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Homosexuality - Biological Hypotheses
If we assume that historically
homosexual individuals have had
lower reproductive success than
heterosexual individuals, how is that
homosexuality is present as a
surprisingly common (1-10%?) trait in
the human population? Difficult to
explain as an evolutionary adaptation.
Homosexuality - Biological Hypotheses
1. Kin selection hypothesis (Wilson
2. Pleiotropy hypothesis (best
example Iemmola & Ciani 2009 –
slide 7)
3. Biological (non-adaptive) side effect
Example: mother immune response
(fraternal birth order) hypothesis
4. Alternative strategy – pursued when
prospects of RS by conventional
strategy is low.
5. Reciprocity in same-sex alliances
(Kirkpatrick 2000)
6. Exotic becomes erotic (Bem 1997)
Kin Selection Hypothesis
Kin Selection Hypothesis
E. O. Wilson, the author of
Sociobiology, noted that homosexuality
seems to be a universal in human
societies, found throughout history and
in all cultures, albeit at relatively low
levels (~ 1 to 10%). Working from
animal analogies – the sterile castes of
eusocial insects (honeybees et al) and
bird “helpers at the nest” – Wilson
proposed that homosexual individuals
help collateral relatives (siblings,
nephews and nieces, cousins) so that
“homosexual genes proliferate through
collateral lines of descent, even if the
homosexuals themselves do not have
Wilson then argued that “this kinselection hypothesis would be
substantially supported if some amount
of predisposition to homosexuality were
shown to be inherited” (1978). He then
cites evidence from twin studies which
suggests the existence of genes for
homosexuality (e.g., monozygotic twins
are more likely to be concordant for the
trait than are dizygotic twins).
But there are problems with this
Problems with Wilson Theory
1. Wilson’s human model does not fit
the animal models which suggested the
kin-selection hypothesis! In the eusocial
insects and the birds and mammals
with ‘helpers’, the non-reproductive
helper is not genetically different from
the reproductives s/he helps! Helping
(or sterility) is a facultative trait in these
species – it has zero heritability. For
example, Queen and worker are
different not because of different genes
they have but because of different
developmental histories.
2. Wilson does not indicate why samesex sexual orientation needs to go
along with helping (it doesn’t, for
example, in the animal models). Why
not just help and skip sex altogether?
Genetic Effects can be Complex
• Pleiotropy = single gene affects
multiple traits
• Epistasis = multiple genes interact
to affect a trait
• multiple traits interact to produce
• therefore, natural selection for
gene combinations
Heritability of Homosexuality
Bailey and Pillard (1991): occurrence of
homosexuality among brothers
• 52% of identical (monozygotic) twins of
homosexual men were likewise
• 22% of fraternal (dizygotic) twins were
likewise homosexual
• 11% of adoptive brothers of homosexual
men were likewise homosexual
Bailey and Pillard (1993): occurrence of
homosexuality among sisters
• 48% of identical (monozygotic) twins of
homosexual women were likewise
homosexual (lesbian)
• 16% of fraternal (dizygotic) twins were
likewise homosexual
• 6% of adoptive sisters of homosexual
women were likewise homosexual
Maternal line and
Class or relatives:
paternal line fecundity
• Mothers (1)
of the two sexual
• Mothers of first borns (1)
orientation groups
• Maternal aunts (0.75)
(likelihood of sharing
• Maternal uncles (0.25)
• Maternal grandparents (0.5) X-chromosomes)
• Sons & daughters of maternal grandparents (0.25-1)
• Paternal aunts (0)
• Paternal uncles (0)
• Paternal grandparents (0)
• Sons & daughters of paternal grandparents (0)
1. Mothers (1)
2. Mothers 1st (1)
3. Mat aunts (0.75)
4. Mat uncs (0.25)
5. Mat grdps (0.5)
6. s&d of mat grps
7. Pat aunts (0)
8. Pat uncs (0)
9. Pat grdps (0)
10.s&d of pat grps
Iemmola, F. & Ciani, A. C. (2009)
Immune Response Theory of
Male Homosexuality
Immune Response Theory of
Male Homosexuality
The maternal immune hypothesis was
never intended to account for the sexual
orientation of all homosexual men. Half
or more of all homosexual men have
zero older brothers, demonstrating that
other etiological factors must be in play…
Bogaert, A. F. (2006). Biological versus nonbiological older
brothers and men’s sexual orientation. Proceedings of the
National Academy of Sciences, 103, 10771-10774.
Bogaert, A. F. (2006). Biological versus nonbiological older
brothers and men’s sexual orientation. Proceedings of the
National Academy of Sciences, 103, 10771-10774.
Immune Response Theory of
Male Homosexuality
Immune Response Theory of
Male Homosexuality
Note: This is from original paper (Blanchard & Bogaert 1996).
Subsequent research has removed the large-family confound and
shown that older brothers but not older sisters have this effect.
G estimates the likelihood that if you are a
homosexual male, it can be attributed to your
having had older brothers. And the more older
brothers you had, the more likely it is.
Overall they have estimated that as a
statistical predictor of homosexuality in males,
the older brother factor accounts for 14%
(1/7th) of the variance.
This is certainly compatible with the immune
response theory, for other factors could protect
the mother or the female fetus against the
immune reaction.
Exotic becomes Erotic (Bem 1996)
• provides same basic account for both
opposite-sex and same-sex erotic desire,
and for both men and women.
• proposes that biological variables do not
code for sexual orientation per se but for
childhood temperaments that influence a
child’s preferences for sex-typical or sexatypical activities.
• These preferences lead children to feel
different from opposite-sex or same-sex
peers—to perceive them as “exotic.”
• This, in turn, produces heightened
physiological arousal that subsequently gets
eroticized to that same class of peers:
Exotic becomes erotic.
• The theory claims to accommodate both the
empirical evidence of the biological
essentialists and the cultural relativism of
the social constructionists.
Exotic becomes Erotic
The temporal
sequence of
events leading
to sexual
orientation for
most men and
according to
A → B.
Role of genes is
to set
B → C.
Exotic becomes Erotic
C → D. Gender-conforming children will feel
different from opposite-sex peers, and gendernonconforming children will feel different from
same-sex peers.
D → E. These feelings of being different
produce heightened physiological arousal.
For male-typical child, it
may be felt as antipathy or
contempt in presence of
girls (“girls are yucky”); for
the female-typical child, as
timidity or apprehension in
presence of boys. Theory
claims that every child –
conforming or nonconforming – experiences
heightened, nonspecific
physiological arousal in
presence of peers from
whom he/she feels different.
E → F.
Exotic becomes erotic
not too much data…