Studies of the causes of homosexual orientation
More detailed coverage of 3
birth order, existing families, & genes.
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
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
- 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
- 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
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.
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.
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:
- 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
- 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
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.
The following information sources were used to prepare and update the above
essay. The hyperlinks are not necessarily still active today.
- 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.
- R. Blanchard, "Fraternal birth order and the maternal immune hypothesis
of male homosexuality," Hormones & Behavior, 2001, 40, Pages 105 to 114.
- R. Blanchard & L. Elllis, "Birth weight, sexual orientation, and the sex
of preceding siblings," Journal of Biosocial Science, 2001, 33: Pages 451 to
- 60 Minutes episode, 2006-MAR-12.
- Dean Hamer & Peter Copeland "The Science of Desire: The Search for the
Gay Gene & the Biology of Behavior", Simon & Schuster (1994)
- 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.
- Mike Haley, "Straight Answers: Exposing the myths and facts about
homosexuality," Focus on the Family, Love Won Out series, (2000), Page 9.
- Kerby Anderson, "Homosexual Theology," at: http://www.probe.org/docs/homotheo.html
Copyright © 1997 to 2013 by Ontario Consultants on Religious Tolerance
Latest update: 2013-JAN-21
Author: B.A. Robinson