Young’s 1959 paper electrified scientists interested in hormones and behavior. By the mid – 1960s the research literature was filled with articles validating the O/A hypothesis in rats, hamsters, mice, and monkeys. The hypothesis had become a theory and then a concept.103 And as a concept, it extended well beyond coital behaviors. As the years passed, scientists applied it to nest build­ing, maternal care, aggression, open field activity, running in an exercise wheel, play fighting, development of a sweet tooth (adult female rats like sweets more than their male counterparts do), conditioned taste avoidance, maze learning, and brain asymmetries.104 The fact that the O/A hypothesis built on Jost’s already accepted accounts of anatomical development, the the­ory’s apparently widespread applicability, and its socially acceptable focus on

heterosexual development were all key factors contributing to its rapid ac­ceptance.105

Young’s ideas not only set the research agenda in his own field. During the 1960s, he led the way in a major shift in theories ofbehavior. Whereas earlier he and other researchers had emphasized the importance of individual (ge­netic) variability, physiological complexity, and environment in the develop­ment of sexual behaviors, now social and biological scientists took up his call to focus on the hormonal causes of gendered behavior. Young himself played a key role in arguing that research into the importance of hormones for the development of animal mating behaviors shed light on the human condition.

One can see this shift in Young’s thinking and its application to humans in his 1961 comprehensive review of ‘‘The Hormones and Mating Behavior.’’ Although he here recounts earlier experiments demonstrating individual vari­ability in rat and guinea pig behavior, as well as work demonstrating the im­portance of experience in the development of sexual behaviors, he seemed more impressed with the dramatic findings that prenatal hormones also influ­enced mating behaviors. He reemphasized the potentially long arm of the O/ A theory, suggesting that it would apply to a variety of nonreproductive behav­iors for which sex differences had been found. And, while acknowledging the widespread belief in ‘‘psychological factors’’ in the development of human sexual behaviors, he nevertheless envisioned a sweeping change: if, as he pre­dicted, prenatal hormones turned out to affect a multitude of behaviors, it would ‘‘bond. . . the work of experimental embryologists. . . and the work of the psychologists and psychiatrists’’ who needed to understand the devel­opment of neural tissues.106

Near the end ofhis life (he died in 1965), as his former students and post­docs became prominent, Young endorsed the reorganization of disciplinary boundaries that encompassed the study of animal and human behavior. In 1964, in a lead article in Science, the official journal of the American Associa­tion for the Advancement of Science, Young, Charles Phoenix, and Robert Goy wrote: ‘‘Without discounting the influence of psychologic factors, which we know is great, or the need for carefully recorded observations ofbehaviors, we expect that, increasingly, the materials and techniques used will be those of the neurologist and biochemist.’’ Indeed, by the end of the 1960s, knowl­edge about the development of sexually dimorphic behaviors had shifted sig­nificantly. Individual genetic differences and the importance of social interac­tions (even for rodents) became less visible.107 Hardly anyone mentioned the fact that males that had been prenatally ‘‘organized’’ by testosterone still needed postnatal organization in the form of social contact. As a result, male and female rodent behaviors, as well as those of humans, for whom they served as a model, emerged as more stereotyped than they had previously seemed, and as more rigidly determined by prenatal hormonal environments.

This happened despite efforts by many prominent researchers to stave off single-factor models of development. Speaking at an interdisciplinary confer­ence that included experts on human development, Charles Phoenix ‘‘hoped that the concept of the organizing action of prenatal androgen will not give rise to time-worn arguments of heredity versus environment or be conceived of as a fatalistic theory that renders useless the need for studying the effect of the environment on the development of normal sexual behavior.’’ But the developers of the O/A theory were unable to integrate their findings on early hormone effects with their findings on environmental determinants of sexual behavior. In fact, their working model of development got in the way. It has the same difficulties as the sex/gender account of human bodies. Develop­ment and experience, nature and nurture, are never separate, but always co­produce one another. Thus Phoenix’s concluding sentence in this passage only reinstates the problem he wished he and others could avoid: ‘‘What is sug­gested here is a mechanism whereby the information encoded in genetic mate­rial is translated into morphology and, ultimately, behavior.’’ In this sentence, the body comes first, then experience may be imposed upon it. With such a model it is never possible to escape ‘‘the time-worn arguments of heredity versus environment.’’108

