SIX STUDIES INTO THE  CAUSES OF HOMOSEXUALITY

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

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Conflicting quotes:

Overview:

Religiously conservative information sources generally reject the belief that homosexuality is caused or set up by genes. Information sources sponsored by gays, lesbians, religious liberals, mental health professionals and human sexuality researchers generally stress the genetic cause of sexual orientation.

We have described a number of studies which show that the propensity towards homosexuality is determined either:

We are always watching for new studies into this phenomenon. We have found none that contradict the above conclusion.

A number of studies on identical twins have indicated that perhaps 10% of people are born with the genetic predisposition to develop a homosexual orientation in adulthood. However, this predisposition may or may not be triggered, either before birth or during early childhood by a factor in the environment which is currently unknown. Thus, having a homosexual orientation is not completely determined by one's genes. However, current evidence is that it is certainly outside of a person's conscious control.

Religious conservatives generally reject these studies. They deny that people are set up by their genes to develop a homosexual orientation. They generally believe that gays and lesbians choose their orientation. One psychologist, Joseph Nicolosi, who is the president of the National Association of Research and Therapy of Homosexuality (NARTH) teaches that:

This theme has been picked up by the Fundamentalist Christian group, Focus on the Family. Focus holds "Love won out" conferences across North America which teach their beliefs that "homosexuality is preventable and treatable." These beliefs are found in many religiously conservative information sources. ₆

Prejudice against gays and lesbians: 

Many people believe that prejudice against gays and lesbians will melt away if the public were to accept the belief that a person's sexual orientation is mainly determined by genes. The rationale is that if a person is a gay or lesbian because of their genetic makeup, then their orientation is not a choice; it is something beyond of their control. This belief appears to be contradicted by a 1999-APR web-site poll conducted by Time.com. It dealt with "Genetics and Homosexuality," and asked the question: "Would your attitude towards homosexuality change if it were proven to be genetically determined?" Response as of 2000-MAR-6 was: 75% no, 21% yes and 4% not sure.

Also consider that:

Study based on fingertip ridges:

J.A.Y. Hall and D. Kimura at the University of Western Ontario at London ON Canada found a relationship between the number of fingertip ridges on men and their sexual orientation. ₇ They compared the number of ridges on the index finger and thumb of the left hand with the number on the corresponding fingers of the right hand. They found that 30% of the homosexuals tested had a surplus of ridges on their left hand, whereas only 14% of the heterosexuals did. This is a particularly interesting finding, because fingerprints are fully determined in a fetus before the 17th week of pregnancy, and do not change thereafter, through birth, infancy, childhood, youth and adulthood. This would seem to prove that for at least some adult homosexuals, their sexual orientation was pre-determined before birth, perhaps at conception; certainly by the end of the 4th month of pregnancy. _8,9

Study based on finger-lengths:

A California psychologist, Marc Breedlove, "conducted his research at three street fairs in the San Francisco Bay area in the fall of 1999." Each study participant had his or her hand photocopied on a portable copy machine to record finger length. Participants also filled out a questionnaire on sexual orientation and birth order. 720 volunteers participated. They found that lesbians tended to have shorter index fingers (relative to their ring fingers) than did heterosexual women. They also found that gay males tended to have shorter index fingers (relative to their ring fingers) than heterosexual males. The relative size of a person's fingers is determined well before birth. These findings imply that sexual orientation is at least partly decided before birth -- perhaps at conception when a person's unique DNA is established. ₁₀

Studies based on fraternal birth order:

In the mid 1990s, researcher Ray Blanchard studied families in which there is a male child with a homosexual orientation. He found that a gay man is more likely to have older brothers than older sisters. He found that the probability that a male child will grow up as a homosexual increases by about 33% for each brother born before he was. Blanchard suggests that this effect may be caused by an immune response within the mother during pregnancy. _11,12,13 According to the program 60 minutes, recent studies have shown that this effect only happens among right-handed brothers. ₂₈

Studies based on existing families:

Psychologist Michael Bailey of Northwestern University and Psychiatrist Richard Pillard of Boston University studied the sexual orientation of male siblings raised together since birth. ₉ He found that if one was homosexual then the chance of their sibling being homosexual was:

This study also points to a very strong genetic factor at the time of conception. This type of study tends to have the same flaws as in the identical twin studies described below. The second twin might not be willing to admit to being homosexual. One sibling could be bisexual and identify themselves to the interviewer as a homosexual; the other could also be bisexual and be behaving as a homosexual.

