.

  • The author conducted a meta-analysis of 14 samples with 10,413 males. Results showed a higher number of older brothers for self-identified homosexual males.

Bogaert, A. F. 1997. Birth order and sexual orientation in women. Behavioral Neuroscience. 111(6): 1395–1397.

  • This paper discusses the birth order effect on sexual orientation in females based on a sample of 257 self-identified homosexual and 5,008 self-identified heterosexual females.  No significant birth order effect was found.

Bogaert, A. F.; S. Bezeau; M. Kuban; et al. 1997. Pedophilia, sexual orientation, and birth order. Journal of Abnormal Psychology. 106(2.): 331-335.

  • This study utilized 338 pedophiles. Findings indicate that self-identified homosexual pedophiles had later sibling birth order than self-identified heterosexual pedophiles.

Bogaert, A. F.  1998. Birth order and sibling sex ratio in homosexual and heterosexual non-White men. Archives ofSexual Behavior. 27(5):  467–473.

Bogaert, A. F.  2000. Birth order and sexual orientation in a national probability sample. The Journal of Sex Research. 37(4): 361-368.

Bogaert, A. F.  2002. Recent research on sexual orientation and fraternal birth order. The Canadian Journal of Human Sexuality. 11(2): 101-107.

  • This is a review article of research done on the fraternal birth order effect in male sexual orientation. It explores both biological and psychosocial explanations for this effect.

Bogaert, A. F.  2003. Interaction of older brothers and sex-typing in the prediction of sexual orientation in men. Archives of Sexual Behavior. 32(2): 129-134.

Bogaert, A. F.  2005. Gender role/identity and sibling sex ratio in homosexual men. Journal of Sex & Marital Therapy. 31: 217-227.

  • The author studied the correlation between sibling sex ratio and childhood gender identity. No correlation was found.

Cantor, J. M.; R. Blanchard; A. D. Paterson; et al. 2002. How many gay men owe their sexual orientation to fraternal birth order? Archives of Sexual Behavior. 31(1): 63-67.

  • This study attempts to determine the percentage of homosexual men the birth order effect accounts for.

Jones, M. B. and R. Blanchard. 1998. Birth order and male homosexuality: extension of Slater’s Index. HumanBiology. 70(4): 775-787.

McKnight, J. and J. Malcolm. 2000. Is male homosexuality maternally linked? Psychology, Evolution & Gender. 2: 229-239.

Zeh, J. A., and D. W. Zeh. 2005. Maternal inheritance, sexual conflict and the maladapted male. Trends in Genetics. 21(5): 281-286.


H.  Neurohormonal Influences: Sexual Identity and Prenatal Determination

Cohen, K. M. 2002. Relationships among childhood sex- atypical behavior, spatial ability, handedness, and sexual orientation in men. Archives of Sexual Behavior. 31(1): 129-143.

Dorner, G.; G. Gotz; W. Rohde; et al. 2001. Genetic and epigenetic effects on sexual brain organization mediated by sex hormones. Neuroendocrinology Letters. 22: 403-409.

  • The authors examined the effects of  the chemical DDT on fetal brains. Their finding showed an increased prevalence of polycystic ovaries and transssexualism among those exposed.

Rahman, Q. and G. D. Wilson. 2003. Born gay? The psychobiology of human sexual orientation. Personality and Individual Differences. 34: 1337-1382.

Savic, I.; H. Berglund; and P. Lindstrom. 2005. Brain response to putative pheromones in homosexual men. Proceedings of the National Academy of Sciences of the United States of America. 102(20): 7356-7361.


I.  Brain Differences: General

Matsumoto, Akira. 1999. Sexual Differentiation of the Brain. Boca Raton, FL: CRC Press LLC.

Swaab, D. F.  2004. Sexual differentiation of the human brain and gender. Endocrine Abstracts. 9:

Swaab, D. F. 2004. Sexual differentiation of the human brain: relevance for gender identity, transsexualism and sexual orientation. Gynecological Endocrinology. 19: 301-312.

  • The authors maintain that evidence supporting the importance of postnatal social factors in development of sexual orientation is lacking.  They suggest that Testosterone plays a major role in the development of the male fetal brain, male gender identity, and male sexual orientation.


J.  Brain Differences: Hypothalamus

Swaab, D. F.; C. J. Wilson; F. Chung; P. M. Kruijveret; et al. 2001. Structural and functional sex differences in the human hypothalamus. Hormones and Behavior. 40:  93-98.

