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 condition is known as complete androgen insensitivity or CAIS.
“Our findings clearly rule out a direct effect of the Y chromosome in producing masculine patterns of response,” said Kim Wallen, an Emory University professor of psychology and behavioural neuroendocrinology.
“It’s further evidence that we need to revamp our thinking about what we mean by ‘man’ and ‘woman,'” said Wallen, who conducted the research with Stephan Hamann, Emory professor of psychology, and graduate students in their labs.
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.
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 oestrogens that their testes produce so they develop physically as women and undergo a feminising 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. Essentially, they have this default female pattern and it’s as though they were never exposed to androgen at all,” Wallen said.
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.
For the 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 said.
“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,” said Hamann.
The study was published in the journal Hormones and Behaviour.