Historically, studies of the role of endogenous hormones in developmental differentiation of the sexes have suggested that mammalian sexual differentiation is mediated primarily by testicular androgens, and that exposure to androgens in early life leads to a male brain as defined by neuroanatomy and behavior. The female brain has been assumed to develop via a hormonal default mechanism, in the absence of androgen or other hormones. Ovarian hormones have significant effects on the development of a sexually dimorphic cortical structure, the corpus callosum, which is larger in male than in female rats. In the females, removal of the ovaries as late as Day 16 increases the cross-sectional area of the adult corpus callosum. Treatment with low-dose estradiol starting on Day 25 inhibits this effect. Female callosa are also enlarged by a combination of daily postnatal handling and exogenous testosterone administered prior to Day 8. The effects of androgen treatment are expressed early in development, with males and testosterone-treated females having larger callosa than control females as early as Day 30. The effects of ovariectomy do not appear until after Day 55. These findings are more consistent with other evidence of a later sensitive period for ovarian feminization as compared to androgenic masculinization.

Studies of callosal sexual differentiation have concentrated on global measures of callosal size, using the rat as a model for studies of potential hormonal mechanisms. It is time to shift the study of callosal sexual differentiation to a more cellular level. Finally, there are potential problems with using the female rat as the primary model for understanding hormonal mechanisms during postnatal life.

I discuss Fitch & Denenberg's argument that no correction for brain size is needed when assessing callosal size. Morphometric criteria may not be sufficient to determine whether corrections are needed. Functional studies of callosal transfer will ultimately specify whether corrections for size are necessary in each case.

The precise relationship between callosal morphology and specific connectivity is not yet known. Callosal axons are often presumed to be arranged according to their origin. In humans, this is true for the genu and the splenium, which convey axons from the prefrontal and occipital cortices, respectively, but not for the body, where axons from wide parts of the cortex are intermingled.

Evidence accumulates that pre- and postnatally circulating estrogens play an active role in the differentiation of the female brain: the susceptible period for feminization of the brain seems to extend far beyond the period during which masculinization of the brain occurs. Thus, there is a need to reevaluate the widely accepted “concept of basic femaleness” in sexual brain differentiation.

New knowledge about the sexual differentiation of the brain profoundly changes our understanding of basic topics in brain development such as the false dichotomy between long-lasting and transient effects of hormones on neural activity, the importance of ovarian hormones in brain development, the plasticity of neural structures throughout the life span, and the way measurement issues affect research conclusions.

Fitch & Denenberg have shown that manipulations of ovarian and testicular hormones early in development can influence the adult size of the corpus callosum in the rat. The human corpus callosum is highly variable in size and shape, but data are only now beginning to emerge on whether sex steroids influence callosal differentiation in humans. I describe recent data from our own laboratory and suggest avenues for future research.

The activational and organizational hormone effects as originally defined do not conflict with activational influences on brain structure. Ovarian hormonal influences on the rodent corpus callosum could be activational rather than organizational. The masculinization/feminization distinction in brain structure and the timing of sex differences in visuo-spatial abilities need to be clarified.

Fitch & Denenberg provide excellent evidence for the existence of dynamically complex interactions between the structural and functional development of the nervous system. They are to be congratulated for showing how subtle social variables (e.g., handling) may not only influence hormonal “cascade effects” on the developing nervous system, but may also alter the structure of brain tissue, such as the corpus callosum.

Data do exist to support the fact that the corpus callosum is relatively larger in women than in men. The corpus callosum is an integral part of the brain, and contrary to Fitch & Denenberg's examples of “pseudostatistics,” is not an extrinsic structure when determining its relative size.

Developmental counts of axons in the splenium of the rat corpus callosum are compatible with the hypothesis that estrogen may be acting late in development to sculpt the female nervous system.

There is some very limited evidence for a role of estrogens in human psychosexual masculinization; its interpretation is uncertain. Fitch & Denenberg's demonstration of a role for estrogens in the behavioral feminization of nonhuman mammals implicitly suggests an answer to a riddle posed by the syndrome of congenital adrenal hyperplasia in women.

Indirect routes by which gonadal hormones influence sexual differentiation are considered. In rats, differentiation may depend on the way in which the mother responds to the hormonal condition of her pups, and this has implications for the interpretation of the data for humans. Interaction between gonadal hormones and light experience in chicks is compared with the mammalian systems covered in Fitch & Denenberg's review.

The fact that the female mammalian brain remains responsive to estrogens throughout life may open the way for other instigators of neuronal plasticity, making the female brain different from that of the male in its response to the actions of a number of hormones, to injury and to aging.

Sex dimorphism occurs when group means differ by four or more standard deviations. However, the average size of the corpus callosum is greater in males by about one standard deviation in rats, 0.2 standard deviation in humans, and virtually zero in mice. Furthermore, variations in corpus callosum size are related to brain size and are not sex specific.

The Fitch & Denenberg target article focuses almost exclusively on short gestation mammals, which differ substantially from long-gestation mammals in the timing and type of hormonal contribution to their sexual differentiation. Conclusions regarding the role of ovaries in female sexual differentiation may accordingly apply to only a limited number of species. Specific criticisms of the organizational effects of hormones stem from an incomplete reading of the original literature. The mechanisms proposed in this target article reflect an extension of the principle of hormonal organization, not a revolutionary restructuring.

We argue that estrogen feminization of the brain is the result of a series of events initiated by differential androgen exposure. There is no need to postulate a feminizing process parallel to androgen-induced masculinization to explain the findings.

A number of commentators agree that the evidence reviewed in the target article supports a previously unrecognized role for ovarian hormones in feminization of the brain. Others question this view, suggesting that the traditional model of sexual differentiation already accounts for ovarian influence. This position is supported by various reinterpretations of the data presented (e.g., ovarian effects are secondary to the presence/absence of androgen, ovarian effects are smaller than testicular effects, ovarian effects are not organizational). We discuss these issues, and reiterate our position that evidence of neurobehavioral ovarian effects is incompatible with the currently accepted model of sexual differentiation. Other points regarding species generalizations, the direct versus indirect action of estrogen, and nonhormonal mechanisms of sexual differentiation are also discussed. Finally, we address the controversial issue of using ratio scores in the assessment of the human corpus callosum (where CC scores are divided by an index of brain size). Future applications to human research are also discussed.

Mazur & Booth fail to consider the conceptual complexities of dominance; it is unlikely that there is a motive to dominate in animals. Also, the lack of empirical evidence for a causal link between testosterone and dominance is obscured by the narrative reviewing procedure, which is prone to bias.

In men, high levels of endogenous testosterone (T) seem to encourage behavior intended to dominate – to enhance one's status over – other people. Sometimes dominant behavior is aggressive, its apparent intent being to inflict harm on another person, but often dominance is expressed nonaggressively. Sometimes dominant behavior takes the form of antisocial behavior, including rebellion against authority and law breaking. Measurement of T at a single point in time, presumably indicative of a man's basal T level, predicts many of these dominant or antisocial behaviors. T not only affects behavior but also responds to it. The act of competing for dominant status affects male T levels in two ways. First, T rises in the face of a challenge, as if it were an anticipatory response to impending competition. Second, after the competition, T rises in winners and declines in losers. Thus, there is a reciprocity between T and dominance behavior, each affecting the other. We contrast a reciprocal model, in which T level is variable, acting as both a cause and effect of behavior, with a basal model, in which T level is assumed to be a persistent trait that influences behavior. An unusual data set on Air Force veterans, in which data were collected four times over a decade, enables us to compare the basal and reciprocal models as explanations for the relationship between T and divorce. We discuss sociological implications of these models.