478 CHAPTER 11
or are early manifestations of the disorder. In the following sections we examine
these neurological, psychological, and social factors.Neurological Factors
Researchers know little about neurological factors that contribute to gender identity
disorder. However, they have begun to document differences in specifi c brain struc-
tures in people who have the disorder versus those who do not, and have evidence
that hormones during fetal development play a role in producing this disorder. In
addition, genes can play a large role in contributing to this disorder.Brain Systems and Neural Communication
The brains of transsexuals differ from typical brains in ways consistent with their
gender identity. In particular, Kruijver and colleagues (2000) examined a specifi c
type of neuron in a brain structure called the bed nucleus of the stria terminalis
(which is often regarded as an extension of the amygdala). Typically, males have
almost twice as many of these neurons as females do. In this study, the number of
these neurons in the brains of male-to-female transsexuals was in the range typically
found in female brains, and the number in the brains of female-to-male transsexuals
was in the range typically found in male brains.
How might such brain alterations arise? Research suggests that prenatal exposure
to hormones may affect later gender identity (Bradley & Zucker, 1997; Wallien et al.,
2008; Zucker & Bradley, 1995). In particular, maternal stress during pregnancy could
produce hormones that predispose a person to gender identity disorder (Zucker &
Bradley, 1995). In rats, maternal stress raises the level of cortisol in both the pregnant
rat and any male pups she is carrying. Such an increase in cortisol can disrupt the
timing of prenatal developmental events in a way that leads to low levels of androgen
in the fetus and changes certain brain areas (Ward, 1992; Zucker & Bradley, 1995,
p. 155). The mother’s male pups later exhibit less “male” behavior (less initiation of
copulation, less rough play) and more “female” behavior (a sexual posturing typical
of female but not male rats). One study found similar results in humans but with a
different hormone, testosterone (which underlies many male sexual characteristics):
Fetal levels of testosterone—measured from amniotic fl uid—were positively associ-
ated with later stereotypical “male” play behavior in girls and, to a lesser extent, in
boys; the higher the level of testosterone in the fl uid, the more “male” play the chil-
dren exhibited when they were between 6 and 10 years old (Auyeung et al., 2009).Genetics
A hint that genetic factors contribute to gender identity disorder was reported by
Sadeghi and Fakhrai (2000). These researchers described two 18-year-old female
identical twins, both of whom had requested sex reassignment surgery. These young
women had cross-dressed since childhood and had the hallmarks of gender identity
disorder. That same year, Green (2000) reported ten pairs of close relatives (siblings or
parent-child pairs) who had in common gender identity disorder. These case reports
are intriguing, but they do not carry the force of full-scale studies such as the one
conducted by Coolidge and colleagues (2002). These researchers studied 314 children
who were either identical or fraternal twins and concluded that as much as 62% of
the variance in gender identity disorder can be chalked up to genes! If this result is
replicated, it will provide strong support for these researchers’ view that the disorder
“may be much less a matter of choice and much more a matter of biology” (p. 251).
However, even in this study, almost 40% of the variance was ascribed to the effects of
nonshared environment, and thus genes—once again—are not destiny.
In short, neurological factors—brain differences, prenatal hormones, and ge-
netic predispositions—may contribute to gender identity disorder, but the presence
of any one of these factors does not appear to be suffi cient to lead to this disorder
(Di Ceglie, 2000).Psychological Factors: A Correlation With Play Activities?
In general, correlational studies have found that boys are more likely than girls to
engage in rough-and-tumble play and to have a higher activity level. Biological boysP SN