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The biological basis of human sexual orientation: is there a role for epigenetics?
[congenital adrenal hyperplasia]
Sexual
orientation
is
one
of
the
largest
sex
differences
in
humans
.
The
vast
majority
of
the
population
is
heterosexual
,
that
is
,
they
are
attracted
to
members
of
the
opposite
sex
.
However
,
a
small
but
significant
proportion
of
people
are
bisexual
or
homosexual
and
experience
attraction
to
members
of
the
same
sex
.
The
origins
of
the
phenomenon
have
long
been
the
subject
of
scientific
study
.
In
this
chapter
,
we
will
review
the
evidence
that
sexual
orientation
has
biological
underpinnings
and
consider
the
involvement
of
epigenetic
mechanisms
.
We
will
first
discuss
studies
that
show
that
sexual
orientation
has
a
genetic
component
.
These
studies
show
that
sexual
orientation
is
more
concordant
in
monozygotic
twins
than
in
dizygotic
ones
and
that
male
sexual
orientation
is
linked
to
several
regions
of
the
genome
.
We
will
then
highlight
findings
that
suggest
a
link
between
sexual
orientation
and
epigenetic
mechanisms
.
In
particular
,
we
will
consider
the
case
of
women
with
congenital
adrenal
hyperplasia
(
CAH
)
.
These
women
were
exposed
to
high
levels
of
testosterone
in
utero
and
have
much
higher
rates
of
nonheterosexual
orientation
compared
to
non-
CAH
women
.
Studies
in
animal
models
strongly
suggest
that
the
long
-term
effects
of
hormonal
exposure
(
such
as
those
experienced
by
CAH
women
)
are
mediated
by
epigenetic
mechanisms
.
We
conclude
by
describing
a
hypothetical
framework
that
unifies
genetic
and
epigenetic
explanations
of
sexual
orientation
and
the
continued
challenges
facing
sexual
orientation
research
.