Introduction and caveat

In popular parlance, the question is whether homosexuality is due to nature or nurture. Frustratingly to those who want a simple answer NOW, there is currently no definitive answer from scientific research, nor is there likely ever to be a simple answer.

In fact, the question itself may be wrongly phrased. The implicit assumption that heterosexuality does not need explaining, but homosexuality needs justification, is by itself already prejudicial. Not only do we not know how homosexual orientation comes about, we know even less about how heterosexual orientation comes about!

Thus, before moving on, it is important to be aware of our own motives for wanting to know where sexual orientation comes from. It is a valid area of scientific enquiry, for, as humans, we are curious and want to know how things have come to be the way they are. However, it has to be stressed that it impossible to understand homosexual orientation alone. The field of enquiry should really be sexual orientation generally, if we are to achieve any real understanding of ourselves as a species.

Unfortunately, this subject is often wrapped up with the politics of sexuality. There is a loose assumption that whether sexuality is determined by biological factors or choice can settle questions of morality, ethics, social and cultural values, even law.

People Like Us makes no such claim. We believe in the value of knowledge for itself. We also believe that the equal treatment of all persons regardless of sexual orientation and gender identity, is founded on the morality of human dignity, and the superior social and cultural values of personal liberty, respect for others and their differences, and freedom of conscience. It does not change the arguments whatever one makes of the science.

 
The science to date, briefly

Since the later 1980s, there have been some interesting leads from neuroanatomy, endocrinology and twin studies. These studies have established a few intriguing correlations which merit further study, but definitive cause-and-effect pathways have not been shown. Yet, enough results have accumulated such that, at this point, one can say that biology plays a significant role. 

However, many people confuse biology with genetics. While the twin studies have shown a good correlation between genetic closeness and homosexuality, indicating a role for genes, they have also shown that genes cannot explain everything. There are probably other biological mechanisms at work.

On the other hand, there have been a paucity of results from psychological studies indicating socialisation as an origin of homosexual orientation. (In fact, there have been far more interesting results from psychology about homophobia than about homosexuality.) However, at this point, everything is so tentative, no theory has been ruled out except the most egregiously simplistic.

Among the most egregiously simplistic are the old ideas that homosexuality is due to dominant mothers and absent fathers, or to being abused when young. Not only has there not been a shred of evidence for these theories despite having been around for a century, it should be obvious that there is something simplistic about them when one realises they only address male homosexuality.

It is likely that there will be a variety of causative factors interacting in complex ways. Moreover, the latest studies to do with fingerprints, birth order and hand morphology are suggestive of the possibility that causative factors for gay men may be different from those for gay women. This is also indicated from studies that have shown that while most gay men feel that their sexual orientation is innate, this is not as often reported from gay women.

To quote one of the references below (the paper: Exotic becomes erotic),

Women's sexual orientations are more fluid than men's. Many studies, including a national random survey of Americans (Laumann, Gagnon. Michael & Michaels 1994), have found that women are more likely to be bisexual than exclusively homosexual, whereas the reverse is true for men. Nonheterosexual women are also more likely to see their sexual orientations as flexible, even "chosen", whereas men are more likely to view their sexual orientations in essentialist terms, as inborn and unchangeable (Whisman, 1996). For example, men who come out as gay after leaving heterosexual marriages or relationships often describe themselves as having "finally realized" their "true" sexual orientations. Similarly situated lesbians, however, are more likely to reject the implication that their previous heterosexual relationships were inauthentic or at odds with who they really were: "That's who I was then, and this is who I am now."

Many who have studied sexuality have cautioned that to understand homosexual men, one has to see them primarily as men; to understand homosexual women must involve seeing them first and foremost as women.

 
Useful URLs

 
Biological factors that can influence homosexuality (2001) by Christina Harms

This is a good overview of the biological studies to date. It covers how testosterone and estrogen in adults have been debunked as causes. But it discusses how these hormones in utero may have had an effect on developing foetal brains. The theory is that the neuronal pathways in the brains are organised in a certain way during foetal life, which manifests themselves as sexual orientation after puberty.

This paper also does an overview of neuroanatomical findings and touches on twin studies as well. Finally it brings up up to date on the Xq28 region of the 26th chromosome, which ahs been suggested as a genetic marker that is somehow related to homosexual oreintation in males.

 
Homosexuality: Nature or Nurture (2003)

Another overview of the science to date. It reaches back to the Kinsey reseach and some early psychological studies. Then it touches on the findings from neuroanatomy, twin studies and Xq28. It points out that "there is no evidence, social or biological, to support that homosexual children were raised differently than were the heterosexual ones."

Finally, it touches on social theories, particularly those of Halperin and Foucault.

 
The Genetic basis for homosexuality

This webpage touches on brain structure and chromosomal linkage studies, but contains a good account of the twin studies and how to interpret their conclusions.

 
Twin studies and homosexuality

This webpage gives a succinct account of the twin studies done by Bailey, Pillard and others. The headline findings were:

• 52% of identical (monozygotic) twins of homosexual men were likewise homosexual
• 22% of fraternal (dizygotic) twins were likewise homosexual
• 11% of adoptive brothers of homosexual men were likewise homosexual

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

• 48% of identical (monozygotic) twins of homosexual women were likewise homosexual (lesbian)
• 16% of fraternal (dizygotic) twins were likewise homosexual
• 6% of adoptive sisters of homosexual women were likewise homosexual

Bailey, J. M. and D. S. Benishay (1993), "Familial Aggregation of Female Sexual Orientation," American Journal of Psychiatry 150(2): 272-277.

It also discusses the methods used in the study and the possible pitfalls. More on the research methods can be found at http://www.utexas.edu/courses/bio301d/Topics/Gay/Text.html

 
Exotic becomes erotic (2000)

This is a speculative theory attempting to explain how biological findings (which currently dominate the research literature on the determination of sexual orientation) can lead to homosexual or heterosexual orientation. It provides a conceptual thesis for a developmental pathway that links biological predisposition to orientation as manifested in adult life.

 
Not in your genes

This webpage is a review of the book The Mismeasure of Desire by Edward Stein, which gives a little insight into Stein's arguments. They are that "Biological arguments for lesbian and gay rights fail theoretically, legally, and pragmatically," and that "strong ethical, legal, and political arguments are needed."

"The results of scientific research are basically irrelevant to such arguments."

Stein critiques many of the scientific studies, and even the conventional wisdom of what sexual orientation is. Through these critiques, it puts our understanding of scientific research in better perspective.

 
The biological evidence challenged (1994)

The webpage is from a PBS program, but is based on an article that first appeared in the Scientific American vol 270 (May 1994). The author, William Byne is a psychiatrist and neuroanatomist, but his point is that the salient questionis not whether biology is involved, but HOW it is involved. All psychological phenomena are ultimately biological. Along the way, however, he critiques neuroanatomical findings and animal studies and cautions us from drawing too broad conclusions from them.

 
Beyond moral panic: why science should investigate sexual orientation (2000)

This webpage focuses on the ethical issues that arise from scientific research into sexual orientation, such as prenatal intervention, abortion, and change of sexual orientation in adults.