The Essentialist School looks to explain homosexual orientation by means of biology and nature.

For many years attempts have been made to alter sexual orientation. These have ranged from forced brothel trips through aversion (electric-shock or chemical) treatment, drugs and hormone injections, to castration of gay men and clitoridectomy of lesbian women. All these ‘treatments’ would be considered highly unethical today, and, indeed, they did not prove successful.

Sexual orientation appears to be highly resistant to change. This suggests to some that there must be a biological cause that is beyond personal control.

Studies have been done to assess this, but it should be noted from the outset that these are few in number. Research into sexuality is understandably not a high priority for funding providers in comparison with, for instance, diseases such as HIV/AIDS.

Nonetheless, instances of anatomical, genetic and endocrine (hormonal) evidence do support a biological component in sexual orientation.

The first test for biological determinism was undertaken in 1957 by Evelyn Hooker. A psychologist, Hooker conducted a series of evaluations using three projective tests (Rorschach, Thematic Apperception Test (TAT), and Make-A-Picture-Story (MAPS) Test) on 30 homosexual and 30 heterosexual males. The two groups had been cross-matched for age, intelligence quotient (IQ) and education. Expert psychologists performed a blind analysis of the results (they did not know from which group a set of results had been obtained) and could find no significant difference in the answers given by the two groups for any of the tests.

Hooker concluded that homosexuality was not a clinical entity and was not involved in psychopathology.[1] This paved the way for the APA to change its stance on homosexuality in later years.

In 1990 the neurobiologist Professor D.F. Swaab of the University of Amsterdam started a controversy in the Netherlands with his research. He found that the suprachiasmatic nucleus, a small region of the hypothalamus, was larger in a group of homosexual men than in the control group. This, he suggested, may constitute evidence for a biological basis for homosexuality.[2]

Two years later Laura Allen and Roger Gorski made a similar discovery. In their study the anterior commissure of the hypothalamus was found to be significantly larger in homosexual subjects.[3] 

Neither of these two areas of the hypothalamus are related to sexual behaviour. The behaviour is not driving these size differences. The differences are genetic and stem from subtle differences in neuroendocrinology (hormone levels) during brain development in foetal growth.

A third study by Simon LeVay, a neuroscientist at the Salk Institute, indicates that yet another region of the hypothalamus (the third interstitial notch of the anterior hypothalamus (INAH3)) was approximately half the size in homosexual men and heterosexual women than that found in heterosexual men.[4] Again, the suggestion was made that there may be a biological basis for sexual orientation.

It should be noted that there is criticism of these studies. For instance, all the homosexual subjects in LeVay’s study had died of AIDS and there is a question over whether the drugs used in treating AIDS may affect brain size and shape. In addition, LeVay is himself gay, and some have questioned his objectivity.[5] However, these criticisms are not wholly convincing. Some of the heterosexual subjects also had died of AIDS, for instance.

The same year that LeVay’s study was published, a psychologist, J Michael Bailey, and psychiatrist, Richard Pillard, published the results of a twins study. They studied the sexual orientation of monozygotic (identical), dizygotic (fraternal) and non-related adopted brothers.

As identical twins share the same genetic makeup you would expect a strong concordance between the pairs of twins if sexuality has a genetic determiner. Some expect a perfect correlation. As David W. Torrance recently wrote, “..if sexual orientation is determined by genes, the figure should be 100% [correlation between both brothers being gay] because identical twins have identical genes.”[6]

That is, however, an overstatement. What you would expect is a higher concordance for identical twins, lesser for fraternal or siblings, lesser still for half-siblings, and the least for adopted children.

To take a few different examples: for diabetes mellitus there is a 50% concordance for identical twins and only 10% for fraternal. For cleft lip it is 40% and 4%. For Schizophrenia, 60% and 10%.[7] In other words, while the genetic component plays an important part, and these traits are inheritable (though not perfectly), the genes are not deterministic.

This trend is precisely what Bailey and Pillard found. Of 167 homosexual men 52% had gay identical twins, 22% had gay fraternal twins and 11% had adopted brothers who were gay.[8] Bailey and Pillard later carried out a similar study on homosexual women and found a similar pattern.[9]

The Bailey and Pillard study has been criticised however because of sample bias. Its subjects were recruited in such a way that it may have drawn individuals who were particularly keen to see a positive result; for example, homosexuals who had a homosexual twin may have been more likely to take part.[10]

Bailey himself has produced a more recent study with somewhat different conclusions. By sending a questionnaire to everyone on the Twin Registry in Australia his results produced a figure of 20% concordance of homosexual orientation for identical male twins, and 0% for fraternal twins. The figures were 24% and 10% respectively for female homosexual identical and fraternal twins.[11]

These new figures are approximately half those in the previous studies which, the authors suggest, may be due to the previous work having inflated concordances as a result of possible sample bias. Nonetheless the same general pattern found in the earlier studies is retained, as one would expect if there was a genetic component in homosexual orientation.