J. M. Bailey, R. C. Pillard and others conducted a similar study of female identical twins raised in the same family, in which one twin identified herself as a lesbian. ₁₄ The results showed that 48% (34 out of 71) of their twins also said that she is homosexual. Again, a very strong genetic component is indicated.

Direct study of genes:

Dean Hamer, and his colleagues at the National Cancer Institute collected family data from the families of 114 gay men. ₁₅ They found out which, if any, of their siblings, parents, cousins, uncles etc. were also gay. They quickly determined that homosexuality runs in families. By itself, these results show nothing. They might indicate that homosexuality is genetically caused, or might show that it is caused by upbringing, by some hormonal levels in the womb or some other environmental factor that appears in some family lines more than other.

Further studies showed that "There were increased rates of gay people among family members genetically related to each other even when raised apart in different households." This reinforced the possibility that genetic factors might cause people to become gay. However, these observations were not conclusive; they were merely suggestive.

What proved the genetic nature of "gayness" was a pedigree test. This type of study examines the sexual orientation of the ancestors of many gay adults - both on their father's and mother's side. Some possible results from a pedigree study on Hamer's sample of gay adults would be:

The researchers found that the second result was observed. A gay male from the population that Hamer studied would notice that more of his mother's brothers will be gay than his father's brothers; so too with the various classes of maternal cousins when compared to his paternal cousins. Thus, much male homosexuality is caused by a gene on the X chromosome. Hamer went on to find the approximate location on the chromosome where the gay-causing gene was located. He found that many of his subjects had an identical sequence on the Xq28 region of their X chromosome. This shows the approximate location of the "gay gene." Researchers speculated that a group of interacting genes (including one in this region) might be found to determine sexual orientation in males. This prediction came to pass. The statistical "p" value is a measure of the significance of a test: the probability that it could have happened by chance. P values less than 0.01 (1%) are considered very significant. The Hamer study had a P factor of 0.00001, and so is considered extremely reliable.

The DNA of 36 pairs of lesbian sisters were also studied; no corresponding pattern has yet been found.

The Hamer group's results on gay males must be regarded as tentative:

Studies on monozygotic twins:

These are twins that resulted from the splitting of a single fertilized egg -- the zygote -- into two separate zygotes with identical genetic structure. It can happen at any time prior to about 14 days after conception. They are commonly called "identical twins." Studying these twins, researchers can determine whether a trait such as homosexuality is determined by the environment or by genetics or by a combination of both. The technique involves the study of pairs of identical twins who were separated at birth and raised in different families without later contact. Being identical twins, they would have the same genetic structure. But being raised independently in different families (often in different states), they would mature (at least after birth) and experience totally unrelated environments, family types, family sizes, parenting methods, level of abuse, etc. Fortunately, a U.S. university maintains an index of twins who were raised separately since birth.

Schizophrenia occurs in about 1% of the adult population. In previous decades, the disorder was believed to be caused by incompetent parenting. Studies of identical twins overturned this belief. They showed that if one twin was schizophrenic, the chances of the other twin having the disorder is 65%. This shows that schizophrenia has a very strong genetic component. ₁₇ Decades ago, autism was also blamed on the parents. Identical twin studies turned up similar results: if one twin was autistic, there was a 68% chance that the other was also autistic. ₁₇ Similarly the cause of homosexuality has been attributed to lack of bonding between a child and the same-sex parent. This belief has been abandoned by almost all mental health professionals. However, many religious conservatives still promote this principle. Studies of identical twins have shown that if one twin is gay, the other has about a 55% of also being gay. Again, there is a very strong genetic component at work.

Assume as a working assumption that 5% of adults are gay. A typical study would involve:

Two possibilities are:

This type of test has been performed by various groups of researchers. The first such study found that 100% of the second twins were also gay. But this study was based on a very small sample size; the results it turned out to be a statistical fluke. Subsequent, larger, tests all reported that somewhat in excess of 50% of the second twins are also gay. This indicates that genes play a very significant role in determining sexual orientation. however, the number is not 100%. This indicates that environment plays a role in determining sexual orientation. It might be an event in the womb, or in early childhood. Other studies, explained below, show that a person's eventual sexual orientation is determined before they reach school age.

