Brief on Sexual Orientation and Genetic Determinism
May 2006

Sexual Orientation Bibliography

Humans display a wide range of sexual feelings and behavior. Nowhere is this more prominent than in American culture which is saturated with images and references to sex and romantic love from television advertisements to billboard displays. Often, our identities as individuals are wrapped up with our romantic tendencies and how these play out in our relationships. Sometimes an individual’s sexual behavior is used as a barometer for his or her moral or religious beliefs. Our collective sense of how human sexuality should be expressed is revealed through the rights and liberties that structure our lives as citizens. 

Public concepts about sexuality have been highlighted recently due to the emergence of new laws. The advent of legal gay marriage in the state of Massachusetts [1] has caused American society to re-examine its notions of sex, identity, and the accompanying taboos against those considered outside the “mainstream.” The push for a broader set of rights and a re-definition of sexual expression, and the accompanying backlash, have taken the country by storm, giving rise to news editorials, constitutional amendment proposals [2] which define marriage as “between a man and a woman,” books on teenage homosexuality [3], and films such as “Brokeback Mountain [4],” that explore same-sex romance. Each of these cultural signs reflects a new environment in which notions of sex and sexual orientation are shaping the changing conversations of our time.

At the same time some scientists are attempting to show that sexual orientation is a fixed reality, one that is pretty much settled well before birth. The idea that our sexual orientation is determined by our biology is related to the idea that sexuality is something that can be categorized in a fixed way. But in reality, this is rarely the case. Dr. Alfred Kinsey’s groundbreaking studies [5] on human sexuality in the late 1940’s and early 1950’s showed that most people express sexual attraction towards their same genders at some point in their lives and that this bisexual orientation is acted upon through fantasies, sexual relationships, and partnerships to differing degrees. Indeed, there are many cases of individuals who marry partners of the opposite sex, have children, then get divorced and choose partners of the same sex. Well-known celebrity Anne Heche, an outspoken advocate of gay rights whose relationship with comedian Ellen Degeneres was the most highly publicized gay relationship in Hollywood during the 1990’s, eventually ended that relationship and married a man [6]. Examples like these, and Kinsey’s findings, suggest that sexual tastes can be expressed in a fluid rather than a fixed manner.

So why are some scientists currently interested in defining sexuality as a fixed property determined by one’s genes? This trend in categorizing human sexuality dates back to the seventeenth century, when sexuality was first critically examined in the West [7]. During this time, a whole range of human sexual practices were dragged “out of the closet,” and examined first by clergy, and then by lawmakers whose intent it was to relegate sex to the matrimonial bed, and then only for the purposes of procreation. One use of religious practices, such as confession, was to discourage “deviant” forms of sexual behavior. Ironically, these practices focused intense attention on the acts themselves and therefore did little to silence them. This resulted in a growing catalogue of deviant sexual behaviors, or “perversions,” out of which rapidly arose a medical industry enlisted to describe their causes. It was during the nineteenth century that homosexual and bisexual practices and sodomy first came to be considered medical “conditions.[8]"

Homosexual behavior was put under the microscope. Those who practiced sex with members of their own gender came to be thought of as biologically different from other “normal” heterosexuals. As homosexuality came to be thought of as abnormal, homosexuals themselves were branded as different. Termed “inverts,” the homosexual was labeled a new species in the taxonomies described by the medical field at the time.