Dean Hamer looked at the possibility of homosexual orientation being linked to the X chromosome. He noted by studying the family trees of openly gay men that there may be a maternal link. He found that 13.5% of the homosexual men’s brothers in the study were also homosexual. The figure for the general population is around 2%.

Hamer took DNA samples from 40 pairs of homosexual brothers and found a concordance in 33 pairs of brothers for 5 molecular markers on a section of the X chromosome called Xq28.[12] This was statistically significant. The study became known as the ‘gay gene study’ in the press. A follow-up study in 1995 confirmed these results.[13] However, a third study in 1999 has not been able to support these results, finding only three shared markers which is approximately what you would expect by chance.[14] More study is needed before anything definitive can be concluded.

The final study that has provided intriguing evidence comes not from humans, but from experiments with rats and mice. In all species the average male and female brains are different, wired in different ways. Initially, however, embryos are neither male nor female. Sex characteristics develop gradually. The view for the past 30 years has been that the body and brain differentiate as a result of secretions of the steroidal hormones testosterone and oestrogen. With respect to brain development, testosterone induces a masculine pattern of development while inhibiting feminine neural and behavioural patterns of differentiation. This process is triggered by the testis-determining gene (SRY) which is located on the Y chromosome.

In the case of brain development, this view has been brought into question by a study from the University of California Los Angeles (UCLA).[15]

The UCLA study identified over 50 candidate genes that are expressed differently in male and female mice brains before the gonads have differentiated, and thus before what was thought to be the stage at which sex differentiation occurred. “This suggests that there are functional differences between male and female brains, which occur independently from hormonal influence. Moreover, these differentially expressed genes are good candidates for a role in brain sexual differentiation and sexual behaviour.”[16]

What does this mean? Eric Vilain has suggested that this study raises the possibility that sexual identity and physical attraction is ‘hard-wired’ by the brain.[17]

The study could also help to explain the phenomenom of transgenderism. In some cases the brain of an embryo might develop, for instance, male, whereas the body could develop female. People could feel that their true gender was not consistent with their bodies.

Having said that, there is much more study required, particularly to understand the interplay between these genes and the steroidal hormones.

Before summarising this section, a short story. An old friend of ours in America was born and raised as a girl. Her family didn’t suspect anything was wrong until she hit puberty. When all the other girls at school were beginning to menstruate, she did not. Her period never came.

Subsequent trips to the doctor found that this girl had an inherited condition called Androgen Insensitivity Syndrome (AIS). This condition occurs in approximately 1 in 20,000 individuals. This girl had X and Y chromosomes and thus was genetically male. Her body, however, was insensitive to androgen, so her external genitalia appeared female, rather than male. Yet because she was genetically male, she had no female reproductive organs.

Feeling that she was a woman, she married and she and her husband have adopted children. Yet genetically, she is male. Is she living in a homosexual relationship?

The complexities of sexuality are brought to mind clearly by such cases.

There is a growing number of studies that provide interesting correlations between biological factors and human sexual orientation. However, none of them show a direct causation.

Devising an experiment that would be able to show direct causation would involve gene splicing in embryos so as to provide a control group. It would be highly unethical to do so, and so any studies undertaken will by necessity deal only with correlations between biological factors and sexual orientation.

There are also ethical concerns raised by devoting too much research into the area of human sexuality. As mentioned earlier, many judge that other areas of research are more pressing at the moment, such as the need to combat diseases. There are other ethical concerns, however. What if a genetic marker was definitively found, for instance? Would someone who is positive for that marker be discriminated against? Would some parents choose to abort if they knew their child had homosexual biological factors?

In summary, the biological data accrued through these studies indicates above all that “the biology of sexuality is very complicated, including many genes, pre-natal hormones, and post-natal hormones. This complexity makes conclusive studies difficult to perform.”[18]

It is, thus, highly unlikely that a simple biological test to determine one’s sexual orientation will be forthcoming for many years, if at all. That is not the same, however, as saying that biological factors do not play a role in determining sexual orientation. The evidence indicates that they probably do, but the evidence is not definitive and it may never be, as a result of the ethical limitations on the experimental methods that can be used.