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Genetic basis for homosexuality

The possibility of a genetic or otherwise biological basis for homosexuality has been the object of a number of studies. Among the most publicised are studies falling into the following three categories:

  1. Studies of brain structure (e.g. Simon LeVay's study of the neurons in the hypothalamus)
  2. Twin studies (e.g. the study of John M. Bailey and Richard Pillard on twin brothers)
  3. Studies of chromosomes (e.g. by Dean Hamer)

These studies lend credence to the idea that there is a genetic component to sexual orientation, or that homosexuality and heterosexuality may be partly the result of some otherwise innate physical characteristic(s). The majority of these studies are far from conclusive and proper scientific discussion over the physiological or genetic causes of homosexuality continues.

Most scientists agree that it is unlikely that there is a single 'gay gene' that determines something as complex as sexual orientation, and that it is more likely to be the result of a collection of factors, some of which may be genetic. Many believe that we do not yet have the scientific knowledge to definitively determine whether sexual orientation is genetic or not, and that we may never be able to do so.

Table of contents
1 Studies of brain structure
2 Twin Studies
3 Chromosome Linkage Studies
4 Politics
5 References and External Links

Studies of brain structure

A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women. There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Swaab and Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure.

However, the best-known work of this type is that of Simon LeVay, reported in "A Difference in Hypothalamic Structure Between Hetero-sexual and Homosexual Men" in the journal Science, August 1991. LeVay studied four groups of neurons in the hypothalamus, called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that this part of the brain played a role in the regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women.

He obtained brains from 41 deceased hospital patients. The subjects were classified as follows: 19 gay men who had died of AIDS, 16 presumed heterosexual men (6 of whom had died of AIDS), and 6 presumed heterosexual women (1 of whom had died of AIDS).

The AIDS patients in the heterosexual groups were all identified from medical records as intravenous drug abusers or recipients of blood transfusions, though only 2 of the men in this category had specifically denied homosexual activity. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been mostly or all heterosexual "on the basis of the numerical preponderance of heterosexual men in the population".

LeVay found no evidence for a difference between the groups in the size of INAH1, INAH2 or INAH4. However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the 6 AIDS patients were included in the heterosexual group. The size of the INAH3 in the homosexual male brains was similar to that in the heterosexual female brains. However, he also found some contrary results:

William Byne and colleagues attempted to replicate the differences reported in INAH 1-4 size using a different sample of brains from 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). They found a significant difference in INAH3 size between heterosexual men and women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men and larger than that of the heterosexual women, though neither difference quite reached statistical significance.

Byne and colleagues also weighed and counted numbers of neurons in INAH3, tests not carried out by LeVay. The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation.

Conclusions

LeVay concluded in his 1991 paper that his results suggested that "sexual orientation has a biological substrate", but later urged against over-interpretation of his results. He stated, "It's important to stress what I didn't find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn't show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain. ...Since I look at adult brains, we don't know if the differences I found were there at birth or if they appeared later."

Also, HIV/AIDS may affect the brain, causing chemical changes. So rather than showing that differences in neurons indicate homosexuality, LeVay's study may be showing that HIV/AIDS causes differences in neurons. It should be noted, however, that neither LeVay nor Byne found an HIV-related difference in INAH3 size.

Twin Studies

One common type of twin study compares the monozygotic (or 'identical') twins of people possessing a particular trait to the dizygotic (non-identical) twins of people possessing the trait. Since monozygotic twins have the same genotype (genetic makeup) while dizygotic twins share only, on average, 50% of their genotype, a difference in the prevalence of the trait in question between these types of twins provides evidence of a genetic component.

A few such studies began to examine homosexuality in the early 20th century, using small, non-random samples. The first relatively large-scale twin study on sexual orientation was reported by Kallman in 1952. Examining only male twin pairs, he found a 100% concordance rate for homosexuality among 37 MZ twin pairs, compared to a 12%-42% concordance rate among 26 DZ twin pairs, depending on definition. In other words, every identical twin of a homosexual subject was also homosexual, while this was not the case for non-identical twins. This study was criticised for its vaguely-described method of recruiting twins and for a high rate of psychiatric disorders among its subjects.

While Kallman's was the largest relevant study until the 1990s, other studies did provide examples of monozygotic twin pairs of both sexes who were discordant for sexual orientation; that is, they found that both male and female homosexuals did sometimes have twins that were not homosexual themselves. The existence of such twins clearly demonstrates that genes are not the only factor involved in determining sexual orientation, at least not for everybody. Environmental factors, in the womb or during life, must play a role.

However, studies such as Kallman's did suggest the existence of a significant genetic component to sexual orientation, and later researchers have attempted to use twin studies to quantify the size of such an effect. The most highly publicised was reported by J. Michael Bailey and Richard Pillard in "A Genetic Study of Male Sexual Orientation" in the Archives of General Psychiatry, December 1991. These researchers recruited gay male subjects through advertisements in the gay media and sent questionnaires, which included questions on sexual orientation, to their male siblings. They reported that:

were also gay themselves. The researchers estimated that the heritability of male homosexuality was between 31% and 74%. A similar study carried out by the same researchers on the siblings of lesbian women reported concordance rates of 48% for MZ twins, 16% for DZ twins, 14% for non-twin sisters and 6% for adoptive sisters. Heritability was estimated as between 27% and 76%.

Being based on samples of people who volunteered for studies specifically targeting gay men and lesbians, these results could only be suggestive. However, a number of more recent studies have examined sexual orientation in large "twin registries" recruited without reference to sexual orientation. Estimates of heritability for male and female homosexuality derived from these are shown below.

