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Katie Miller
Biology 303 Honors
Dr. Bert Ely
November 1, 2012
The Balancing Selection Hypothesis and Explaining the Survival of Homosexual Genes
For many years both science and society alike have grappled with the existence of
homosexuality. What started as something many in Western society shunned and the medical
community classified as a mental disorder, acceptance of homosexuality in humans is now
something very commonplace and, in most places no longer considered a crime or an illness.
However, we as a society are still trying to figure out the origins of this variance in sexual
orientation. Most agree that homosexuality is something an individual is born with and is not a
choice. But, the influence of genes on sexual orientation is something that has not yet been
explained. A curious question arises as explained by Blanchard (2012):
“How can genetic variants that predispose to homosexuality remain at a (seemingly)
constant rate in the population, when the men who carry them are so much less likely
to produce offspring?”
An almost perfectly precise answer to this question is the Balancing Selection Hypothesis. As
defined in Blanchard’s 2012 paper by Schwartz et al. (2010), the Balancing Selection Hypothesis
is explained by saying “the decreased fertility of men with the heritable form of homosexuality is
offset by an increased fertility of biological relatives who carry the same genetic variants.” The
Balancing Selection Hypothesis is a possible answer as to why these genes predisposing
individuals to homosexuality are not decreasing from the population. Blanchard’s 2012 research
used six previous studies’ data to correlate evidence for the Balancing Selection Hypothesis and
built on the research of Camperio-Ciani et al. (2004) and Iemmola and Camperio-Ciani (2009)
that first outlined a correlation between homosexuality and the birth rate in the relatives of
homosexual men.
Blanchard formulated his hypothesis with the help of two papers, one being CamperioCiani et al. (2004) which documented an increase in fecundity (fertility) from the female
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maternal members of the homosexual subjects’ families. The same increase, however, was not
found in the heterosexual subjects’ relatives. In Camperio-Ciani et al. (2004), 98 homosexual and
100 heterosexual men from northern Italy were studied. Each filled out a survey providing their
family size, sexual orientation of male relatives and birth order and gender of siblings. The data
showed that the homosexual men’s maternal side of the family had increased fertility and also
these subjects had more older siblings, in particular more older brothers, than their heterosexual
counterparts. The paternal side of the family did not have an increase from either group. Later
the researchers discussed how this increase in maternal fecundity of homosexual men and no
change in paternal fecundity of heterosexual men showed a correlation that would lead to a
hypothesis suggesting that these genes are linked to the X chromosome (Camperio-Ciani et al.
2004).
The second paper Blanchard (2012) used as the basis of his research was Iemmola &
Camperio-Ciani (2009). In this paper, 250 probands (test subjects) were sampled with 152 being
homosexual. These men were mostly picked from tourist industry jobs, so they likely came from
all over Italy despite the sampling haven taken place in Northern Italy, similar to Camperio-Ciani
et al. (2004). Again the men filled about a survey about their family size and demographics. The
researchers also asked about how many children each group of probands had. The homosexuals
had about one fifth of the children the heterosexuals had, showing the natural decline in passing
on the genes. Once again the data showed that homosexuals had a higher maternal fecundity rate
than the heterosexuals and had a greater difference than with the previous study, which yet again
showed a possible X chromosome link to these genes. However, this study also showed a higher
paternal fecundity rate. The researchers admitted that there was a bit of sampling bias in how
they recruited probands (since they went to bars, resorts, clubs, gyms, etc. to recruit homosexuals
and to the same types of places to recruit heterosexuals, it probably resulted in an oversampling
of non-married heterosexuals) and they stress the importance of repeating the experiment with a
larger sample to eliminate biases and to try to repeat the results (Iemmola & Camperio-Ciani
2009).
The main paper that tested the Balancing Selection Hypothesis is Blanchard 2012.
Blanchard built onto the ideas presented by both Camperio-Ciani et al. (2004) and Iemmola &
Camperio-Ciani (2009) and took it one step further. Instead of testing the probands’ entire family
size, Blanchard (2012) instead focused on the number of siblings each subject had. While just
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testing sibling numbers, the Balancing Selection Hypothesis still applies but this time focused on
the mothers of the probands (Blanchard 2012).