As in the nineteenth century when sex reformers such as Havelock Ellis and Edward Carpenter were arguing in defense of homosexuals based on what was perceived to be an unavoidable biological tendency, today heated debates continue over whether homosexuals are “born that way.”  In fact, groups on both sides of the argument have vigorously argued both for and against the idea that sexuality is biology. Noticeably missing from this debate is the notion, championed by Kinsey, that human sexual expression is as variable among people as many other complex traits. Yet just like intelligence, sexuality is a complex human feature that modern science is attempting to explain with genetics. Research on brain size, hormone levels, finger length, and other biological traits have yet to yield evidence for this, however. It is important to note that traits such as these result from a combination of gene expression and developmental and other environmental factors. Well-known biologist and social theorist, Anne Fausto-Sterling advocates in her book Sexing the Body, for what scientists term a “systems approach” to be applied to our understanding of sexual preference. Rather than determining that this results from purely biological processes, a trait evolves from developmental processes that include both biological and social elements.  In addition, scientists rarely take into consideration sexual preferences that are not described by the two poles heterosexual and homosexual, “in hopes of maximizing the chance that they will find something of interest. [9]" Yet, as the following suggests, to date, no conclusive link between genetics and sexual orientation has been found.


It Runs In the Family

The first major study linking male homosexuality to family inheritance was published in 1986. Using a sample of 50 “straight” (heterosexual) men and 51 “gay” (homosexual) men, researchers James Weinrich and Richard Pillard found that the straight men had far fewer gay brothers than the gay men had. From this, they declared, “there is a significant familial component to male homosexuality.[10]" Other studies with similar designs have confirmed Pillard and Weinrich’s general observation [11,12,13,] including those that show a higher rate of homosexuality among sisters of lesbians [14].

But all of this evidence rests on the notion that sexual orientation is fixed and that individuals are either “straight” or “gay.” Researchers in these studies did not indicate whether individuals in their sample provided an extensive account of their sexual histories and tastes. Rather, individuals in the samples were simply asked to define their sexual orientation. In doing this, researchers set up a false dichotomy whereby the individuals studied are forced into one of two categories neither of which may accurately account for the full range of their sexual expression.

Additionally, it is difficult to conclude with certainty that because a trait shows up more often among biologically-related siblings, it is inherited. In fact, many such traits appear to be linked to distinctly non-biological factors. Diet, drug addiction, religious and political orientation, and career paths are just a few of the behaviors that cluster in families due to shared social influences. Indeed, a shared environment can often produce a clustering of all types of behavior patterns. It is doubtful that any study could be designed with sufficient sensitivity to exclude this possibility.

The methods used in family linkage studies, which depend upon recruitment from gay and lesbian magazines, websites, and organizations, carry a clear risk of ascertainment bias. Gay brother pairs may be more interested in responding than gay men with straight brothers, given the potential for homophobia among siblings and the substantial number of homosexuals who are “in the closet” toward their families. Twins or siblings who are both gay might find the subject interesting or already suspect a genetic basis of their shared sexual orientation, thus making them more likely to participate [15].

In order to exclude these potentially confounding factors, more recent studies have drawn participants from random samples. A study in 2000 of 4,500 twins from the Australian Twin Registry by Bailey and colleagues showed only a 30% rate of homosexuality shared between both male and female identical twins [16].


Searching for the Gay Gene

In 1993 Dean Hamer and colleagues at the National Institutes of Health, claimed to have discovered a gene for homosexuality. Their study, published in Science [17], used a sample of 40 “gay” brother pairs whose sexual orientation was said to be maternally inherited. Hamer and colleagues found that 83% of the pairs shared the same markers on a region of the X chromosome called Xq28. Interestingly, a follow-up study including lesbian sister pairs did not show the same occurrence of shared markers [18]. Though the original study was never replicated, Time magazine featured an article the following week with a bold cover entitled, “Born Gay: Scientists Discover a Genetic Link.” In 1999, in the largest study of its kind to date, George Rice, George Ebers and colleagues at the University of Western Ontario failed to reproduce a statistically significant linkage to the Xq28 marker in a sample of 52 gay brother pairs [19]. Another unpublished study led by Alan Sanders at the University of Chicago produced inconclusive results [20]. Further evidence of a genetic link to homosexuality has not been produced.


