SEVEN MORE STUDIES INTO THE  CAUSES OF HOMOSEXUALITY

Do a person's genes determine if they will become homosexual?

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This is a continuation of another essay which describes six studies into the causes of homosexuality.

Study of eye blinking inhibition:

In 2003-OCT-2, researchers at University of East London and at King's College announced the results of a study which differentiates between homosexuals and heterosexuals. They studied groups of homosexual and heterosexual men and women and found significant differences in an involuntary response to being startled with bursts of loud noises. This is formally called "prepulse inhibition" or (PPI). Subjects were exposed to a low level noise, followed by a strong noise. Researchers measured the strength of the involuntary eye-blink responses. These data were then compared to similar strength measurements taken after exposure to a loud noise without the preceding low level noise. The lower the response, the stronger the level of inhibition. "The reaction of the lesbian test subjects was closer to that which would be expected among straight men. And, gay men reacted closer that of women, although to a lesser extent."

They found that the average PPI was:

Startle responses is known to be an involuntary response rather than learned reaction. It controlled by the limbic system, a region of the brain that also controls sexuality. One of the researchers, Qazi Rahman, said: "The startle response is pre-conscious and cannot be learned...This is very strong evidence that sexual orientation may be 'hard-wired' in this region" of the brain. The researchers claimed that "this study offers the first independent evidence of a non-learned neurological basis for sexual orientation." Dr. Rahman said: "These findings may well affect the way we as a society deal with sexuality and the issues surrounding sexual orientation." _{1 to 4}

NIH study of multiple genetic origins to male homosexuality:

The Hamer study, described above, showed that the Xq28 region of the X chromosome can cause a propensity towards homosexuality. Brian Mustakski of the University of Illinois in Chicago and colleagues at the National Institutes of Health searched for additional genes among other chromosomes which might be also involved in causing male homosexuality.

Two articles analyzed his paper in Human Genetics journal and reached opposite conclusions:

Both articles are correct in what they report. However, they interpret the results to confirm their pre-existing beliefs about the cause of homosexuality

Mustaksi remarked: "It’s the largest molecular genetic study ever done on sexual orientation." He said: "There is no one 'gay' gene. Sexual orientation is a complex trait, so it's not surprising that we found several DNA regions involved in its expression." They found three such locations. They studied the DNA of persons from 146 families in which there was at least two gay brothers. They included brothers from the 40 families use in the Hamer study of 1993, as well as from 106 new families.

Brothers share about 50% of their genes. The study searched for markers -- short strips of DNA that commonly correspond to specific parts of a chromosome. If all of the brothers in the study shared any specific marker significantly more than 50% of the time, then the researchers would have found a genetic link to homosexual orientation.

They detected three chromosomal regions on the human genome more than 50 percent of the time; they were on 7, 8, and 10. Of these, a region 7q36 on chromosome 7 was shared most frequently. Since males "inherited this region from their fathers just as often as their mothers, a finding that suggests genes from both parents can contribute to a son’s sexual orientation." ₅

As in previous studies, they were unable to confirm the results of Hamer's discovery of the Xq28 region of the X chromosome as a cause of homosexuality among their 146 test subjects. However, when they retested the 40 pairs that Hamer had used, they found that Xq28 could contain genes linked to sexual orientation. Mustanski refers to this as "locus heterogeneity" –- a situation in which one gene might influence sexual orientation in one family, but not in another family. Recall that Hamer's study selected only gays who had a surplus of gays in their mother's side of the family. Mustanski said: "The fact that we found suggestive evidence of different areas where there might be genes for sexual orientation builds upon the research that’s been done on family studies, twin studies, and previous molecular genetic studies which are consistently showing evidence of genetic influences."

The chromosome 7 finding may shed new life on a Dutch study. A gene near 7q36 is known to be responsible for the creation of a brain center called the suprachiasmatic nucleus. A Dutch research team in 1990 found that men with a homosexual orientation have a larger suprachiasmatic nucleus than heterosexual men. Mustanski said: "Perhaps this gene results in different brain structure and that brain structure causes someone to be heterosexual or homosexual."