  • The authors discuss hypothesized structural sex differences in the brain, specifically in the hypothalamus.

Swaab, Dick F.  2003. The human hypothalamus: basic and clinical aspects, part 1: nuclei of the human hypothalamus, handbook of clinical neurology. Amsterdam, Netherlands: Elsevier.

Swaab, Dick F.  2004. The human hypothalamus: basic and clinical aspects, part 2: neuropathology of the human hypothalamus and adjacent structures, handbook of clinical neurology.  Amsterdam, Netherlands: Elsevier.

Swaab, D. F. 2004. Book review: The human hypothalamus: basic and clinical aspects. Journal of Neuroendocrinology. 16: 1009-1010.

  • This textbook on the human hypothalamus includes a section that discusses sexual orientation.


K.  Brain Differences: Suprachiasmatic Nucleus

Rahman, Q. and K. Silber. 2000. Sexual Orientation and the Sleep Waking Cycle. Archives of Sexual Behavior. 29: 127–134.

Swaab, D. F., and M. A. Hofman. 1990. An Enlarged Suprachiasmatic Nucleus in Homosexual Men. Brain Research. 537: 41-148.

  • The authors found the Suprachiasmatic Nucleus to be larger in self-identified homosexual men than in self-identified heterosexual men.


L.  Brain Differences: Anterior Commisure

Allen, L. S. and R. A. Gorski.  1991. Sexual dimorphism of the anterior commissure and massa- intermedia of the human brain. Journal of Comparative Neurology. 312:  97-104.

Byne, W.; M. S. Lasco; E.  Kemether; et al. 2000. Theinterstitial nuclei of the human anterior hypothalamus: an investigation of sexual variation in volume and cell size,number and density. Brain Research. 856:  254-258.

  • This study of all four INAH areas utilized a sample of 18 males and 20 females without HIV.  Findings showed that the INAH3 was larger in the sample males than in the sample females. No other sex differences were found.

Byne, W.; S. Tobet; L. A. Mattiace; et al. 2001. The interstitial nuclei of the human anterior hypothalamus: an investigation of variation withsex, sexual orientation and HIV status. Hormones and Behavior. 40: 86-92.

  • This study was conducted with a sample of 34 presumed heterosexual men and women, and 14 presumed homosexual men with and without HIV. The INAH-1 area was larger in participants who were HIV positive.  The INAH-3 was significantly larger in heterosexual men than in women.  Trend shows smaller volume for INAH-3 in homosexual men then heterosexual men, however there was no difference in the number of neurons present.

Kurup, R. K. and P. A. Kurup. 2002.  Hypothalamic digoxin, cerebral dominance, and sexual orientation. Archives of Andrology. 48: 359-367.

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

  • The author studied brain tissue from 41 subjects. Differences were found between males and females and between presumed heterosexual and homosexuals in the INAH 1,2,3,4 regions.  All of the homosexual men in the study had died of AIDS. This study was not replicated.


M.  Left-handedness

Lalumiere, M. L.; R. Blanchard; and K. J. Zucker. 2000. Sexual orientation and handedness in men and women: a meta-analysis. Psychological Bulletin. 126(4): 575-592.

  • This meta-analysis of 20 studies looked at samples totaling 6,987 self-identified homosexuals and 16,423 self-identified heterosexuals. The self-identified homosexuals had a 39% greater chance of being left handed. Left-handedness was greater in men than in women.

Lippa, R.  2003. Handedness, sexual orientation, and gender-related personality traits in men and women.  Archives of Sexual Behavior. 32(2):  103-114.

  • This study found self-identified homosexuals to have a 50% greater chance of non right-handedness. Between the genders there was an 82% greater chance of non right-handedness for homosexual men, versus 22% for homosexual women.

Mustanski, B. S.; J. M. Bailey; and S. Kaspar. 2002. Dermatoglyphics, handedness, sex, and sexual orientation. Archives of Sexual Behavior. 31(1): 113-122.

  • This study found that self-identified homosexual females, but not males, had a higher incidence of left- handedness compared to same-sex self-identified heterosexual controls. 


N.  Dermal Fingerprint Ridges

Green, R., and R. Young. 2000. Fingerprint asymmetry in male and female transsexuals. Personality &Individual Differences. 29: 933–942.

Hall, J. A. Y. and D. Kimura. 1994. Dermatoglyphic asymmetry and sexual orientation in men. Behavioral Neuroscience. 108: 1203–1206.

