Peer-Reviewed Studies on the Origin of Sexual Orientation Since 1990
In 19 peer-reviewed studies, researchers found anatomical and genetic differences between homosexuals and heterosexuals (different brain structures, inner ear anatomy, smell sensitivity, chromosome markers, etc.), observed innate homosexual behavior in animal species (from fish to fruit flies), and evaluated the effects of "conversion therapy."
Each study may provide clues to the origin of sexual orientation and are therefore labeled as Pro, Con, or NC (not clearly pro or con) to our core question, "Is sexual orientation determined at birth?" Of the 19 studies, 15 are pro, one is con, and three are NC. The studies are arranged from most recent to oldest. We may continue to add peer-reviewed studies to this page as we find them.
1. Gay and straight men's faces shaped differently, gay men's faces rated as more masculine
"The main aim of this study was to test for possible differences in facial shape between heterosexual and homosexual men. Further, we tested whether self-reported sexual orientation correlated with sexual orientation and masculinity–femininity attributed from facial images by independent raters. In Study 1, we used geometric morphometrics [a method of examining size and shape] to test for differences in facial shape between homosexual and heterosexual men. The analysis revealed significant shape differences in faces of heterosexual and homosexual men. Homosexual men showed relatively wider and shorter faces, smaller and shorter noses, and rather massive and more rounded jaws, resulting in a mosaic of both feminine and masculine features. In Study 2, we tested the accuracy of sexual orientation judgment from standardized facial photos which were assessed by 80 independent raters... [H]omosexual men were rated as more masculine than heterosexual men, which may explain the misjudgment of sexual orientation. Thus, our results showed that differences in facial morphology of homosexual and heterosexual men do not simply mirror variation in femininity, and the stereotypic association of feminine looking men as homosexual may confound judgments of sexual orientation."
Jaroslava Varella Valentova, et al., "Shape Differences Between the Faces of Homosexual and Heterosexual Men," Archives of Sexual Behavior, Oct. 2013
[Editor's Note: The Huffington Post, on Nov. 7, 2013, published "Could Face Shape Reveal if Someone Is Gay or Straight?" by Cavan Sieczkowski, which stated:
"Jarka Valentova, a researcher on the Czech study, elaborated on the results in an email to The Huffington Post Thursday. 'It's necessary to point out to possible misunderstandings of our results,' she said. 'The fact that we have found some significant morphological differences between homosexual and heterosexual men does not mean that any of the groups is easily recognizable on the street (and our Study 2 actually shows that it's not that easy to guess anyone's sexual orientation without knowing it), or that anything like that should be done (like pointing on people with our illustrations and guessing who is who).'
She also added that the sample size used was small and, in order for this study to ascertain more validity, it would need to be replicated within different populations.
Still, researchers suggest the differences in facial shapes may indicate certain prenatal environmental factors."]
2. Homosexual behavior in male fish makes them more attractive to females, may increase chance of reproduction
"Male homosexual behaviour—although found in most extant clades across the Animal Kingdom—remains a conundrum, as same-sex mating should decrease male reproductive fitness. In most species, however, males that engage in same-sex sexual behaviour also mate with females, and in theory, same-sex mating could even increase male reproductive fitness if males improve their chances of future heterosexual mating. Females regularly use social information to choose a mate; e.g. male attractiveness increases after a male has interacted sexually with a female (mate choice copying). Here, we demonstrate that males of the tropical freshwater fish Poecilia mexicana [Atlantic mollies] increase their attractiveness to females not only by opposite-sex, but likewise, through same-sex interactions. Hence, direct benefits for males of exhibiting homosexual behaviour may help explain its occurrence and persistence in species in which females rely on mate choice copying as one component of mate quality assessment."
David Bierbach, PhD, Christian T. Jung, et al., "Homosexual Behaviour Increases Male Attractiveness to Females," Biology Letters, Feb., 2013
[Editor's Note:Ars Technica, on Dec. 17, 2012, published "What We Know—and Don't Know—About the Biology of Homosexuality" by Kate Shaw, which stated:
"Female mollies engage in what is called 'mate choice copying,' meaning that they prefer to mate with males they have watched copulate with other females...