Estimates of heritability of homosexuality
MaleFemale
Hershberger, 19970%48%
Bailey et al., 200040%0%
Kendler et al., 200028-65% (male & female combined)
Kirk et al., 200030%50-60%

Conclusions

One criticism often levelled at comparisons of MZ/DZ twins in general is that they depend upon an assumption that MZ and DZ twins share a similar amount of their environments with their twins. This assumption has been questioned.

To consider specifically the studies under discussion here, the results of the Bailey and Pillard volunteer studies pose some problems for interpretation. Consider the male study. The higher concordance rate among MZ twins compared to DZ twins is consistent with substantial heritability. However, the lower rate among non-twin brothers compared to DZ twins (both share on average 50% of genes), and the high rate of homosexuality among adoptive brothers of gay men compared to the general population (when adoptive brothers are no more genetically similar than men from different families) provide evidence for environmental factors. It is also worth stressing that the MZ concordance was well short of 100%, which also points to a role for environmental factors.

It is considered likely by some authors, including Bailey, that the heritability of homosexuality has been overstated by volunteer studies. For example, a gay man with a gay brother may be more likely to volunteer for a study of gay men and their brothers than will a gay man with a heterosexual brother, perhaps because he feels that his brother will be more willing to cooperate. The lower heritability estimates from the more recent, probably more representative, studies seem to confirm this.

However, there is still considerable variation even between these studies. It is interesting to note that the Kirk et al. study (see table above) was a reanalysis of the same data used by Bailey et al. (same table), using different definitions of homosexuality. The striking difference in results, particularly for women, underlines the lack of definitive results at this point. One problem is that, for most definitions, the prevalence of homosexuality in the general population is low, which means that registries will contain relatively few twin pairs of which one is gay or lesbian. The lack of statistical power resulting from this may explain some of this lack of consistency. Meta-analysis might be of use in resolving this difficulty.

Overall, data appear to indicate that genetic factors play some role in the development of sexual orientation, but that they probably account for only a minority of variation and that further work will be needed to quantify their influence more precisely.

Any genetic component must be rooted in evolution by natural selection, and many non-scientists assume that a homosexual orientation would necessarily result in decreased reproduction. Gene prevalence, however, and therefore selection, can be influenced by increasing the reproductive success of individuals with whom we share genes in common. While it may be unclear to some how homosexuality could offer a selective advantage to individuals, many theories exist that explain why an inherited tendency toward this orientation might offer a selective advantage to the genes they carry. Most theories speculate that the presence of homosexual members may also promote intragroup harmony. These are theories: hard empirical data is lacking. Apparent homosexual behavior provides a stealth mechanism for slipping past alpha males in some species; like most putative explanations, that does not explain inheritance of female homosexuality. Some twin effects could be the result of their shared environment from conception to birth.

Chromosome Linkage Studies

Methodology

Conclusions

Politics

The issue of genetic or other physiological determinants as the basis of homosexuality is a highly politicised issue. Recent studies have demonstrated that public acceptance towards homosexuality would increase significantly if scientific proof emerged that sexual orientation had a genetic cause or otherwise innate cause. Therefore, both sides have a lot to gain or lose depending on results in this area.

Most objections to the idea of a genetic or innate cause of homosexuality come from religious groups and others in the anti-gay lobby. They seek scientific proof that homosexuality is not determined by genetics or other innate means and interpret scientific results warily. They believe that homosexuality is determined by purely psychological factors, and, more so, that a person's sexuality is a matter of personal choice or of poor upbringing.

Similarly, many gay rights advocates seek scientific proof that homosexuality is determined by genetics or other innate means. However, many do not actually believe the cause(s) of homosexuality to be purely genetic, and instead believe a collection of various factors, including genetics, to be the cause. Most agree that homosexuality is innate.

Many research scientists find themselves in the center of these two camps. They see themselves as neutral observers merely publishing their results as they find them. They often have little control over the public dissemination of their findings. A few scientists capitalise on the large media interest in the subject, publishing dubious or meritless findings with large press conferences, frequently with little or no peer review - in other words, science by press conference.

References and External Links

BBC (April 23, 1999). Doubt cast on 'gay gene'. BBC News.

William Byne (May 1994). The Biological Evidence Challenged. Scientific American, vol. 270, pp. 50-55.

William Byne et al. (2001). The Interstitial Nuclei of the Human Anterior Hypothalamus: An Investigation of Variation with Sex, Sexual Orientation, and HIV Status. Hormones and Behavior, vol. 40, pp. 86-92.

Simon LeVay (1991). A Difference in Hypothalmic Structure Between Heterosexual and Homosexual Men. Science, vol. 253, pp. 1034-1037.

Simon LeVay & Dean H. Hamer (May 1994). Evidence for a Biological Influence in Male Homosexuality. Scientific American, vol. 270, pp. 44-49.

Trisha Macnair (undated). Genetics and human behaviour. BBC Health.

Timothy F. Murphy (Fall 2000). Now What? The Latest Theory of Homosexuality. APA Newsletter on Philosophy and Lesbian, Gay, Bisexual and Transgender Issues.

Nuffield Council on Bioethics (2002). Genetics and human behaviour. London: Author. Chapter 10 discusses sexual orientation.

T. J. Taylor (1992). Twin Studies of Homosexuality. Part II Experimental Psychology Dissertation (unpublished), University of Cambridge, UK.

Rosemary C. Veniegas & Terri D. Conley (2000). Biological Research on Women's Sexual Orientations: Evaluating the Scientific Evidence. Journal of Social Issues, vol. 56, pp. 267-282.

Neil Whitehead & Briar Whitehead (updated March 6, 2002). Twin Studies. Online at http://www.mygenes.co.nz/twin.html.