Dr. Segal

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Dr. Segal has spent the last year studying Juan and Liana's case, and she says their different sexual identities were probably determined in the womb. "We know that their mother had a very stressful pregnancy," Dr. Segal says. "She had a kidney infection. She was in a car accident. We know that certain hormones can be released during pregnancies that could possibly affect the brain of one twin and not the other."

The fact that Juan and so many other transsexual men and women struggle with their feelings at such an early age, Dr. Segal says, indicates that sexual identity is not a choice made in childhood or adolescence. "Juan and Liana were born into a family at the same time, they're identical twins, had many similar experiences—and yet they ended up in such opposite directions. So we really think that whatever is responsible for the difference in behavior was there from the moment of birth."