One problem with this and similar studies is that the researchers can ask what a person's orientation is, but not necessarily get a valid answer; a gay individual may be so firmly "in the closet" that they will not admit even to a stranger what their actual orientation is. Also, most studies classify subjects as either heterosexual or homosexual. A bisexual person might be involved with persons of the opposite gender and consider themselves to be heterosexual; their twin may also be bisexual but be involved with members of the same gender, and identify themselves as homosexual. Another problem is that identical twins share the same environment (their mother's womb) for the 9 months prior to birth. Thus, if there is a environmental factor which determines sexual orientation, it might work on the fetus before birth. This series of replicated studies proves beyond any doubt that at least male homosexuality is largely determined before birth. As the American Psychological Association's statement on homosexuality in 1994-JUL asserts: "Research suggests that the homosexual orientation is in place very early in the life cycle, possibly even before birth..."

This essay continues elsewhere with the results of seven more studies

Related essay:

References:

1. Matt Ridley, "Nature via nurture: Genes, experience, and what makes us human," HarperCollins, (2003). Page 159. Read reviews or order this book safely from Amazon.com online book store.
2. "Statement on Homosexuality," Parents Rights Coalition, 2000-MAY-21, at: http://www.parentsrightscoalition.org/
3. Kevin Schattenkirk, "Being gay - A choice? Debunking some popular views on homosexuality," The Daily University of Washington, 1999, at: http://archives.thedaily.washington.edu/ (No longer available online)
4. "Toward a new national discussion of homosexuality," Focus on the Family, 1998-SEP-15, at: http://www.family.org/cforum/
5. Joseph and Linda Nicolosi, "A parent's guide to preventing homosexuality," InterVarsity Press, (2002). Read reviews or order this book safely from Amazon.com online book store. This book has been given a poor rating (2 stars out of 5) by Amazon.com reviewers.
6. "Love won out," Focus on the Family," at: http://www.family.org/cforum/lwo/
7. Caleb Crain, "Did a germ make you gay," Out magazine, 1999-AUG, Pages 46 to 49.
8. Anne Moir and David Jessel, "Brainsex", Dell Books (1993)
9. Dean Hamer & Peter Copeland "The Science of Desire: The Search for the Gay Gene & the Biology of Behavior", Simon & Schuster (1994)
10. "Male hormone levels in womb may affect sexual orientation, study says," CNN.com Health, 2000-MAR-29, at: http://www.cnn.com/2000/HEALTH/03/29/
11. Op cit, Matt Ridley, Page 161.
12. R. Blanchard, "Fraternal birth order and the maternal immune hypothesis of male homosexuality," Hormones & Behavior, 2001, 40, Pages 105 to 114.
13. R. Blanchard & L. Elllis, "Birth weight, sexual orientation, and the sex of preceding siblings," Journal of Biosocial Science, 2001, 33: Pages 451 to 467.
14. Dean Hamer et al, "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation" Science 261 (1993-JUL-16): pp 321-27.
15. Mike Haley, "Straight Answers: Exposing the myths and facts about homosexuality," Focus on the Family, Love Won Out series, (2000), Page 9.
16. Kerby Anderson, "Homosexual Theology," at: http://www.probe.org/docs/homotheo.html 
17. Op cit, Matt Ridley, Pages 103 &104
18. Mike Haley, "Straight Answers: Exposing the myths and facts about homosexuality," Focus on the Family, Love Won Out series, (2000), Page 9.
19. Kerby Anderson, "Homosexual Theology," at: http://www.probe.org/docs/homotheo.html 
20. Daniel R. Heimbach,  "Notes for The Bible in the Moral War over the Rejection of Homosexuality by the Military Services: A View from Inside the Pentagon," PREMISE, Volume II, Number 7, 1995-AUG-27, Page 10. Online at:  http://capo.org/premise/95/august/
21. Sue Bohlin, "A Christian perspective on homosexuality," at: http://www.northave.org/MGManual/
22. N. E. Whitehead, "The Importance of Twin Studies," NARTH, at: http://www.narth.com/docs/whitehead.html
23. "Family News from Dr. James Dobson," June 2002.
24. Rich Nathan, "Who is my enemy? Welcoming people the church rejects," Zondervan, (2002) Page 185. Read reviews or order this book safely from Amazon.com online book store
25. "Brothers and Twins," 1997-OCT, at: http://www.geocities.com/
26. Sylvia, "The Biolobical Basis of Homosexuality," at" http://www.geocities.com/southbeach/
27. Brian Student, "Biological Factors," at: http://www.godsrainbow.org/
28. 60 Minutes episode, 2006-MAR-12.