Hall, L. S.  2000. Dermatoglyphic analysis of total finger ridge count in female monozygotic twins discordant for sexual orientation. Journal of Sex Research. 37(4): 315-320.

  • The author found no differences in total finger ridge count for female twins concordant for sexual orientation. There were significant differences between the discordant MZ twins.

Hall, L. S.  2002. Dermatoglyphic analysis of monozygotic twins discordant for sexual orientation. In The State of Dermatoglyphics: The Science of Anger and Palm Prints, edited by N. M. Durham, K. M. Fox, and C. C. Plato, 123- 165.

Mustanski, B. S.; J. M. Bailey; and S. Kaspar. 2002. Dermatoglyphics, handedness, sex and sexual orientation. Archives of Sexual Behavior. 31: 107-116.

Slabbekoorn, D.; S. H. M. Van Goozen; G. Sanders; et al. 2000. The dermatoglyphic characteristics of transsexuals: is there evidence for an organizing effect of sexual hormones?Psychoneuroendocrinology. 25: 365–375.


O. Finger Length

Hall, L. S. and C. T. Love. 2003. Finger-length rations in female monozygotic twins discordant for sexual orientation. Archives of Sexual Behavior. 32(1): 23-38.

Jordan, C. C. and S. M. Breedlove. 2000. Finger length ratio and sexual orientation. Nature. 404:  455-456.

Lippa, R. A.  2003. Are 2D: 4D finger-length ratios related to sexual orientation?  Yes for men, no for women. Journal of Personality and Social Psychology. 85(1): 179-188.

  • The author tested the hypothesis that finger length signals sexual orientation and is related to prenatal androgen exposure.   

Rahman, Q. and G. D. Wilson. 2003. Sexual orientation and the 2nd to 4th finger length ratio: evidence for organizing effects of sex hormones or developmental instability? Psychoneuroendocrinology. Vol. 28: 283-303

  • The authors found that self-identified homosexuals, both male and female, had significantly lower finger length ratios. They suggest that this is caused by elevated levels of androgens which results in developmental instability.


P.  Penis Length

Bogaert, A. F. and S. Hershberger. 1999. The relation between sexual orientation and penile size.  Archives of Sexual Behavior. 28(3): 213-321.

  • This study utilized a sample of 5122 males and found that the self-identified homosexual men had larger penises than the heterosexual men.


Q.  Prenatal Effects

Bailey, J. M.; L. Willerman; and C. Parks. 1991. A test of the prenatal stress theory of human male homosexuality. Archives of Sexual Behavior. 20:  277-293.

Dorner, G.; T. Geier; L. Ahrens; et al. 1980. Prenatal stress as possible etiogenic factor of homosexuality in human males. Endokrinologie. 75:  365–36.

Ellis, L. and S. Cole-Harding. 2001. The effects of prenatal stress, and of prenatal alcohol and nicotine exposure, onhuman sexual orientation. Physiology and Behavior. 74: 1–14.

  • 7500 subjects and their mothers provided information regarding their sexual orientation and the mother’s stress, as well as use of alcohol and nicotine during pregnancy. Maternal stress during pregnancy, particularly during the first trimester, was shown to significantly correlate with male homosexuality.


R.  Puberty Onset, Weight and Height

Blanchard, R. and A. F. Bogaert. 1996. Biodemographic comparisons of homosexual and heterosexual men in the kinsey interview data. Archives of Sexual Behavior. 25: 551–579.

Bogaert, A. F. and R. Blanchard. 1996. Physical development and sexual orientation in men: height, weight and age of puberty differences. Personality and Individual Differences. 21: 77–84.

Bogaert, A. F.; C.  Friesen; and P. Klentrou. 2002. Age of puberty and sexual orientation in a national probability sample. Archives of Sexual Behavior. 31(1): 73-81.

Bogaert, A. F. and C. Friesen. 2002. Sexual orientation and height, weight, and age of puberty: new tests from a british national probability sample. Biological Psychology. 59(2): 135-145.

Bogaert, A. F.  2003. Physical development and sexual orientation in women: height, weight, and age of puberty comparisons. Personality, Individual Differences. 24(1): 115-121.

  • This study utilized a female sample size of 5476. Sexual orientation was determined using a self-report method. Those identifying as homosexual were taller and heavier than those identifying as heterosexual. No difference in puberty onset was found.


S.  Other Possible Biological Causes of Homosexuality: Virus Theory

Binstock, T. 2001. An immune hypothesis of sexual orientation. Medical Hypotheses. 57(5): 58-590.

