Transcript homosexuality nature or nurture

Sexual Orientation-is it
Nature or Nurture?
Ela Lutwak
Homosexuality in animals
98% bisexual
(Zuk, 2002)
Great apes
Sheeps
Penguins
Homosexuality in humans
2% of the general population in the United States
are gay, lesbian or bisexual.
1.7% of the general population of Canada
identified as gay, lesbian or bisexual.
Determining an actual number is
complicated by problems defining
these terms and by the fact that
some people are reluctant to
admit their sexual feelings
to those doing surveys.
Historical Perspectives
From antiquity
Concept of inborn homosexual orientation
dates from 1860s
Psychiatric illness needing a cure
Sigmund Freud's basic theory of human
sexuality
Current Theories
Many existing and differing theories,
most of which can be categorized into one
of two generalized categories:
Non-biological theories
( environment)
Psychoanalytic
Behaviorism/socialization
Biological
theories
Genes
Hormones
Anatomy
Brain studies—anatomy
Cognitive studies
Birth order
Non-biological theories
(environment)
Psychoanalytic theories
close-binding mother
unconscious hatred of their mothers
absent or distant father
envy of a brother’s penis
Homosexuality resulted from pathological family relationships
during the oedipal period (around 4-5 years of age(
(Bieber et al., 1962).
"double blind" procedures
have not been used in
clinical psychoanalytic
studies of homosexuality.
homosexuals who
were seeking
treatment or
therapy .
Behaviorism/socialization
parents, teachers, peers, and society
Effect of sexual experiences
Psychoanalytic
studies suffer
from small and
unrepresentative
samples
There are
significant cultural
and historical
differences in the
manifestation of
sexual behavior
Gay genes and evolution
 kin-selection
 balanced polymorphism
 reciprocal altruism - same sex alliances
 The analogy is to the sickle-cell gene
(Miller, 2000)
The evolutionary value of gay genes may become
clearer if and when such genes are identified and
their mechanism of action determined.
Genes
In the fly Drosophila melanogaster the gene
called fruitless, nicknamed fru, influences
sexual development and behavior. Males with
fru gene mutations behave as bisexual they
cannot tell other males from female flies
when courting..
Manoli et al., 2005; Stockinger et al.,
2005
In insects, sex differentiation is largely cellautonomous
In humans sexual differentiation is not cell-autonomous
but depends in large part on circulating sex hormones .
Twins and sibling Studies
brothers of gay men
22%
brothers of heterosexual men
4%
Pillard & Weinrich, 1986
56 Pairs MZ Twins
54 Pairs DZ Twins
57 Pairs Adoptive Sibs
Concordance of
Gay Male Brothers
52 %
22 %
11 %
J.M. Bailey and R.C. . Pillard 1991
Are Genes
involved in
Sexual
Orientation?
Family studies (pedigree): More male
homosexuality on maternal side
X
These were “genes” or “DNA sequences” of
unknown function (the only thing known was
where on the X-chromosome they were
located).
The concordance patterns of each gene
was examined and compared to the degree of
relationship to the self-identified gay male.
Conclusion: higher concordance with close
relatives. An “association” of some gene or
genes on the tip of the X-chromosome with
something the group of gay males shared in
common.
Schematic Xchromosome
Genetic Evidence- Linkage studies
 Xq28 region (Hamer et al., Science 1993 (
Genetics and Male Sexual
Orientation
82%
67%
66%
46%
64%
D. H. Hamer; et al., Science 285, 803a (1999)
New Study Proves that “Gay Gene” Theory Is
Totally Dead
By MassNews Staff
Posted 2-1-2005
Brian S. Mustanski March 2005 Human Genetics
The presumption was that at least one “doctor”
or gay gene would be found.
They found several identical stretches of DNA
that were shared among gay siblings on
chromosomes other than the female X.
About 60% of these brothers shared identical
DNA on three chromosomes - chromosome 7, 8
and 10.
Multiple genes - and not just the sex
chromosomes - are important in sexual
orientation
Hormone & Prenatal Evidence
 Adult hormone levels –no significant differences
 Prenatal hormone level organize the brain in a male/femalespecific fashion
 Maternal stress -The reason is that the stress activates
the fetuses’ stress hormones which in turn lead to a
diminution in the levels of testosterone during a critical
period of brain development.
Birth Order-How many gay men owe
their sexual orientation to fraternal
birth order?
 Latter born sons more likely to be gay
 Mothers develop immunity to H-Y Antigen
o The transfer of anti-male antibodies across the placenta from the
maternal into the fetal compartment
o The transfer of maternal cytokines across the placenta
o Maternal immune reactions affecting the placenta itself.