Gay Brains Look Different

In 1991, neurobiologist Simon LeVay published a study he had conducted on brain tissues from male and female cadavers. The cadavers were presumed to be of 19 “homosexual” and 16 “heterosexual” men as well as 6 “heterosexual” women. In his analysis he found a region at the base of the brain was substantially smaller among gay men compared to heterosexual men, and comparable between gay men and heterosexual women [21]. Though it is unclear how he determined sexual orientation in the first place, LeVay stated in his book The Sexual Brain, that the differences he observed were the result of structural differences in the brains of gay men due to disruption in the development of “separate centers within the hypothalamus for the generation of male-typical and female-typical sexual behavior and feelings [22].”

But, what is a gay cadaver? How was it decided that these bodies represented individuals who would be considered homosexual or heterosexual? It was revealed that sexual histories were unavailable for 14 of the 16 ‘presumed heterosexual’ male cadavers, so it is impossible for LeVay to have accurately defined the sexual orientation of these subjects. Additionally, it seems possible, under these circumstances, that the sexual orientation of the other individuals may not have been accurately accounted for. Notably, all 19 of the ‘presumed homosexual’ men had died of AIDS. Despite this important fact, LeVay does not discuss the possible physical changes that may have occurred to their brains because of this, nor does he include a sample of “homosexual” men who had not suffered from the disease. Finally, while, as a group, the size of the INAH-3 in “gay” male cadavers tended to be smaller than that in “straight” male cadavers, the range of sizes in the two groups was nearly identical. Some gay cadavers had a larger INAH-3 than heterosexual cadavers. Not surprisingly, other studies have failed to replicate LeVay’s findings [23]. Yet, despite the lack of clarity revealed by his data, the study garnered enormous press attention, including a cover story in Newsweek.


Fraternal Birth Order

Studies performed over the last several decades have found that gay men tend to have a greater number of older male siblings than do straight men. In a recent analysis of data from 14 studies involving 10,000 individuals, a group led by Ray Blanchard, a professor of psychology at the University of Toronto, determined that a large number of older brothers is a significant predictor of male homosexuality [24]. This has also been found to be true for birth weight. Some studies have shown that gay men with older brothers weigh an average of 170 grams less at birth than heterosexual men [25].

But what does this mean? Blanchard and others have suggested a “maternal immune” effect whereby mothers develop an increasing immunological response to successive pregnancies involving male-specific antigens. As the mother’s immune system comes to recognize these antigens in successive pregnancies, it is assumed that she produces antibodies that affect the sexual differentiation of the brain in subsequent pregnancies involving male fetuses [26]. There is an absence of evidence linking either male birth order to maternal immune responses or maternal immune responses to male homosexuality. And other explanations for the ‘fraternal birth order effect’ are not difficult to imagine. It is certainly possible that social influences linked to family environments with multiple older male siblings may impact the development of a child’s gender identity and later sexual expression, yet these perspectives have gone unstudied.


We are sexual beings, yet this does not mean that we are born homosexual, bisexual, or heterosexual. Our sexual expression can change over time, towards different people, through different experiences. A lack of understanding about this type of human variability often leads to a perspective that our genes define who we are. Each of the above areas of research displays findings that hinge on the assumption that a given individual’s sexual expression neatly fits into the categories “straight” and “gay.” By not considering evidence of human sexual fluidity, debates regarding origins and biology are not substantial or complete. Current efforts fail to tell the whole story. And even if we were to accept that the assigned sexual orientation of the individuals participating in these studies accurately reflected their lifelong expression, conclusive proof of a link between this and their genes has yet to be found.

As discussions about the origins and character of human sexuality rage within the scientific community, the social climate surrounding it is changing. Several European countries have legalized same sex unions, considering these a state matter rather than a religious one. In the United States, these issues are somewhat less clear and thus our society has put sex and sexual orientation on the docket. Accordingly, the social urgency to answer questions regarding sexual orientation has pushed a greater interest in the “science” of it. Yet a narrow focus on the variability of sexual expression threatens to cloud the issue altogether. Without giving proper attention to the mutability of human sexual expression, questions regarding its origins and character cannot be answered.



