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."