But homosexual behavior among males is also common among mollies. This behavior, called 'nipping,' occurs when a male nips at the genitals of another male...
...[V]ideo playback tests showed that the perceived attractiveness of subordinate males skyrocketed after females watched them engage in sexual behavior... Moreover, it didn’t matter whether males nipped at other males or copulated with females; engaging in any sexual behavior at all increased female preference for the males.
In nature, subordinate males are generally the ones engaging in homosexual nipping behavior. The researchers suggest that this behavior might be an alternative strategy: if less desirable males can't get a boost in sexual attractiveness by actually mating, homosexual nipping behavior may be another way to get the girl."]
3. Homosexuality passed down via "epi-marks" (physical changes in genetic material), not genes
"Pedigree and twin studies indicate that homosexuality has substantial heritability in both sexes, yet concordance between identical twins is low and molecular studies have failed to find associated DNA markers. This paradoxical pattern calls for an explanation. We use published data on fetal androgen signaling and gene regulation via nongenetic changes in DNA packaging (epigenetics) to develop a new model for homosexuality."
William R. Rice, PhD, Urban Friberg, and Sergey Gavrilets, "Homosexuality as a Consequence of Epigenetically Canalized Sexual Development," Quarterly Review of Biology, Dec. 2012
[Editor's Note:Ars Technica, on Dec. 17, 2012, published "What We Know—and Don't Know—About the Biology of Homosexuality" by Kate Shaw, which stated:
"Although scientists have spent the last few decades scouring our genome for a 'gay gene,' William Rice, Urban Friberg, and Sergey Gavrilets suggest in The Quarterly Review of Biology that homosexuality may have its roots in epigenetics, rather than in genetics...
[T]he researchers suggest, epigenetic inheritance via 'epi-marks' might be responsible for sexual orientation. Epi-marks are physical changes in our genetic material (such as chemical modification or changes in DNA packaging proteins) that regulate gene activity without actually changing the sequence of bases...
The researchers argue that cross-sex epigenetic inheritance—either from mother-to-son or father-to-daughter—could contribute to homosexuality. A female-specific epi-mark that persisted in male offspring that inherited it would trigger feminization—and possibly sexual attraction to males. A similar scenario could happen if a male-specific epi-mark was inherited by a daughter, causing masculinization and, potentially, sexual attraction to other females...
However... none of these hypotheses have been tested with real data."]
4. Maternal relatives of homosexual men produce more children than maternal relatives of heterosexual men
"Our analysis showed that both mothers and maternal aunts of homosexual men show increased fecundity [ability to reproduce] compared with corresponding maternal female relatives of heterosexual men. A two-step statistical analysis... showed that mothers and maternal aunts of homosexual men (i) had fewer gynecological disorders; (ii) had fewer complicated pregnancies; (iii) had less interest in having children; (iv) placed less emphasis on romantic love within couples; (v) placed less importance on their social life; (vi) showed reduced family stability; (vii) were more extraverted; and (viii) had divorced or separated from their spouses more frequently...
Our findings are based on a small sample and would benefit from a larger replication, however they suggest that if sexually antagonistic genetic factors that induce homosexuality in males exist, the factors might be maintained in the population by contributing to increased fecundity greater reproductive health, extraversion, and a generally relaxed attitude toward family and social values in females of the maternal line of homosexual men."
Andrea S. Camperio Ciani, PhD, Lilybeth Fontanesi, MS, et al., "Factors Associated with Higher Fecundity in Female Maternal Relatives of Homosexual Men," Journal of Sexual Medicine, Nov. 2012
[Editor's Note: The Life's Little Mysteries blog stated in the June 11, 2012 article "Why Are There Gay Men?" by Natalie Wolchover:
"...[C]onsidering that the trait [male homosexuality] discourages the type of sex that leads to procreation — that is, sex with women — and would therefore seem to thwart its own chances of being genetically passed on to the next generation, why are there gay men at all?...
