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 girl/boy or 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. In this eye-opening exploration of the science of sex, Gerald N. Callahan, PhD, challenges our notion of two opposite sexes. Human sex is more than it appears to be. Using a brief history of sex from the ancient Greeks to the geneticists of the twentieth century, he shows us how our understanding of the sexual development of human beings is constantly evolving.

By sharing the extraordinary stories of people living with intersex conditions such as hermaphroditism, Klinefelter syndrome, and androgen insensitivity syndrome, Between XX and XY reveals that the path of sexual development is as varied as humans themselves.