  • Presents the hypothesis that sexual orientation is encoded within immune-cell subsets of mucosal and epithelial tissues.

Web article:



III. Cognitive Differences

A.  Differences in Spatial Competencies

Neave, N., Menaged, M., and Weightman, D. R. “Sex differences in cognition: the role of testosterone and sexual orientation, Brain & Cognition, Vol. 41, (1999). 245-262

  • The authors found differences in performance on spatial and verbal tasks between the genders. They also found differences in the amount of salivary testosterone during performance of the spatial tasks. They concluded that testosterone might play a role in the performance of spatial tasks.

Rahman, Q. Wilson, G. D., and Abrahams, S. “Sexual orientation related differences in spatial memory,” Journal of the International Neuropsychological Society, Vol. 9, (2003). 376-383

  • This study utilized a sample of 240 men and woman. The authors found that the females had significantly better spatial memory than the males, with self-identified homosexual males scoring similarly to self-identified heterosexual females. No difference in performance was found between self-identified homosexual and heterosexual females.


B.  Visuomotor Targeting Tasks

Hall, J. A. Y., Kimura, D.  “Performance by homosexual males and females on sexually dimorphic motor tasks,” Archives of Sexual Behavior, Vol. 24, (1995). 395–407

Loehlin, J. C., and McFadden, D. “Otoacoustic emissions, auditory evoked potentials, and traits related to sex and sexual orientation,” Archives of Sexual Behavior, Vol. 32, No. 2. (2003). 115-127

McFadden, D., and Pasanen, E. G.  “Comparison of the auditory systems of heterosexuals and homosexuals: click-evoked otoacoustic emissions,” Proceedings of the National Academy of Sciences, Vol. 95, (1998). 2709–2713

McFadden, D., and Pasanen, E. G.  “Spontaneous otoacoustic emissions in heterosexuals, homosexuals and bisexuals,” Journal of the Acoustical Society of America, Vol. 105, (1999). 2403-2413


C. General Neuropsychological Findings

Neave, N., Menaged, M., and Weightman, D. R. “Sex differences in cognition: the role of testosterone and sexual orientation, Brain & Cognition, Vol. 41, (1999). 245-262


D. Color Preferences

Ellis, L. and C. Ficek. 2001. Color preferences according to gender and sexual orientation. Personality and Individual Differences. 31: 1375-1379.

  • This study of college students found that the males had a stronger preference for shades of blue than the females.  Findings indicated no color preference differences between self-identified heterosexuals and homosexuals of either gender.



IV.  Behavioral Differences

A.  "Butch" Females

Singh, D., Vidaurri, M., and Zambarano, R. J. “Lesbian erotic role identification: behavioral, morphological and hormonal correlates,” Journal of Personality and Social Psychology, Vol. 76, (1999). 1035–1049.


B.  Childhood Sex-Typed Behavior

Bailey, J. M. and K. J. Zucker. 1995. Childhood sex-typed behavior and sexual orientation: a conceptual analysis and quantitative review. Developmental Psychology. 31: 43-55.

  • In a review of several studies, the authors found a correlation between childhood sex-typed behavior and sexual orientation.

Green, R. 1978. Sexual identity of 37 children raised by homosexual or transsexual parents. The American Journal of Psychiatry. 135:  692-697.



V.  Ethical, Legal, and Social Issues

A. General

Billings, P. 1993. Genetic discrimination and behavioral genetics: the analysis of sexual orientation. In Intractable neurological disorders, human genome, research, and society,edited by N. Fujiki and D. Macer.

Diamond, L. M. 2003. Was it a phase?  Young women’s relinquishment of lesbian/bisexual identities over a 5- year period. Journal of Personality and Social Psychology. 83(2): 352-364.

  • Analyzes the transitions women made in a 5-year period from homosexual to heterosexual or asexual identities.

Gabard, D. L. 1999. Homosexuality and the human genome project: private and public choices. Journal of Homosexuality. 37(1): 25-51.

Garnets, L. D. 2000. Understanding women’s sexualities and sexual orientation: an introduction. Journal of Social Issues. 56(2): 181-192.

  • The author discusses 3 theories regarding how parents’ sexual orientation affects their child’s sexual orientation, the role of gender non-conformity and how womens’ sexuality will affect public policy.

Gottschalk, L. 2003. From gender inversion to choice and back changing perceptions of the etiology of lesbianism over three historical periods. Women’s Studies International Forum. 26(3): 221-233.