...For several years, studies led by Andrea Camperio Ciani at the University of Padova in Italy and others have found that mothers and maternal aunts of gay men tend to have significantly more offspring than the maternal relatives of straight men...
The theory holds that the same genetic factors that induce gayness in males also promote fecundity (high reproductive success) in those males' female maternal relatives. Through this trade-off, the maternal relatives' 'gay man genes,' though they aren't expressed as such, tend to get passed to future generations in spite of their tendency to make their male inheritors gay."]
5.Homosexual men and women have brain activity and structure similar to the opposite sex
"Cerebral responses to putative pheromones and objects of sexual attraction were recently found to differ between homo- and heterosexual subjects. Although this observation may merely mirror perceptional differences, it raises the intriguing question as to whether certain sexually dimorphic features in the brain may differ between individuals of the same sex but different sexual orientation... HeM [heterosexual men] and HoW [homosexual women] showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM [homosexual men] and HeW [heterosexual women]. No cerebellar asymmetries were found. Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the connections were more widespread from the left amygdala; in HoW and HeM, on the other hand, from the right amygdala... The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities."
Ivanka Savic-Berglund, MD, PhD, and Per Lindström, "PET and MRI Show Differences in Cerebral Asymmetry and Functional Connectivity Between Homo- and Heterosexual Subjects," Proceedings of the National Academy of Sciences of the United States of America, June 16, 2008
[Editor's Note:National Geographic, on June 16, 2008, published the article "Gay Men, Straight Women Have Similar Brains," by James Owen, which stated:
"In some ways the brains of straight men and lesbians are on similar wavelengths, the research suggests. Likewise, gay men and straight women appear to have similar brains, in some respects. The findings are new evidence that homosexuals may be born with a predisposition to be gay...
Differences both in the brain activity and anatomy were observed in a study involving 90 men and women, including homosexuals and heterosexuals of both genders...
Researchers focused in particular on the amygdala, an almond-shaped structure inside each brain hemisphere associated with processing and storing emotions.
In homosexuals, brain activity most closely matched that of heterosexuals of the other sex.
For example, the study found that straight men and gay women are both wired for a greater 'fight or flight' response than gay men or straight women, the team reports this week in the journal Proceedings of the National Academy of Sciences.
Also, homosexual men and straight women showed significantly more neural connections across the two brain hemispheres than heterosexual men did."]
6.More older brothers increases probability of homosexuality in men
"In this article, I demonstrate that the number of biological older brothers, including those not reared with the participant (but not the number of nonbiological older brothers), increases the probability of homosexuality in men. These results provide evidence that a prenatal mechanism(s), and not social and/or rearing factors, affects men’s sexual orientation development...
A mother's body may have a memory for male (but not female) fetuses because she herself is female, and thus, her immune system may interpret and remember male ... fetuses as foreign."
Anthony Bogaert, PhD, "Biological Versus Nonbiological Older Brothers and Men's Sexual Orientation," Proceedings of the National Academy of Sciences, June 28, 2006
[Editor's Note: The Los Angeles Times stated on June 27, 2006 in the article "Study Links Male Gays, Birth of Older Brothers:
"Having one or more older brothers boosts the likelihood of a boy growing up to be gay -- an effect due not to social factors, but biological events that occur in their mother's womb, according to a study published today.
In an analysis of 905 men and their siblings, Canadian psychologist Anthony Bogaert found no evidence that social interactions among family members played a role in determining whether a man was gay or straight.
The only significant factor was the number of times a mother had previously given birth to boys, according to the report in the Proceedings of the National Academy of Sciences.
The so-called fraternal birth order effect is small: Each older brother increases the chances by 33%. Assuming the base rate of homosexuality among men is 2%, it would take 11 older brothers to give the next son about a 50-50 chance of being gay.
