Brain structure: Remove section. Studies are questionable, discredited, and/or contradict each other.
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Hundreds of species of mammals, birds, fish, reptiles, amphibians, insects, and other invertebrates have been observed engaging in sexual activity, courtship, affection, pair bonding, or parenting among same-sex animal pairs.
Hundreds of species of mammals, birds, fish, reptiles, amphibians, insects, and other invertebrates have been observed engaging in sexual activity, courtship, affection, pair bonding, or parenting among same-sex animal pairs.
The nature of the activity varies greatly between species, and the neuroscientist Simon LeVay cautioned that "Although homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity."{{ref|Simon LeVay, ''Queer Science: The Use and Abuse of Research into Homosexuality'', 1996, MIT Press, Cambridge, Massachusetts, p.207. Quoted in the Wikipedia article, [https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals Homosexual behavior in animals]}}
The nature of the activity varies greatly between species, and the neuroscientist Simon LeVay cautioned that "Although homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity."{{ref|Simon LeVay, ''Queer Science: The Use and Abuse of Research into Homosexuality'', 1996, MIT Press, Cambridge, Massachusetts, p.207. Quoted in the Wikipedia article, [https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals Homosexual behavior in animals]}}
−
Further, homosexual behaviour in animals is often not as a result of sexual motivation—ritualised aggression is one
reason
{{ref|1={{author|Edward O.|Wilson}}, [http://books.google.de/books?id=v7lV9tz8fXAC&pg=PA281&lpg=PA281#v=onepage&q&f=false ''Sociobiology, The New Synthesis''] Harvard College: Boston, 1975; p. 281.}}
—and is more common in captive animals than in the wild.
{{ref|
1=
{{author|
D.
|
Symons
}}
1979
.
The Evolution
of
Human Sexuality, Oxford University Press
,
New York, p. 60
.}}
+
Further, homosexual behaviour in animals is often not as a result of sexual motivation—ritualised aggression is one
reason—and is more common in captive animals than in the wild.
{{ref|1={{author|Edward O.|Wilson}}, [http://books.google.de/books?id=v7lV9tz8fXAC&pg=PA281&lpg=PA281#v=onepage&q&f=false ''Sociobiology, The New Synthesis''] Harvard College: Boston, 1975; p. 281.}}{{ref|{{author|
Jerry
|
Bergman
}}
, [http://creation
.
com/creationism-and-the-problem-
of
-homosexual-behaviour Creationism and the problem of homosexual behaviour]
,
''Journal of Creation'' 9(1):121–130 April 1995
.}}
* The only species (aside from humans) in which exclusive homosexual orientation has been reported is domesticated '''sheep''' (''Ovis aries'').{{ref|1=Aldo Poiani, [http://books.google.ca/books?id=EftT_1bsPOAC&pg=PA179#v=onepage&q&f=false ''Animal Homosexuality: A Biosocial Perspective''], Cambridge University Press, Cambridge, UK, 2010, pp.179-185.}} Again citing Simon LeVay, "About 10% of rams (males) refuse to mate with ewes (females) but do readily mate with other rams."{{ref|Simon LeVay, ''Gay, Straight, and The Reason Why: The Science of Sexual Orientation'', 2011, Oxford University Press, Cambridge, Massachusetts, pp.70-71. Quoted in the Wikipedia article, [https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals Homosexual behavior in animals]}}
* The only species (aside from humans) in which exclusive homosexual orientation has been reported is domesticated '''sheep''' (''Ovis aries'').{{ref|1=Aldo Poiani, [http://books.google.ca/books?id=EftT_1bsPOAC&pg=PA179#v=onepage&q&f=false ''Animal Homosexuality: A Biosocial Perspective''], Cambridge University Press, Cambridge, UK, 2010, pp.179-185.}} Again citing Simon LeVay, "About 10% of rams (males) refuse to mate with ewes (females) but do readily mate with other rams."{{ref|Simon LeVay, ''Gay, Straight, and The Reason Why: The Science of Sexual Orientation'', 2011, Oxford University Press, Cambridge, Massachusetts, pp.70-71. Quoted in the Wikipedia article, [https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals Homosexual behavior in animals]}}
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== Genetics ==
== Genetics ==
−
Twin studies suggest
a
moderate
genetic
influence on the expression of homosexuality
in
humans, explaining about
a
third
of
the variance
.
{{ref|N.Långström
,
"Genetic
and
environmental effects on same-sex sexual behavior: a population study of twins in Sweden"
, ''
Arch Sex Behav'
',
Vol
.
39, No
.
1
,
pp. 75–80. February
,
2010
.}}
+
The history of the search for a biological basis for homosexuality is peppered with bias and poor research claiming to have found
a genetic
basis.
−
Chromosome linkage studies
have implicated a marker on the X chromosome at Xq28
, although other genes must be involved as well
.{{ref|1=D.H.Hamer ''et al.'', [http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=8332896 "A linkage between DNA markers on the X chromosome and male sexual orientation"], ''Science'', Vol. 261, No. 5119, PP. 321-7. July, 1993.}}{{ref|
B
.
S
.
Mustanski
, [http://
mypage
.
iu
.edu/
~bmustans
/
Mustanski_etal_2005
.
pdf
"
A genomewide scan of
male
sexual orientation
"], ''
Hum
.
Genet
.'', Vol.