Also, Blanchard (2012) only used first-born homosexual and heterosexual probands to
eliminate any bias from the Fraternal Birth Order effect (FBO effect). Blanchard (2012) cites the
Zietsch et al. 2008 definition of FBO as an effect “… where males with a greater number of
older brothers and sisters are more likely to be homosexual …” The direct mechanics of why the
FBO has an influence with homosexuality is not elaborated in these papers, but both previous
papers, Camperio-Ciani et al. 2004 and Iemmola & Camperio-Ciani 2009, show a trend where
homosexuals who have more siblings tend to have many more older brothers and sisters than
younger (Camperio-Ciani et al. 2004 and Iemmola & Camperio-Ciani 2009). So to eliminate any
bias from the FBO effect, Blanchard (2012) only used first-borns because the Balancing
Selection Hypothesis should still have applied (Blanchard 2012).
In his study, Blanchard (2012) used data from six previous papers(he participated in all
six). These papers all have slight differences among how their data was presented but otherwise
have almost the same main idea:
1) Blanchard and Bogaert 1996a (first-borns 18 and older sampled to prevent incomplete
sibships)
2) Blanchard and Bogaert 1996b (used a full heterosexual group instead of subdividing to match
each homosexual group)
3) Blanchard and Lippa 2007 (combined bisexual and homosexual probands into one group
instead of keeping them separate)
4) Blanchard and Zucker 1994 (used this studies data; in the original study the family size mean
was calculated including the probands in the family size, but it was corrected for use in
Blanchard 2012)
5) Blanchard et al. 2006 (eliminated one group sampling because it did not include data on
younger siblings. This study only uses 2,764 of the 3,146 probands of the original study)
6) Blanchard et al. 1998 (this study did not compare number of siblings of the homosexuals and
heterosexuals against each other; this error was corrected for Blanchard 2012)
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Blanchard’s results were quite surprising and unexpected. Having only tested first-born
homosexual and heterosexual probands, the results showed either that the heterosexuals had
more siblings than the homosexuals or that the data was not statistically significant. Also, the
mothers of both groups of probands had statistically more children than average (about 2.1 births
for industrialized countries) which showed that the probands sampled have some sort of increase
in maternal fecundity rate yet the heterosexuals still had more siblings than the homosexuals.
These results were in direct contrast to the Balancing Selection Hypothesis (Blanchard 2012).
As Blanchard (2012) quoted himself in the discussion section, “What can explain the
discrepancy?” Blanchard threw around a number of ideas including, but not limited to, the FBO
effect, sampling bias, too small of a sample and not taking into account birth weight of the
probands to test for discrepancies in the womb. He also said this study would need to be repeated
so as to replicate the results. He believed that replication would be easy considering that there are
already lots of data archived on this subject that could be examined and compiled into another
study. Blanchard supported more research on the subject because until this paper was published,
the two previous papers, Camperio-Ciani et al. 2004 and Iemmola & Camperio-Ciani et a. 2009,
confirmed the Balancing Selection Hypothesis (Blanchard 2012).
Whether there was an error in Blanchard’s 2012 paper or not, the Balancing Selection
Hypothesis is one of the most comprehensive and plausible hypotheses proposed in recent years
to explain how a genetic aspect related to homosexuality in humans could survive in the
population. Researchers should still consider it a valid direction of thinking until more definitive
answers come to light, especially with data both confirming and denying this hypothesis.
Literature Cited
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1. Blanchard, R. 2012. Fertility in the Mothers of Firstborn Homosexual and Heterosexual Men.
Archives of Sexual Behavior 41:551–556.
2. Schwartz, G., Kim, R. M., Kolundzija, A. B., Rieger, G., & Sanders, A. R. (2010).
Biodemographic and physical correlates of sexual orientation in men. Archives of Sexual
Behavior, 39, 93–109.
3. Camperio-Ciani, A., Corna, F., & Capiluppi, C. (2004). Evidence for maternally inherited
factors favouring male homosexuality and promoting female fecundity. Proceedings of
the Royal Society of London B, 271, 2217–2221.
4. Iemmola, F., & Camperio-Ciani, A. (2009). New evidence of genetic factors influencing
sexual orientation in men: Female fecundity increase in the maternal line. Archives of
Sexual Behavior, 38, 393–399.
5. Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J. H., Keller, M. C., Macgregor, S., et
al. (2008). Genetic factors predisposing to homosexuality may increase mating success in
heterosexuals. Evolution and Human Behavior, 29, 424–433.