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Studies of the causes of homosexual orientation

More detailed coverage of 3 studies based
on birth order, existing families, & genes.

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This topic is continued from an earlier essay

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Study 3: 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. 1,2,3 According to the program 60 Minutes, recent studies have shown that this effect only happens among right-handed brothers. 4

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Study 4: 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. 5 He found that if one was homosexual then the chance of their sibling being homosexual was:

  • Fifty two percent for their identical twin, who shared 100% of the same genes. This result was essentially identical to the separated identical twin studies described below. It shows that if one identical male twin is gay, then the other twin will probably also be gay -- whether raised in the same family or raised by different families.

  • Twenty-two percent for their non-identical twin, with whom they share half of their genes.

  • About 10% for adopted or non-twin brothers with whom they share none to half of the their genes.

This study also points to a very strong genetic factor at the time of conception. This type of study tends to have the same potential for inaccuracy as in the identical twin studies described below. The second twin might not be willing to admit to being homosexual. Alternately, 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. 6 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.

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Study 5: Based on direct study of genes:

Dean Hamer, and his colleagues at the National Cancer Institute collected family data from the families of 114 gay men. 7 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 about the cause of homosexuality. They might indicate that homosexuality is genetically caused, or might show that it is caused by upbringing, or 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:

  1. An approximately equal number of gays might be found on the mothers' side of the family, when compared to the fathers' side. Some gays may have many gay ancestors on their mother's side, whereas other gays may see the same effect on their father's side. But when all the results were lumped together, if the total numbers would be about equal, then the results could point to:
    • "Gayness" being caused by environmental factors, or

    • "Gayness" being caused by some gene on a chromosome other than the X chromosome.

  2. A much larger number of gays might be found on the mother's side of the family. This would show conclusively that not only was the gene passed genetically, but that it is located somewhere on the X chromosome - since men always get their X chromosome only from their mother. This is called the "maternal effect." It is well known in genetics.

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 was found.

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

  • Dr. Alan Sanders, professor of psychiatry at the University of Chicago attempted to replicate the results on 54 sets of gay brothers. He announced at the American Psychiatric Association's annual conference in 1998-JUN that he was unsuccessful. "No [genetic] marker data reached statistically significant criteria." The results of the pedigree test on Sander's subjects is not known.

  • George Rice's selected subjects from carefully selected families in which the pedigree test was the reverse of Hamer's: gays were found in the father's line, not the mother's line. As expected, the markers that Hamer found were not found by Rice. 8 One might conclude that:
  • Some gays have many gay ancestors in their mother's line but not in their fathers' line. This was the group from which Hamer drew his subjects. They share genes on their X chromosome that apparently can cause a gay orientation.

  • Other gays have the reverse pedigree pattern: they have many gay ancestors in their father's line but not their mother's. One would not expect them to have gay-causing genes on their X chromosome. Rice was unable to find any.

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More studies are described in the next essay

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References used:

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

  1. Matt Ridley, "Nature via nurture: Genes, experience, and what makes us human," HarperCollins, (2003). Page 161. Read reviews or order this book safely from Amazon.com online book store.
  2. R. Blanchard, "Fraternal birth order and the maternal immune hypothesis of male homosexuality," Hormones & Behavior, 2001, 40, Pages 105 to 114.
  3. R. Blanchard & L. Elllis, "Birth weight, sexual orientation, and the sex of preceding siblings," Journal of Biosocial Science, 2001, 33: Pages 451 to 467.
  4. 60 Minutes episode, 2006-MAR-12.
  5. Dean Hamer & Peter Copeland "The Science of Desire: The Search for the Gay Gene & the Biology of Behavior", Simon & Schuster (1994)
  6. 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.
  7. Mike Haley, "Straight Answers: Exposing the myths and facts about homosexuality," Focus on the Family, Love Won Out series, (2000), Page 9.
  8. Kerby Anderson, "Homosexual Theology," at: http://www.probe.org/docs/homotheo.html 
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Copyright © 1997 to 2013 by Ontario Consultants on Religious Tolerance
Latest update: 2013-JAN-21
Author: B.A. Robinson

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