(Blanchard and Klassen 1997,
Blanchard 2001, Cantor 2002).
Prenatal hormones
Body anatomy
penis size
finger length ratios
fingerprints
Brain anatomy
Suprachiasmatic nucleus
INAH1
Brain function
Auditory system
Sexual arousal
Neurotransmitter function
Odor responses
Cognitive and personality
traits
Visuospatial tasks
Object location memory
Verbal fluency
Aggressiveness
Handedness
Penis size
Finger-length ratios
Fingerprints
Our erotic personalities are as
unique as our fingerprints?
Williams, T. J .et al )2000( Nature
Human Brain Anatomy and
Sexual Orientation
INAH1-4 regions (interstitial nuclei
of the anterior hypothalamus)

examined homosexual men, heterosexual men and women
 area INAH3 differentiated groups (Simon LeVay, 1991)
 LeVay found structural differences in the size of
the hypothalamus of gay and straight men (1991).
 LeVay used HIV patients in study. HIV impacts
brain negatively.
 Byne, et al (2001) failed to replicate LeVay’s
findings and said “sexual orientation cannot be
reliably predicted on the basis of INAH3 volume
alone.”
Researchers
Brain region
INAH1
INAH2
INAH3
INAH4
Swaab & Fliers,
1985
Larger in
men
Not studied
Not studied
Not studied
Allen et al,
1989
No sex
difference
Larger in men than in
some women
Larger in men
No sex
difference
LeVay, 1991
No sex
difference
No sex difference
Larger in heterosexual men than in women or
homosexual men
No sex
difference
Does IHAH-3 have more cells in
the brains of straight males?
Critique of Brain Studies
Selection of samples:
• All from autopsies
• Retrospective assignment of sexual orientation
• Confounding variable of HIV infections
Analysis
• Microscopic measurements
• Very tiny differences
• Wide statistical variations
Comparisons: gay males = females
Lack of replication by
other scientists
LeVay said about his research:
 "It's important to stress what I didn't find. I did
not prove that homosexuality is genetic, or find a
genetic cause for being gay. I didn't show that gay
men are born that way, the most common mistake
people make in interpreting my work. Nor did I
locate a gay center in the brain. ...Since I look at
adult brains, we don't know if the differences I
found were there at birth or if they appeared
later.“
Nimmons, D. (1994). Sexual brain. Discover, 5, 3.
Non-biological theories
vs biological theories
hormonal changes
a close-binding mother
facial beauty
 Some scientists believe sexual
attractions are primarily determined by
genetics and/or prenatal factors
 Others believe that people mainly
develop their sexual attractions
throughout their life via a combination
of life experiences and personal traits.
 No gay gene has been discovered
 Environmental circumstances have not
been identified that always lead to same
sex attractions for all people.
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Evidence exists to support both points
of view, but there is not enough
evidence to be conclusive about either
viewpoint.
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Thank you
for listening
References
 Bailey, J. M., M. P. Dunne, and N. G. Martin. 2000. Genetic and
environmental influences on sexual orientation and its correlates in an
Australian twin sample. J. Personality Social Psychology 78:524-536.
 Blanchard, R. 2001. Fraternal birth order and the maternal immune
hypothesis of male homosexuality. Hormones and Behavior 40: 105-114.
 Blanchard, R. and P. Klassen. 1997. H-Y antigen and homosexuality in men.
J. Theor. Biol.185: 373-378.
 Blanchard, R. (2004). Quantitative and theoretical analyses of the relation
between older brother and homosexuality in men. Journal of Theoretical
Biology, 230:173-187.
 Byne, W., M. S. Lasco, E. Kemether, A. Shinwari, M. A. Edgar, S. Morgello,
L. B. Jones, and S. Tobet. 2000. The interstitial nuclei of the human
anterior hypothalamus: An investigation of sexual variation in volume and
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 Cantor, J. M., R. Blanchard, A. D. Paterson, and A. F. Bogaert. 2002. How
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 LeVay, S. 1991. A difference in hypothalamic structure between
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LeVay, S. 2003b. The biology of sexual orientation. Available from
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Mustanski, B. S., Dupree, M. G., Nievergelt, C. M., Bocklandt, S.,
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sexual orientation. Human Genetics.
Pillard, R. C. and J. M. Bailey. 1998. Human sexual orientation has a
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Roselli, C. E., K. Larkin, J. A. Resko, J. N. Stellflug, and F.
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Swaab, D., W. C. J. Chung, F. P. M. Kruijver, M. A. Hoffman, and T.
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