Richard Pillard, a psychiatrist at Boston University specializing in  sexual orientation suggests that Mustanki's study should be repeated with a larger number of genetic markers. He said that this study: "Doesn’t come up with a definitive answer. The first step is to say, 'yes there’s a genetic contribution.' The next step is to say, 'well how are these genes expressed in the brain, as we presume they must be'?"

Mustanski concluded that: "There’s a converging line of evidence between the hormonal studies, the genetic studies, and the neuroanatomical studies. My research has identified candidate genes within these new chromosomal regions that could link together all of these different findings." ₅

Warren Thockmorton and Ray Durwood are professors at the Grove City College -- a conservative Christian college which rejects "relativism and secularism." ₇ They reviewed the Mustanki study and said: "The authors describe in the article three non-X chromosomal 'new regions of genetic interest' (7q36, 8p12, and 10q26). In the authors’ view, a noteworthy aspect of the study as follows: 'Our strongest finding was on 7q36 with a combined mlod score of 3.45 and equal distribution from maternal and paternal allele transmission. This score falls just short of Lander and Kruglyak's (1995) criteria for genomewide significance.' They go on to say 'two additional regions (8p12 and 10q26) approached the criteria for suggestive linkage' - again pointing out that neither was statistically significant." ₆

If one assumes that the propensity towards homosexual orientation is determined by an interaction of many genes, then no region on any one chromosome can be expected to be conclusively proven. The finding that the chromosome 7 was very close to significant probably shows that it must be significant in some male homosexuals.

It is worth noting that most studies into the causes of homosexuality seem to show that about 10% of individuals are set up to be potentially homosexual by some gene or combination of genes. However, only about 55% of them are triggered by an unknown environmental factor during early childhood so that they will develop as homosexuals after puberty. The Mustanski study shows that, if this theory is correct, some combination of multiple genes is involved, rather than a single gene.

LaVay study of brain structure:

Simon LeVay, a Neuroanatomist at the Salk Institute in California published a study in 1991 which examined the brains of men, many of whom had died of AIDS. He found that the INAH 3 (a structure within the hypothalamus) differed in size between heterosexual and homosexual men. This suggested to the researcher that "sexual orientation has a biological substrate". ₈ This study has been criticized on a number of grounds. Many of his subjects died of AIDS; perhaps the difference in structure size was caused by the disease, or the medications that they took, not the sexual orientation. Others have suggested that homosexual or heterosexual activity may have determined the size of the structure, rather than vice-versa.

It would seem that the study proves nothing. However, it appears to be frequently cited in books and on the Internet.

Hearing sensitivity:

A group of researchers at the University of Texas found structural differences in the inner ears between lesbians and heterosexual women. On average, women have more sensitive cochlea amplifiers than men; they are able to detect softer sounds in a very quiet room. The researchers found that lesbians had inner ear characteristics that were more like those of men. The structure of the inner ear forms before birth and is affected during pregnancy by androgens. ₉ These findings indicate that sexual orientation is at least partly decided before birth -- perhaps at conception.

Ear emissions:

Everyone's inner ears produce very weak sounds called "spontaneous otoacoustic emissions." These cannot normally be heard by other persons, but can be detected by sensitive instruments. The Washington Post reported that:

Dennis McFadden and Edward G. Pasanen of the University of Texas in Austin and colleagues compared the emissions of 60 homosexual and bisexual women with those of 57 heterosexual women. As a group, the homosexual and bisexual women's emissions were slightly more like that of men: less frequent and weaker than those of the heterosexual women, the researchers found.

One explanation could be that homosexual and bisexual women were exposed to slightly different levels of hormones when they were developing in the womb, causing subtle changes in their development, the researchers say.

"For us, the most plausible explanation is that the inner ears of the non-heterosexual women were partially masculinized at some time in development, possibly at the same time that whatever brain structures are responsible for sexual orientation were also masculinized," says McFadden, whose study is featured in the April [1999] issue of the Journal of the Acoustical Society of America. ₁₀

Response to pheromones by gay men:

Researchers of the Karolinska Institute in Stockholm, Sweden. divided 36 subjects into three groups of 12 each: heterosexual men, heterosexual women and homosexual men. All were healthy, unmedicated, right handed, and HIV negative, of similar age and of comparable educational level. Each subject sniffed a series of odors, while PET scans were taken of their brain activity.

