WOMEN WITH Y CHROMOSOME DON''T THINK LIKE MEN
Women born with a rare
condition that gives them a Y chromosome don't only look like women physically,
they also have the same brain responses to visual sexual stimuli, a new study
shows.
The journal Hormones
and Behavior published the results of the first brain imaging study of
women with complete androgen insensitivity, or CAIS, led by psychologists at
Emory.
"Our findings
clearly rule out a direct effect of the Y chromosome in producing masculine
patterns of response," says Kim Wallen, an Emory professor of psychology
and behavioral neuroendocrinology. "It's further evidence that we need to
revamp our thinking about what we mean by 'man' and 'woman.'"
Wallen conducted the
research with Stephan Hamann, Emory professor of psychology, and graduate
students in their labs. Researchers from Pennsylvania State University and
Indiana University also contributed to the study.
The Y chromosome was
identified as the sex-determining chromosome in 1905. Females normally have an
XX chromosome pair and males have an XY chromosome pair.
By the 1920s,
biochemists also began intensively studying androgens and estrogens, chemical
substances commonly referred to as "sex hormones." During pregnancy,
the presence of a Y chromosome leads the fetus to produce testes. The testes
then secrete androgens that stimulate the formation of a penis, scrotum and
other male characteristics.
Women with CAIS are
born with an XY chromosome pair. Because of the Y chromosome, the women have
testes that remain hidden within their groins but they lack neural receptors
for androgens so they cannot respond to the androgens that their testes
produce. They can, however, respond to the estrogens that their testes produce
so they develop physically as women and undergo a feminizing puberty. Since
they do not have ovaries or a uterus and do not menstruate they cannot have
children.
"Women with CAIS
have androgen floating around in their brains but no receptors for it to
connect to," Wallen says. "Essentially, they have this default female
pattern and it's as though they were never exposed to androgen at all."
Wallen and Hamann are
focused on teasing out neural differences between men and women. In a 2004
study, they used functional magnetic resonance imaging (fMRI) to study the
neural activity of typical men and typical women while they were viewing photos
of people engaged in sexual activity.
The patterns were
distinctively clear, Hamann says. "Men showed a lot more activity than
women in two areas of the brain -- the amygdala, which is involved in emotion
and motivation, and the hypothalamus which is involved in regulations of
hormones and possibly sexual behavior."
For the most recent
study, the researchers repeated the experiment while also including 13 women
with CAIS in addition to women without CAIS and men.
"We didn't find
any difference between the neural responses of women with CAIS and typical
women, although they were both very different from those of the men in the
study," Hamann says. "This result supports the theory that androgen
is the key to a masculine response. And it further confirms that women with
CAIS are typical women psychologically, as well as their physical phenotype,
despite having a Y chromosome."
A limitation of the
study is that it did not measure environmental effects on women with CAIS.
"These women look the same as other women," Wallen explains.
"They're reared as girls and they're treated as girls, so their whole
developmental experience is feminized. We can't rule out that experience as a
factor in their brain responses."
The findings may have
broader applications in cognition and health. "Anything that we can learn
about sex differences in the brain," Wallen says, "may help answer
important questions such as why autism is more common in males and depression
more common in females."
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