116
, No.
4
, pp.
272–8
.
March
,
2005
.}}
+
+
Homosexual Simon LeVay published a paper
in
1991 claiming to show
a
difference in brain sizes
of
heterosexual and homosexual men
.
+
However
,
his subjects were dead, so he was unable to ask them about their sexual preferences
and
made questionable assumptions
,
and didn
'
t take into account that brains can alter according to one
'
s experiences.{{ref|Dale O
'
Leary
,
[http://daleoleary
.
wordpress
.
com/2011/11/05/the-born-that-way-myth-2/ The Born that Way Myth]
,
{{date|November 5
,
2011}}
.}}
+
+
In 1993 a group of researchers led by homosexual Dean Hamer claimed to
have
found evidence that
implicated a marker on the X chromosome at Xq28.{{ref|1=D.H.Hamer ''et al.'', [http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=8332896 "A linkage between DNA markers on the X chromosome and male sexual orientation"], ''Science'', Vol. 261, No. 5119, PP. 321-7. July, 1993.}}
+
However, other researchers were unable to replicate his results, and questions were raised about the integrity of Hamer's research.
{{ref|
Ryan Sorba, [http://www
.
wnd
.
com/2010/06/161549/ The 'Gay Gene' Hoax], WND, {{date|06/03/2010}}.}}{{ref|Dan Pierce
, [http://
www
.
bio.davidson
.edu/
courses/genomics/2002/pierce
/
gaygene
.
htm The Gay Gene: Assertions, Retractions, and Controversy].}}
+
Rice and others, for example, concluded that their results
"
do not support an X-linked gene underlying
male
homosexuality
"
.{{ref|G. Rice, C. Anderson, N. Risch, and G. Ebers, [http://www.ncbi.nlm.nih.gov/pubmed/10213693 Male homosexuality: absence of linkage to microsatellite markers at Xq28
], ''
[[Science (journal)|Science]]'', {{date|23 April 1999}}
.
}}
+
+
A more recent twin study has suggested a moderate genetic influence on the expression of homosexuality in humans, explaining about a third of the variance in males and about half that in females, although confidence intervals were wide, and non-genetic environmental factors such as pre-birth hormone levels were not ruled out
.
+
Nevertheless, environmental causes remained the main factor.{{ref|N.Långström, [http://1-media-cdn.foolz.us/ffuuka/board/tg/image/1375/46/1375467417264.pdf Genetic and environmental effects on same-sex sexual behavior: a population study of twins in Sweden], ''Arch Sex Behav
'', Vol.
39
, No.
1
, pp.
75–80
.
February
,
2010.}}{{ref|Warren Throckmorton, [http://www.patheos.com/blogs/warrenthrockmorton/2008/07/09/baileyinterview/ Swedish twin study: Q & A with J. Michael Bailey], ''patheos'', {{date|July 9, 2008}}
.}}
A group of geneticists at the Korea Advanced Institute of Science and Technology altered the sexual preferences of female mice by removing a single gene linked to reproductive behavior. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase (FucM) were attracted to male mice.{{ref|D. Park ''et al.'', [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912782/ "Male-like sexual behavior of female mouse lacking fucose mutarotase"], ''BMC genetics'', Vol. 11, p. 62.}}
A group of geneticists at the Korea Advanced Institute of Science and Technology altered the sexual preferences of female mice by removing a single gene linked to reproductive behavior. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase (FucM) were attracted to male mice.{{ref|D. Park ''et al.'', [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912782/ "Male-like sexual behavior of female mouse lacking fucose mutarotase"], ''BMC genetics'', Vol. 11, p. 62.}}
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The complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females of the common fruit fly ''Drosophila melanogaster''.{{ref|Pavlou HJ, and Goodwin SF. "Courtship behavior in Drosophila melanogaster: towards
The complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females of the common fruit fly ''Drosophila melanogaster''.{{ref|Pavlou HJ, and Goodwin SF. "Courtship behavior in Drosophila melanogaster: towards
a 'courtship connectome'". ''Curr Opin Neurobiol.'' 2013 Feb; 23(1):76-83.}}
a 'courtship connectome'". ''Curr Opin Neurobiol.'' 2013 Feb; 23(1):76-83.}}
−
−
== Brain structure ==
−
−
Several researchers have found differences between structures in the brains of heterosexuals and homosexuals, including
−
* a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men,{{ref|Swaab DF, Hofman MA, [http://linkinghub.elsevier.com/retrieve/pii/0006-8993(90)90350-K "An enlarged suprachiasmatic nucleus in homosexual men"], ''Brain Res.'' Vol. 537, No. 1–2, pp. 141–8. December, 1990}}
−
* a difference related to sexual orientation in the size of the anterior commissure,{{ref|Allen LS, Gorski RA, [http://www.pnas.org/content/89/15/7199.full.pdf+html "Sexual orientation and the size of the anterior commissure in the human brain"], ''Proc. Natl. Acad. Sci. U.S.A.'', Vol. 89, No. 15, pp. 7199–202. August 1992.}} and
−
* a difference in the size of the INAH3 group of neurons in the hypothalamusbetween homosexual and heterosexual men.{{ref|LeVay S, "A difference in hypothalamic structure between heterosexual and homosexual men", ''Science'', Vol. 253, No. 5023, pp. 1034–7. August 1991.}}
==References==
==References==