The chemical model of sex was a rather radical model because it reduced the differences between the sexes to one hydroxyl group: men and women differ only in the relative amounts of their sex hormones. This does not mean that sex endocrinology did not emphasize differences between the female and the male body. In addition to the chemical model, sex endocrinology also provided a model in which sex differences came to be conceptualized in terms of the rhythm of hormone production. Biologists and gynecologists specified the quantitative theory of sex differences in terms of the notion that the male and the female body differ from each other with respect to their hormone regulation. Gynecologists used the hormonal blood test to specify the nature of hormone regulation in the female and the male body. Based on this test, the female body became characterized by its cyclic hormonal regulation and the male body by its stable hormonal regulation. In this biological context, sex differences thus became conceptualized in terms of cyclicity versus stability.

This association of femininity with cyclicity was not entirely new. In the second half of the nineteenth century, psychiatrists became interested in the “periodic madnesses” of their female patients and ascribed these to the cyclic nature of their menstruation. At the turn of the century, the German psychiatrist Krafft-Ebing questioned the “mental integrity” of menstruating women (Baart and Bransen 1986:8). This association of femininity with cyclicity was thus compatible with notions about femininity that already existed in culture and medical practice. What is new is that sex endocrinologists transformed this notion of cyclicity into a basic model for understanding the specific nature of the physical features of the female body. They linked cyclicity with a chemical substance, regulating the development of a wider variety of functions than just reproductive functions. This extension of the concept of cyclicity is exemplified in the handbook Female Sex Endocrinology (1949):

the complete physical and mental structure of a woman is, just as her reproductive functions, exposed to cyclic stimulation of sex hormones. This is not only manifest in the cyclic changes in the endometrium, but also in fluctuations in temperament, changes in breast size, heart beat, and so on.

(Binsberg 1949:1)

Sex endocrinologists thus redefined the meanings assigned to femininity, emphasizing cyclicity as the key concept for understanding the female body.10

In this case, we see how sex endocrinology incorporates cultural norms and —at the same time—actively reshapes these ideas. A similar pattern can be distinguished in the manner in which sex endocrinology conceptualized the female body in terms of its reproductive functions. In the hormone model, the female body became increasingly defined as a reproductive entity. The image of the female body as primarily a reproductive body has a long history manifest in many cultures. Foucault described how in the late eighteenth and early nineteenth centuries scientists incorporated this cultural notion into the medical sciences. In this period, medical scientists increasingly defined the female body as an object of sexuality and reproduction (Foucault 1984:104, 115, 119). Sex endocrinologists integrated the notion of the female body as a reproductive body into the hormonal model, but not without thoroughly changing it. They introduced highly technical tools to investigate the female reproductive functions to an extent that was not possible before. Sex endocrinologists shaped medical practice most profoundly, because they provided the medical profession with tools to intervene in features that had been considered inaccessible prior to the hormonal era. The introduction of diagnostic tests and drugs enabled the medical profession to intervene in the menstrual cycle and the menopause, thus bringing the “natural” features of reproduction and aging into the domain of medical intervention.