  • A history of how lesbianism, over time, has come to be seen as biological in origin.

Landen, M. and S. Innala. 2002. The effect of a biological explanation on attitudes towards homosexual persons.  A Swedish national sample study. Nordic Journal Psychiatry.56(3): 181-186.

Lippa, R. A. 2002. Gender related traits of heterosexual and homosexual men and women. Archives of Sexual Behavior. 1(1): 83-98.

Mustanski, B. S. and J. M. Bailey. 2003. A therapist’s guide to the genetics of human sexual orientation. Sexual and Relationship Therapy. 18(4): 429-436.

  • This paper discusses different biological theories of homosexuality and whether having a biological causation makes homosexuality moral.

Otis, M. D. and W. F. Skinner. 2004. An exploratory study of differences in views of factors affecting sexual orientation for a sample of lesbians and gay men. Psychological Reports. 94(3): 1173-1179.

  • This survey found that gay men were more likely to view sexual orientation as genetic compared to lesbians.

Peplau, L. A. and L. D. Garnets. 2000. A new paradigm for understanding women’s sexuality and sexual orientation. Journal of Social Issues. 56(2): 330-350.

  • The authors state that research has shown no strong links between biology and female sexual orientation.

Schuklenk, U. 1997. Science, gay genes, and the third sex. Gay and Lesbian Humanist.

Schuklenk, U.; E. Stein; J. Kerin; et al. 1997. The ethics of genetic research on sexual orientation. Hastings Center Report. 27(4): 6-13.

Spitzer, R. L. 2003. Can some gay men and lesbians change their sexual orientation?  Two hundred participants reporting a change from homosexual to heterosexual orientation.Archives of Sexual Behavior. 32(5): 403-417.

  • 200 participants were studied who had attempted a transition from homosexual to heterosexual identity. Females reported more transitions than males. The changes were brought about as a result of reparative therapy.

Veniegas, R. C., and T. D. Conley. 2000. Biological research on women's sexual orientations: evaluating the scientific evidence. Journal of Social Issues. 56(2): 267-282.

  • This is a review paper on biological studies linking genetics with sexual orientation.


B.  General Legal Issues

Ball, C. A. and J. F. Pea. 1998. Warning with Wardle: morality, social science, and gay and lesbian parents. University of Illinois Law Review. 253-388.

  • This article analyzes Wardle’s argument that gay and lesbian parents’ sexual orientation is harmful to their children. The authors argue that sexual orientation alone is not an appropriate barometer for parenting skills.

Braman, D. 1999. Of race and immutability. UCLA Law Review. 46: 1375-1465.

  • This article reviews the Supreme Court's rulings on race and immutability and how these affect the rights of individuals based on their sexual orientation.

Halley, J. E. 1994. Sexual orientation and the politics of biology: a critique of the argument from immutability. Stanford Law Review. 46: 503-561.

  • This article argues that pro-gay legal arguments which rely on arguments of biological causation should be abandoned. Instead, pro-gay essentialists and constructivists should design legal strategies that emphasize the political dynamics that inevitably attend sexual orientation identity regardless of cause.

Hua, N.  2002. Same- sex sexual harassment under title vii: the line of demarcation between sex and sexual orientation discrimination. Santa Clara Law Review. 43: 249-280.

  • The author argues that same-sex sexual harassment requires a different framework for evaluation than opposite-sex sexual harassment. This paper also discusses the differences between gender-based harassment and sexual orientation-based harassment.

Knauer, N. J.  2003. Law and sexuality: A review of lesbian, gay, bisexual and transgender legal issues. Law and Sexuality. 12: 1-93.

Marcosson, S. A. 2001. Constructive immutability. University of Pennsylvania Journal of Constitutional Law. 3: 646-730.

  • The author considers the implications of a jurisprudential reliance on constructive immutability in the specific context of litigation challenging discrimination against sexual and gender minorities.

Mucciaroni, G. and M. L. Killman. 2004. Immutability, science, and legislative debate over gay, lesbian and bisexual rights. Journal of Homosexuality. 47(1): (2004) 53-77.

  • The authors review ten legislative debates occurred both before and after biological research into homosexuality had been conducted.  Their findings suggest that science has not had a profound impact on the discussions regarding the origins of sexual orientation among legislators.

Ortiz, D. R. 1993. Creating controversy: essentialism and constructivism and the politics of gay identity. Virginia Law Review. 79:  1833-1849.