Bogaert... said he didn't know what biological mechanism was behind the fraternal effect, which he and a colleague first identified 10 years ago.
The leading theory is that women's bodies react to male fetuses' proteins as foreign, making antibodies to fight them, Bogaert said.
Such antibodies could affect the developing fetus, and the more times a woman has carried boys, the stronger the antibody response would be."]
7. Lesbian women and heterosexual women respond differently to pheromones
"In contrast to heterosexual women, lesbian women processed AND [a progesterone derivative] stimuli by the olfactory networks and not the anterior hypothalamus. Furthermore, when smelling EST [an estrogen-like steroid], they partly shared activation of the anterior hypothalamus with heterosexual men. These data support our previous results about differentiated processing of pheromone-like stimuli in humans and further strengthen the notion of a coupling between hypothalamic neuronal circuits and sexual preferences."
Ivanka Savic-Berglund, MD, PhD, Hans Berglund, and Per Lindström, "Brain Response to Putative Pheromones in Lesbian Women," Proceedings of the National Academy of Sciences of the United States of America>, May 23, 2006
[Editor's Note:New Scientist, on May 8, 2006, published the article "Clue to Sexual Attraction Found in Lesbian Brain" by Patrick Barry, which stated:
"Lesbian and heterosexual women respond differently to specific human odours, a brain-scanning study has found. The homosexual women showed similar brain activity to heterosexual men when they inhaled certain chemicals, which may be pheromones, the researchers say.
'But our study can't answer questions of cause and effect,' cautions lead researcher Ivanka Savic at the Karolinska Institute in Stockholm, Sweden. "We can't say whether the differences are because of pre-existing differences in their brains, or if past sexual experiences have conditioned their brains to respond differently."...
When the heterosexual women smelled AND their brains showed activity in the anterior hypothalamus, a region of the brain thought to process sexual cues. But EST only produced activity in the olfactory region of their brains, the area that processes smells.
The lesbians, however, only showed activity in the olfactory region whichever odour they smelled."]
8. Gene splicing produces female-female courtship in fruit flies
"All animals exhibit innate behaviors that are specified during their development. Drosophila melanogaster [fruit fly] males (but not females) perform an elaborate and innate courtship ritual directed toward females (but not males). Male courtship requires products of the fruitless (fru) gene, which is spliced differently in males and females. We have generated alleles of fru that are constitutively spliced in either the male or the female mode. We show that male splicing is essential for male courtship behavior and sexual orientation. More importantly, male splicing is also sufficient to generate male behavior in otherwise normal females. These females direct their courtship toward other females (or males engineered to produce female pheromones). The splicing of a single neuronal gene thus specifies essentially all aspects of a complex innate behavior."
9. Gay men respond to pheromones in the same way as straight women
"These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes."
Ivanka Savic-Berglund, MD, PhD, Hans Berglund, and Per Lindström, "Brain response to Putative Pheromones in Homosexual Men," Proceedings of the National Academy of Sciences of the United States of America, May 17, 2005
[Editor's Note: The New York Times noted in a May 10, 2005 article "For Gay Men, an Attraction to a Different Kind of Scent," by Nicholas Wade:
"Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual men respond differently to two odors that may be involved in sexual arousal, and that the gay men respond in the same way as women. The new research may open the way to studying human pheromones, as well as the biological basis of sexual preference. Pheromones, chemicals emitted by one individual to evoke some behavior in another of the same species, are known to govern sexual activity in animals, but experts differ as to what role, if any, they play in making humans sexually attractive to one another."]
10. Lesbian women show "masculinized" eye blinking when startled
"Prepulse inhibition (PPI) refers to a reduction in the startle response to a strong sensory stimulus when this stimulus is preceded by a weaker stimulus--the prepulse. PPI reflects a nonlearned sensorimotor gating mechanism and also shows a robust gender difference, with women exhibiting lower PPI than men. The present study examined the eyeblink startle responses to acoustic stimuli of 59 healthy heterosexual and homosexual men and women. Homosexual women showed significantly masculinized PPI compared with heterosexual women, whereas no difference was observed in PPI between homosexual and heterosexual men. These data provide the first evidence for within-gender differences in basic sensorimotor gating mechanisms and implicate the known neural substrates of PPI in human sexual orientation."