Ivanka Savic, lead author and associate professor of neurology at the Institute said: "The experimental conditions were standardized and identical in all subjects. When adding to the improbability of chance activation by [testosterone] in homosexual men in the brain area very similar to heterosexual women, it seems convincing that we detected an undistorted physiological response."

Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada, who was not part of the Swedish research team, commented: "It is one more piece of evidence ... that is showing that sexual orientation is not all learned." She said the findings clearly show a biological involvement in sexual orientation.

The study was published in the Proceedings of the National Academy of Science on 2005-MAY-10. _11,12

Response to pheromones by lesbians:

A research team led by Ivanka Savic at the Karolinska Institute repeated the above study on lesbian subjects. They found that lesbians' brains react differently to sex hormones than those of heterosexual women.

Their study was published in the Proceedings of the National Academy of Sciences on 2006-MAY-09. According to Associated Press: Both this study and the previous one on gay males "...add weight to the idea that homosexuality has a physical basis and is not learned behavior." Sandra Witelson, a specialist on brain anatomy and sexual orientation at McMaster University in Hamilton, ON, Canada, who was not part of the research effort, said "The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation." ₁₃

Response to the smell of sweat:

Neuroscientist Charles Wysocki led a team at the Monell Chemical Senses Center in Philadelphia, PA. They studied the response of 82 heterosexual men, gay men, heterosexual women and lesbians to the odor of underarm sweat. The samples were collected from 24 donors who varied in sex and sexual orientation. They found that:

This study has two conclusions:

Wysocki said: "Our findings support the contention that gender preference has a biological component that is reflected in both the production of different body odors and in the perception of and response to body odors."

The Associated Press reported that: "In particular,....[Wysocki] said, finding differences in body odors between gay and straight individuals indicates a physical difference. It's hard to see how a simple choice to be gay or lesbian would influence the production of body odor, he said." ₁₁

Related essay:

References:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

1. "Sexual orientation 'hard-wired' before birth - startling new evidence revealed in the blink of an eye," University of East London, 2003-OCT-2, at: http://www.uel.ac.uk/
2. Peter Moore, "Gay in the blink of an eye," RainbowNetwork.com, undated, at: http://www.rainbownetwork.com/
3. Patrick Letellier, "Study hints sexual, gender identity in genes," PlanetOut, 2003-OCT-22, at: http://www.planetout.com/
4. "Sexual Orientation Related Differences in Prepulse Inhibition of the Human Startle Response" Behavioral Neuroscience magazine, 2003-OCT, published by the American Psychological Association.
5. Ariel Whitworth, "Studies Suggest Multiple Genes Contribute to Sexual Orientation," Gay Life, 2005-FEB-18, at: http://www.bgp.org/
6. "New Genetics Study Undermines Gay Gene Theory," LifeSite, 2005-FEB-10, at: http://www.lifesite.net/
7. "The Mission of Grove City College," at: http://www.gcc.edu/
8. Simon LeVay, "The Sexual Brain," MIT Press, (1994). Read reviews or order this book safely from Amazon.com online book store
9. "Lesbian Ears", The Globe and Mail, Toronto ON, 1998-MAR-19.
10. "Sexuality and Ear Emissions," Science Notebook, Washington Post, 1999-APR-05, Page A07. Online at: http://equake.geol.vt.edu/
11. "Gay Men Respond Differently to Pheromones," Associated Press, 2005-MAY-09, at: http://start.earthlink.net/
12. Oliver Moore, "Response to scent is linked to sexual tendency. Researchers study brains of gay men and straight women," The Globe and Mail, 2005-MAY-10, Page A1. Online at: http://www.theglobeandmail.com/ This is a temporary listing.
13. Randolphe Schmid, "Sexuality may be in the genes - Study. Lesbians react similarly to men. Researchers test brain differences," Associated Press, 2006-MAY-09.