The argument was built on the conclusion, derived from studies in animals, that sexual differentiation progresses independently in different brain tissues, enabling genetically- and environmentally-induced variation in sexual differentiation of different tissues within the same brain (e.g., ) can change the form of specific brain characteristics (e.g., size, number of neurons, dendritic morphology, number and size of axons, density of receptors) from the “male” form to the “female” form or vice versa, but that this happens independently or exclusively in select brain regions. Yet, following exposure to 15 minutes of stress, dendrites from stressed males had the “female” form (i.e., high density of spines), whereas dendrites from stressed females had the “male” form (i.e., low density of spines).
For example, Shors and colleagues  found a sex difference in the density of apical dendritic spines on pyramidal neurons in the CA1 area of the hippocampus, with dendrites from male rats having fewer spines compared to dendrites from female rats (see Figure ] with pictures obtained from Prof. In contrast, in the basal dendrites of the same neurons there was no sex difference in intact rats, but a sex difference emerged following the 15 minutes of stress, as the latter resulted in increased spine density in males but not in females (see Figure An example of interaction between sex and environment in determining the structure of brain features. Golgi impregnation of apical dendrites in area CA1 of the hippocampus of male and female rats that did or did not undergo 15 minutes of stress 24 hours before their brains were removed (The pictures were received from Prof. Shors and are from the study reported in Figure 3 in ). The mean and standard error of the mean density of apical and basal dendritic spines on pyramidal cells in area CA1 of the hippocampus of male and female rats that did or did not undergo 15 minutes of stress 24 hours before their brains were removed.
About 1% of the human population is identified as “intersex” because of either having an intermediate form at one or more levels, or having the “male” form at some levels and the “female” form at other levels.
These two types of “intersex” reflect the facts, respectively, that the different levels of 3G-sex are not completely dimorphic nor perfectly consistent.
In such a system there would be “male”, “female” and “intersex” subjects, with the latter characterized by having the “male” form at some levels and the “female” form at other levels (an example for such a subject is represented by the pink and blue short bars). A system with high dimorphism at some levels and partial consistency between levels.
As a result, humans are divided into men and women and brains into male brains and female brains (e.g., ).Significant differences are marked with asterisks (adopted with permission from Figure 4 in [Shors et al’s study demonstrates that one should be cautious in the use of the terms “male” form and “female” form when considering brain features, because what is “male” and what is “female” may be different under different environmental conditions.What is “male” and what is “female” may also be different at different stages across the life span (e.g., ).3G-sex is a categorization system in which ~99% of human subjects are identified as either “male” or “female”, and identification with either category entails having all the characteristics of that category (i.e., “female” = XX, ovaries, uterus, fallopian tubes, vagina, labia minora and majora, clitoris, and “male” = XY, testes, prostate, seminal vesicles, scrotum, penis).That 3G-sex is such a powerful categorization system relies on two characteristics.