“On October 10, 1970, the day she was born, she was named Dorothy Maree Alaniz—a baby girl. Curiously, though, no one filled out a birth certificate that day. When the certificate was finally filed on November 5, the name on it was Rudolph Andrew Alaniz. Within less than one month after her birth, this girl became a boy.” Every year in the United States, more than two thousand children are born with an intersex condition or disorder of sex development. What makes someone a boy or a girl? Is it external genitalia, chromosomes, DNA, environment, or some combination of these factors? Not even doctors or scientists are entirely clear. What is clear is that sex is not an either-or proposition: not girl/boy, XX/XY, switching between two poles like an on-off switch on a radio. Rather, sex is like the bass and treble knobs on that radio. Between XX and XY provides a fascinating look at the science of sex and what makes people male or female. There are people born XXY, XXXY, or XXXXY, or with any number of variations in X or Y chromosomes, but those who do not fit into society’s preconceived notions about sex often face a difficult path in life. Dr. Callahan explores why humans are so attached to the idea of two sexes, and examines our obsession with sex and sexual intercourse through the ages.