1 “Massachusetts court rules ban on gay marriage unconstitutional,” Retrieved December 2, 2005 from:

2 “President Calls for Constitutional Amendment Protecting Marriage,” Retrieved January 10, 2006 from:

3 Savin-Williams, Ritch C. .2005. The New Gay Teenager. Cambridge: Harvard University Press.

4 Retrieved January 10, 2006 from:

5 Kinsey, Alfred C. et al. 1948/1998. Sexual Behavior in the Human Male. Phildelphia: Bloomington, IN: Indiana University Press.

Kinsey, Alfred C. et al. 1953/1998. Sexual Behavior in the Human Female. Phildelphia PA: W.B. Saunders; Bloomington: Indiana University Press.

6 Stockwell, Anne. 2001. The Agony and the Ecstacy of Anne Heche. The Advocate. November 6. (retrieved January 10, 2006 from: http:/ ; Friedman, Roger. 2006. Anne Heche: Book Her Ready to Talk. (retrieved January 10, 2006 from:,2933,9679,00.html).

7 Foucault, Michel. 1980. The History of Sexuality, Volume 1: An Introduction. New York: Vintage Books.

8 Foucault, Michel. 1980. The History of Sexuality, Volume 1: An Introduction. New York: Vintage Books.

9 Fausto-Sterling, Anne.  2000. Sexing the body: how biologists construct human sexuality. New York, NY: Basic Books.

10 Pillard, Richard and Weinrich, James. 1986. Evidence of familial nature of male homosexuality. Archives of General Psychiatry. 43: 808-812.

11,12,13 Pillard, Richard and J. M. Bailey. 1998. Human sexual orientation has a heritable component. Human Biology. 70: 347-65; Bailey, J. Michael, et al. 1999. A family history study of male sexual orientation using three independent samples.  Behavior Genetics.  29;  79-86; Bailey, J. Michael and R. Pillard. 1991. A genetic study of male sexual orientation.  Archives of General Psychiatry. 48: 1089-96.

14 Pattattucci, A. and D. Hamer. 1995.  Development and familiality of sexual orientation in females. Behavior Genetics. 25: 407-20; Bailey, J. M. and B. Benishay. 1993. Familial aggregation of female sexual orientation. American Journal of Psychiatry. 150:  272-77.

15 Hubbard, Ruth and Elijah Wald. 1999. Exploding The Gene Myth. Boston:  Beacon Press.

16 Bailey, J. M., et al. 2000.  Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78: 524-36.

17 Hamer, D; S. Hu; V. Magnuson; N. Hu; and A. Pattatucci. 1993. A linkage between DNA markers on the X chromosome and

male sexual orientation. Science. 261:  321-27.

18 Hu, S.; A. Pattattuci; Patterson, C. Li; L. Fulker; D. Cherny; S., L. Kruglyak; and D. Hamer. 1995. Linkage between sexual orientation and chromosome Xq28 in males but not in females. Nature Genetics. 11: 248-56.

19 Rice, G., C. Anderson; N. Risch; and G. Ebers. 1999. Male homosexuality: absence of linkage to microsatellite markers at Xq28. Science. 284: 665-67.

20  Wickelgren, I. 1999. Discovery of gay gene questioned. Science. 284: 571.

21 Levay, S. 1991. A difference in hypothalamic structure between heterosexual and homosexual men. Science. 253: 1034-37.

22 Levay, Simon. 1993. The Sexual Brain. Cambridge: Bradford Books.

23  Byne W., S. Tobet; L. Mattiace; M. Lasco; E. Kemether; M.  Edgar; S. Morgello; M. Buchsbaum; and L. Jones. 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.

24 Blanchard, R.  2004. Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology. 230:  173-87.

25 Blanchard, R. and L. Ellis.  2001. Birth weight, sexual orientation, and the sex of preceding siblings. Journal of Biosocial Science. 33: 451-67.

26 Blanchard, R. 2004. Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology. 230: 173-87.

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