Dr. Nancy Segal

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Dr. Nancy Segal is a psychologist and director of the Twin Studies Center at California State University at Fullerton. She's author of the book Entwined Lives: Twins and What They Tell Us About Human Behavior. She says that although identical twins have the same DNA, differences develop as early as the first few days following conception.

What about after birth—are mom or dad to blame for a child's sexual identity crisis? Twins studies indicate that the answer is no.

"It's unlikely that parents create differences between twins," Dr. Segal says. "Why would parents have two identical female twins and treat one like a little boy and one like a little girl? It just doesn't make sense. ... I think that the more we study [transgender men and women], parents will understand that it's nothing they did."