[Editor's Note: The University of East London stated in an Oct. 2, 2003 press release titled "Sexual Orientation 'Hard-Wired' Before Birth - Startling New Evidence Revealed in the Blink of an Eye":
"The team discovered significant differences in the response between male and female, and heterosexual and homosexual subjects. Because the startle response is known to be involuntary rather than learned, this strongly indicates that sexual orientation is largely determined before birth...
Recent studies report links between foetal development, testosterone levels and adult sexuality, but this study offers the first independent evidence of a non-learned neurological basis for sexual orientation.
Dr Rahman said, 'The startle response is pre-conscious and cannot be learned. It is mediated by an ancient region of the brain called the limbic system which also controls sexual behaviour. This is very strong evidence that female sexual orientation at least may be 'hard-wired' in this region.'"]
11. Predominantly homosexual men and women change orientation after "reparative therapy" (study retracted*)
"This study tested the hypothesis that some individuals whose sexual orientation is predominantly homosexual can, with some form of reparative therapy, become predominantly heterosexual...
The majority of participants gave reports of change from a predominantly or exclusively homosexual orientation before therapy to a predominantly or exclusively heterosexual orientation in the past year. Reports of complete change were uncommon. Female participants reported significantly more change than did male participants. Either some gay men and lesbians, following reparative therapy, actually change their predominantly homosexual orientation to a predominantly heterosexual orientation or some gay men and women construct elaborate self-deceptive narratives (or even lie) in which they claim to have changed their sexual orientation, or both. For many reasons, it is concluded that the participants’ self-reports were, by-and-large, credible and that few elaborated self-deceptive narratives or lied. Thus, there is evidence that change in sexual orientation following some form of reparative therapy does occur in some gay men and lesbians."
Robert L. Spitzer, MD, "Can Some Gay Men and Lesbians Change Their Sexual orientation? 200 Participants Reporting a Change from Homosexual to Heterosexual Orientation," Archives of Sexual Behavior, Oct. 2003
[*Editor's Note: Originally a "con" study, Dr. Spitzer has retracted his conclusions and we now categorize the study as "not clearly pro or con." Dr. Spitzer made the retraction in an Apr. 2012 letter to Ken Zucker, the editor of Archives of Sexual Behavior, quoted by TruthWinsOut.org in its Apr. 25, 2012 report "Exclusive: Dr. Robert Spitzer Apologizes to Gay Community for Infamous 'Ex-Gay' Study":
"I offered several (unconvincing) reasons why it was reasonable to assume that the subject’s reports of change were credible and not self-deception or outright lying. But the simple fact is that there was no way to determine if the subject’s accounts of change were valid.
I believe I owe the gay community an apology for my study making unproven claims of the efficacy of reparative therapy. I also apologize to any gay person who wasted time and energy undergoing some form of reparative therapy because they believed that I had proven that reparative therapy works with some 'highly motivated' individuals."]
12. Lesbian women more than twice as likely as heterosexual women to have polycystic ovary syndrome
[Editor's Note: This peer-reviewed study by Rina Agrawal, MD, PhD, et al. was presented on June 30, 2003 at the annual conference of the European Society of Human Reproduction and Embryology, Madrid, Spain. We have been unable to locate a text version of the study. BBC News stated in the June 30, 2003 article "Hormone Link to Lesbianism" by Martin Hutchinson:
"Lesbians are more than twice as likely to suffer from a hormone-related condition [polycystic ovary syndrome], fueling theories that hormones play a role in developing their sexuality...
Full-blown polycystic ovarian syndrome was present in 38% of lesbians, and 14% of the heterosexual women...
