Androgen Insensitivity Syndrome (AIS) is a human disorder in which an individual's genetic sex (genotype) differs from that individual's observable secondary sex characteristics (phenotypes). A fetus with AIS is genetically male with a 46,XY genotype. The term 46,XY refers to the chromosomes found in most cells of the fetus. Most cells have a total of 46 autosomes, or non-sex chromosomes, and a pair sex chromosomes, XX for genetic females, or XY for genetic males. Due to a defect on the androgen receptor gene (AR) located on the X chromosome, a fetus with AIS cannot process male sex hormones or androgens. The effect on the fetus is that, compared to genetically male fetuses without AIS, it doesn't develop normal male phenotypes. The resistance to androgens affects all of the fetus's organs during embryonic development and during puberty. Although genetically male, persons with AIS can be socially raised as either female or male (sex-of-rearing) yet identify with a gender discordant with their sex-of rearing. AIS and other states of intersexuality challenge physicians, scientists, and society to evaluate definitions of sex.

Carl Richard Moore was a professor and researcher at the University of Chicago in Chicago, Illinois who studied sex hormones in animals from 1916 until his death in 1955. Moore focused on the role of hormones on sex differentiation in offspring, the optimal conditions for sperm production, and the effects of vasectomy or testicular implants on male sex hormone production. Moore's experiments to create hermaphrodites in the laboratory contributed to the theory of a feedback loop between the pituitary and fetal gonadal hormones to control sex differentiation. Moore showed that the scrotal sac controls the temperature for the testes, which is necessary for sperm production. He also helped distinguish the hormones testosterone, and androsterone from testicular extracts.

Hermaphrodites and the Medical Invention of Sex, by historian of science Alice Domurat Dreger, was published in 1998 by Harvard University Press. In the book, Dreger describes how many doctors and scientists treated human hermaphrodites from the late nineteenth century to the early twentieth century. She states that during this time period, many physicians and scientists struggled to determine the nature sex, and to support a classification of sex as male or female, many physicians and scientists resorted to viewing a person's gonads for identification of his or her sex. At the time that this book was published, Dreger was a faculty associate at the Center for Ethics and Humanities in the Life Sciences at the College of Medicine, University of Michigan, Michigan.