  • This paper investigates the debate regarding the causes of homosexuality.

Simon, N. C. 1996. The evolution of gay and lesbian rights: reconceptualizing homosexuality in Andbowers v. Hardwick from a sociobiological perspective. Annual Survey of American Law. 105:

  • The author discusses different sociobiological theories of homosexuality; also reviewed are how these theories are applied to legal issues involving lesbian and gay rights.

Spitko, E. G. 1996. A biological argument for gay essentialism-determinism: implications for equal protection and substantive due process. University of Hawaii Law Review. 18: 571-609.

  • This article surveys recent developments in research into the biological causes of sexual orientation and discusses their importance for two areas of federal constitutional law.

Wardle, L. D. 1996. A critical analysis of constitutional claims for same-sex marriage. Brigham Young University Law Review. 1-74.

  • The paper reviews constitutional claims for legalizing same sex marriage with regards to the equal protection clause and the fundamental rights doctrine

Yoshino, K. 1996. Suspect symbols: the literary argument for heightened scrutiny for gays. Columbia Law Review. 96: 1753- 1842.

  • This paper engages the question of whether homosexuals should receive heightened scrutiny with regards to the equal protection clause.


C.  Pre-Implantation Genetic Diagnosis

Dahl, E.  2003. Ethical issues in new uses of pre-implantation genetic diagnosis: should parents be allowed to use pre-implantation genetic diagnosis to choose the sexual orientation of their children? Human Reproduction. 18(7): 1368-1369.

Greenberg, A. S. and J. M. Bailey. 2001. Parental selection of children’s sexual orientation. Archives of Sexual Behavior. 30(4): 423-437.

Stein, E. 1998. Choosing the sexual orientation of children. Bioethics. 12(1): 1-24.

Ten, C. L. 1998. The use of reproductive technologies in selecting the sexual orientation, the race, and the sex of children. Bioethics. 12(1): 45-48.


VI. Other Resources

A.  Books

Bailey, J. Michael.  2003. The man who would be queen: the science of gender-bending and transsexualism. Washington, DC: Joseph Henry Press.

Burr, Chandler. 1997. A separate creation: the search for the biological origins of sexual orientation, New York, NY: Hyperion Books.

Cabaj, Robert P. and Terr S. Stein. 1996. Textbook of homosexuality and mental health. Arlington, VA: American Psychiatric Publishing.

Caramagno, Thomas C.  2002. Irreconcilable differences? Intellectual stalemate in the gay rights debate. Westport, CT: Praeger Publishers.

Cohler, Bertram J. and Robert M. Galatzer-Levy. 2000. The course of gay and lesbian lives: social and psychoanalytical perspectives. Chicago, IL: University of Chicago Press.

Friedman, Richard C. and Jennifer I. Downey. 2002. Sexual orientation and psychoanalysis: sexual science and clinical practice. New York, NY: Columbia University Press.

Hamer, Dean and Peter Copeland. 1994. Science of desire: the gay gene and the biology of behavior. New York, NY: Simon and Schuster.

Hamer, Dean and Peter Copeland. 1999. Living with our genes: why they matter more than you think. New York, NY: First Anchor Books.

Lahey, Kathleen. 1998. Are we persons yet? Law and sexuality. Toronto, CA:  University of Toronto Press.

LeVay, Simon. 1993. The sexual brain. Cambridge, MA: MIT Press.

LeVay, Simon. 1996. Queer science: the use and abuse of research into homosexuality. Cambridge, MA: MIT Press.    

LeVay, Simon and Sharon Valente. 2002. Human sexuality. Sunderland, MA: Sinauer Associates.

Lipkin, Arthur. 2001. Understanding homosexuality, changing schools. Boulder, CO: Westview Press.

McKnight, Jim 1997. Straight Science. New York, NY: Routledge.

Money, John. 1988. Gay, straight and in-between: the sexology of sexual orientation. New York, NY: Oxford University Press.

Rosario, Vernon A.  1997. Science and homosexualities. New York, NY: Routledge.

Jones, Stanton L. and Mark A. Yarhouse.  2000. Homosexuality: the use of scientific research in the church’s moral debate, Downers Grove, IL: InterVarsity Press.


B. Websites

Born Gay: Pro or Con?

Netherlands Institute for Brain Research

The website of researcher J. Michael Bailey

The website of researcher Brian S. Mustanksi

Religious Tolerance

Ethics Update website

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