Lead researcher Dr. Rina Agrawal said that the results suggested 'significantly greater' rates of hormone imbalance in the lesbian women.
She said that while there was no evidence that polycystic ovaries could be implicated as a cause of lesbianism, it was possible that this hormone imbalance could be linked to both the medical condition and sexuality."]
13. Researchers "switch" homosexual behavior on and off in male fruit flies
"It is reported here that male–male courtship behavior is evoked instantaneously in the fruit fly Drosophila by conditional disruption of synaptic transmission...
Sexual orientation of the fruit fly Drosophila has a genetic basis, which is evidenced by particular genetic variants that exhibit aberrant bisexual orientation. Viable mutant alleles of the fruitless gene (fru) show a variety of courtship abnormalities including vigorous male–male courtship. When male fru mutants are grouped together, they form 'courtship chains' in which a courting male is courted by other males, leading to a line of flies... Although the nature of these genetic variants is characterized at a molecular level, the mechanistic basis as to how these genes are involved in determining sexual orientation remains elusive. This is partly because little is known about the neuronal circuitry that controls the actual manifestation of male reproductive behavior."
Toshihiro Kitamoto, PhD, "Conditional Disruption of Synaptic Transmission Induces Male–Male Courtship Behavior in Drosophila," Proceedings of the National Academy of Sciences of the United States of America, Oct. 1, 2002
[Editor's Note: The Dayton Daily News noted in a Sep. 16, 2002 article "Scientists Turn On a Fruit Fly's Turn-On":
"Scientists say they have found a way to 'switch' homosexual behavior on and off in male fruit flies.
The researchers were able to do this by temporarily disrupting synaptic transmissions in the flies.
Previous research indicated that the sexual orientation of fruit flies is genetically determined, but the brain pathways for controlling sexual preference weren't clear."]
14. Change in sexual orientation reported after "conversion therapy" or "self-help"
"We present the results of a survey of 882 dissatisfied homosexual people whom we queried about their beliefs regarding conversion therapy and the possibility of change in sexual orientation... Of the 882 participants, 726 of them reported that they had received conversion therapy from a professional therapist or a pastoral counselor. Of the participants 779 or 89.7% viewed themselves as 'more homosexual than heterosexual,' 'almost exclusively homosexual,' or 'exclusively homosexual' in their orientation before receiving conversion therapy or making self-help efforts to change. After receiving therapy or engaging in self-help, 305 (35.1%) of the participants continued to view their orientation in this manner. As a group, the participants reported large and statistically significant reductions in the frequency of their homosexual thoughts and fantasies that they attributed to conversion therapy or self-help. They also reported large improvements in their psychological, interpersonal, and spiritual well-being. These responses cannot, for several reasons, be generalized beyond the present sample, but the attitudes and ideas are useful in developing testable hypotheses for further research."
Joseph Nicolosi, PhD, A. Dean Byrd, PhD, and R.W. Potts, "Retrospective Self-Reports of Changes in Homosexual Orientation: A Consumer Survey of Conversion Therapy Clients," Psychological Reports, June 2000
15. Homosexual men reported larger penis size than heterosexual men
"The relation between sexual orientation and penile dimensions in a large sample of men was studied. Subjects were 5122 men interviewed by the Kinsey Institute for Research in Sex, Gender, and Reproduction from 1938 to 1963... Penile dimensions were assessed using five measures of penile length and circumference from Kinsey's original protocol. On all five measures, homosexual men reported larger penises than did heterosexual men.
Explanations for these differences are discussed, including the possibility that these findings provide additional evidence that variations in prenatal hormonal levels (or other biological mechanisms affecting reproductive structures) affect sexual orientation development."
