  For example, feminists have scrutinized reductionism with regard to the role of hormones in developmental biology in one of the most ubiquitous model organisms, rats (Birke 1986, Longino and Doell 1983, Longino 1990). In an early model of this system, which Longino and Doell called the linear-hormonal model, prenatal and perinatal hormone levels are assumed to be the basis for behavioral sex differences. The assumption was that a gene on the Y chromosome triggers the development of testes, and that the hormones released by these brand new testes affect the structure of the rodent brain during a critical developmental period. The presence of testes and the hormones that they produce were assumed to cause the male brain to develop such that the male performs ‘stereotypical’ male sexual behavior; in the absence of testes and the hormones that they produce the female brain to develops in such a way as to cause the female to perform ‘stereotypical’ female behavior. Longino points out that in this explanatory model it is assumed that there is a “unidirectional and irreversible sequence of (biochemical) events” (Longino 1990, 135).