Normally, development of testes and are driven by specific genes in the Y chromosome. One hormone gets rid of the female internal structures and cells in the testes generate testosterone to form the internal and external male genitals.
In CAIS, normal testes are formed, but there is no penis, nor are there internal female organs. The undifferentiated "cleft" appears as a vagina.
"The newborn looks like a perfectly normal girl," said Spack.
CAIS can be diagnosed at several stages, most often in women over 35 during an amniocentesis, chromosome analysis and an ultrasound. Sometimes, it's never discovered until puberty, when there is no menstruation, or during hernia surgery.
"Twice a year, I get a call from the operating room that there is a perfectly normal little girl with a hernia in the groin," said Spack. "The surgeon looks in the hernia sac and it's an obvious testis, which can be confirmed by frozen section biopsy."
Upon further examination, the vagina is shortened with no cervix at the back. "Some girls don't get looked at until they are 15 or 16 even if they are sexually involved."
"The vagina is expansive and can accommodate so their sexual potential is not a big issue," said Spack.
"Our chromosomes don't tell us who we are," Dr. Arlene Baratz, a Pittsburgh breast radiologist who has two CAIS daughters, told ABCNews.com last year. "We expect XX is pink and a girl and XY is blue and a boy, but we know from children with gender identity conditions that is not always the case, even when their bodies are perfectly typical."
When Baratz's daughter Katie was 6, doctors discovered small testes in a hernia sac. Today, in her late 20s, she hopes one day to adopt a child.
"These girls look completely female and they are girls," said her mother. When these babies are gender assigned as female, 99 percent of them go on to feel like women when they grow up.
Unlike Wallis, girls with CAIS tend to be tall -- she was 5-feet, 7-inches tall. "They also tend to be buxom," said Spack. "Wallis was flat as a board."
They also have "scant genital hair" and rarely need to shave their underarms and legs. Their mothers, as carriers of the genetic mutation, tend to have less sexual hair than sisters who are not carriers, according to Spack.
As for raspy voices and male musculature, as was described in the Wallis biography, that is not the case, he said. The testes, if present, compensate for lack of testosterone and -- in a biological contradiction -- produce "sky high" testosterone that converts to estradiol, which feminizes these women.
After puberty, doctors usually, but not always, remove the testes because they can become cancerous, and intersex women take estrogen supplements to maintain feminization.
As for the Wallis biography, the author suggests her personality traits confirmed she was intersex -- a premise that Spack finds blatantly sexist by today's mores.
Sebba says that a German graphologist who analyzed her handwriting determined Wallis was, "a woman with a strong male inclination in the sense of activity, vitality and initiative. She must dominate, she must have authority, and without sufficient scope for her powers can become disagreeable. She is ruled by contradictory impulses."
But being "brash" and "having balls" and liking oral sex don't define intersexuality, according to Spack.
Reports about Wallis in British tabloids like the Daily Mail made Spack "recoil a bit," he said.
"She is just a woman," he said. "Don't tell me she doesn't have a right to be as feminine or masculine as any other woman with her demeanor ... Don't blame it on her hormones."