Anthony Bogaert, PhD, and S. Hershberger, "The Relation Between Sexual Orientation and Penile Size," Archives of Sexual Behavior, June 1999
16. Cochlea (auditory system of the inner ear) "partially masculinized" in lesbian women
"Click-evoked otoacoustic emissions (CEOAEs) are echo-like waveforms emitted by normal-hearing cochleas in response to a brief transient. CEOAEs are known to be stronger in females than in males. In this experiment, the CEOAEs of homosexual and bisexual females were found to be intermediate to those of heterosexual females and heterosexual males. A parsimonious explanation is that the auditory systems of homosexual and bisexual females, and the brain structures responsible for their sexual orientation, have been partially masculinized by exposure to high levels of androgens prenatally. No difference in CEOAEs was observed between homosexual and heterosexual males."
Dennis McFadden, PhD, and Edward G. Pasanen, "Comparison of the Auditory Systems of Heterosexuals and Homosexuals: Click-evoked Otoacoustic Emissions,"Proceedings of the National Academy of Sciences of the United States of America, Mar. 3, 1998
[Editor's Note: Associated Press noted in the article "Study Says Physical Differences Between Straight and Gay Women," published by the Augusta Chronicle on Mar. 3, 1998:
"Researchers say they have found the first strong evidence of a physical difference between lesbians and straight women -- a finding that the inner ears of gay women work more like those of men.
The discovery adds new support to the theory that sexual orientation may be predisposed before birth."]
17. Male homosexuality correlated with inheritance of "polymorphic markers" on the X chromosome
"The role of genetics in male sexual orientation was investigated by pedigree and linkage analyses on 114 families of homosexual men. Increased rates of same-sex orientation were found in the maternal uncles and male cousins of these subjects, but not in their fathers or paternal relatives, suggesting the possibility of sex-linked transmission in a portion of the population. DNA linkage analysis of a selected group of 40 families in which there were two gay brothers and no indication of nonmaternal transmission revealed a correlation between homosexual orientation and the inheritance of polymorphic markers on the X chromosome in approximately 64 percent of the sib-pairs tested. The linkage to markers on Xq28, the subtelomeric region of the long arm of the sex chromosome, had a multipoint lod score of 4.0 (P = 10(-5), indicating a statistical confidence level of more than 99 percent that at least one subtype of male sexual orientation is genetically influenced."
Dean H. Hamer, PhD, Stella Hu, et al., "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation," Science, July 16, 1993
18. Twin siblings of homosexual males more likely to be homosexual than non-twin siblings
"Homosexual male probands with monozygotic cotwins, dizygotic cotwins, or adoptive brothers were recruited using homophile publications. Sexual orientation of relatives was assessed either by asking relatives directly, or when this was impossible, asking the probands. Of the relatives whose sexual orientation could be rated, 52% (29/56) of monozygotic cotwins, 22% (12/54) of dizygotic cotwins, and 11% (6/57) of adoptive brothers were homosexual. Heritabilities were substantial under a wide range of assumptions about the population base rate of homosexuality and ascertainment bias. However, the rate of homosexuality among nontwin biological siblings, as reported by probands, 9.2% (13/142), was significantly lower than would be predicted by a simple genetic hypothesis and other published reports."
19. Nucleus of the hypothalamus of heterosexual men over twice as large as in women and homosexual men
"The anterior hypothalamus of the brain participates in the regulation of male-typical sexual behavior. The volumes of four cell groups in this region [interstitial nuclei of the anterior hypothalamus (INAH) 1,2,3, and 4] were measured in postmortem tissue from three subject groups: women, men who were presumed to be heterosexual, and homosexual men... As has been reported previously, INAH 3 was more than twice as large in the heterosexual men as in the women. It was also, however, more than twice as large in the heterosexual men as in the homosexual men. This finding... suggests that sexual orientation has a biological substrate...
The discovery that a nucleus differs in size between heterosexual and homosexual men illustrates that sexual orientation in humans is amenable to study at the biological level, and this discovery opens the door to studies of neurotransmitters or receptors that might be involved in regulating this aspect of personality. Further interpretation of the results of this study must be considered speculative."
Simon LeVay, PhD, "A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men," Science, Aug. 30, 1991