While the O/A theory grew deep roots during the 1960s, by the mid – 1970s accepted definitions of rodent masculinity and femininity had come under fire—both from Frank Beach and from some of his students, among whom were those inspired by the newly emerged women’s liberation move – ment.109 The role of estrogen in establishing both masculine and feminine behaviors again became a topic for debate, and the possibility emerged that masculinity and femininity ran in parallel, rather than oppositional lines.110 As founding editor of the new journal Hormones and Behavior, which rapidly became the premier location for publishing articles on hormones and sexual behavior and at conferences, Beach attacked the O/A theory.111 Immediate responses in print or in explicit experimental deed were sparse, and he him­self fell nearly silent about it afterward—as if the tide were too strong and the personal ties to those who disagreed too dear for even such a renowned scientist to swim against.112

Beach continued, however, to believe in a bisexual model of adult develop­ment. Reminding readers that untreated adult females would not only mount other animals but would thrust and show a pattern of intromission, he con –

eluded that ‘‘the female rat’s nervous system is capable of mediating all of the masculine responses with the noteworthy exception of ejaculation.’’113 If one wanted to understand the relationships between hormones and behavior, he concluded it would be better to study the immediate factors leading to a par­ticular behavior than to construct what he called ‘‘imaginary brain mecha­nisms.’’ Nevertheless, by the 1970s Beach had qualified what had been learned about ‘‘a basic bisexuality of the brain.’’ He agreed with Young that in genetic males the suite of masculine behaviors was easier to activate than the female repertoire, while the converse held for females. In addition, in both sexes the female repertoire was more sensitive to estrogen stimulation, while the male behavioral suite responded more easily to androgen.114 For Beach, a ‘‘basic bisexuality’’ did not mean a lack of sex differences.

While Beach published a few more papers critical of experimental proce­dures used to study prenatal sex hormone effects,115 others examined the effects of hormones on genital development more carefully.116 A report that neonatal androgen produced measurable anatomical differences in the brain’s hypothalamus, however, seemed to confirm the organizational hypothesis.117 Still, matters were more complex than originally envisioned. A summary of a work session intended to produce a state-of-the-art account of hormones and sex differences concluded: ‘‘despite such evidence against complete deter­minative influence of peripheral structures, the expression of adequate sexual behavior clearly is partly dependent upon adequate peripheral structures. When observed, suppression of behavior must be carefully interpreted and assurances provided’’ before concluding that the central nervous system is the only culprit.118

Eventually Beach accepted the evidence that prenatal hormones could per­manently affect brain development. Nevertheless, he continued to remind anyone who would listen that the hormone/behavior interaction was com­plex, depending on genetic makeup, an individual’s current emotional and physical condition, and personal history.119 By 1981, the psychologist Harvey Feder, one of the new generation of hormone researchers, found that drawing analogies with Jost’s anatomical studies was ‘‘no longer helpful. . . and may even be counterproductive.’’120 In the decade since Beach’s critique, evidence had accumulated showing prenatal hormonal effects on brain anatomy. But the relationship between anatomical effects and behavior remained (and still remain) unclear.121 Beach was right that the basic wiring for suppressed behav­iors exists into adulthood; Young was right that they need special circum­stances to be called into play. Despite the fact that some of Beach’s criticisms had not withstood the test of time, problems continued to arise for the O/A.

Many of them had, in one way or another, to do with how best to conceptual­ize and experiment on gender differences. In the 1970s the long arm of the women’s movement moved into the rat lab.