The term one-sex theory refers to the belief that there was only one sex and it was male. The theory originated in the writings of Aristotle and Galen; they had postulated a structural homology between the sexual organs of men and women whereby they were basically the same, except that those of men lay outside the body while those of women lay inside it and were, naturally, reversed: a vagina was a penis turned inside, the ovaries were the testicles, and so on. Similarly, bodily fluids (semen, blood, milk) were basically the same, being composed of the same fungible matter. The difference between men and women was not, therefore, one of kind (two different types of beings), but of degree (various types of the same being).
The theory postulated that, in the final stages of gestation immediately preceding birth, heat drove the sexual organs out of the fetus's body and created a man; should there not be enough heat, an incompletely formed male (that is, a female) would be born. According to this model, females were thus imperfectly formed males, with all the social and cultural consequences that followed, including exclusion from the highest ecclesiastical, political, or intellectual positions in their society, subservience and obedience to male kins, severe restrictions in legal and economic matters, and so forth.
An important corollary proposed that if, at puberty, sufficient heat were applied, a "girl" could force her sexual organs out of her body and become a "boy." In the sixteenth century, anecdotal accounts attesting to such transformations abound. One of the most famous is the case of Marie, a French shepherdess from Vitry-le-François who, at age fifteen, while chasing some pigs in the heat of the summer, jumped over a small creek and, landing heavily on the other side, so ruptured her ligaments that her sexual organs fell out and she instantly became a man, Germain, who then lived as a male for the rest of his life. The story is recounted by, among others, the physicians Jacques Ferrand and Amboise Paré, and the philosopher Michel de Montaigne (1533–1592). Other stories, such as the one told by a certain Antoine Loqueneux to Amatus Lusitanus (1511–1568), attribute the change to the "heat of passion"—a girl in bed with a chambermaid is so sexually aroused that she suddenly ejects a male member from her body and carries on life (and, one assumes, sexual activity) as a male.
In his groundbreaking volume Making Sex: Body and Gender from the Greeks to Freud (1990), Thomas Laqueur highlights this theory and suggests that it was the fundamental operative model for understanding sex and sexuality not only in the Renaissance but even as far as the eighteenth century. A chorus of scholars (Katherine Park, Robert Nye, Michael Stolberg, and Donald Beecher, among others) have argued strongly against it, however, pointing out that already by 1600 the Aristotelian-Galenic one-sex model had been completely debunked and abandoned not only by European thinkers but, more importantly, by the medical profession itself. Its reaffirmation by Laqueur and others in the late twentieth century is, according to some, more grounded in contemporary theoretical battles than in the realities of Renaissance culture or science.
Beecher, Donald. 2005. "Concerning Sex Changes: The Cultural Significance of a Renaissance Medical Polemic." Sixteenth Century Journal 36(4): 991-1016.
Laqueur, Thomas. 1990. Making Sex: Body and Gender from the Greeks to Freud. Cambridge, MA: Harvard University Press.
Park, Katherine, and Robert Nye. 1991. "Destiny is Anatomy." The New Republic (18 Feb. 1991): 53-57.
Stolberg, Michael. 2003. "A Woman Down to Her Bones: The Anatomy of Sexual Difference in the Sixteenth and Early Seventeenth Centuries." Isis 95: 274-299.