In 1993 Debbie Hartman was sure she was hearing things in her hospital room. She had just undergone a caesarean section, and the doctors were saying the baby was healthy but they weren't sure whether it was a boy or a girl. "I thought the drugs were making me hallucinate," she recalls. In fact, she was hearing just fine. But nothing about her child's biology — from the chromosomes to the reproductive tissue — conformed to the standard demarcations we have come to expect between the male and female sexes. In the language of developmental biologists, the baby was "intersexual."...