Margaret Mead's argument that gender identity is learned in sexless minds separate and distinct from sex-specific bodies legitimized the "sex/gender system" in feminist theory. In this system, sex refers to physiological differences in the domain of the body and gender to learned behavior in the domain of the mind.

Since this "two-domain" distinction obviated the connection between biological reality and gender identity, it allowed gender identity to be viewed as a product of patriarchal cultural narratives - stories, myths, legends and the like invented by men in order to control and oppress women.

In S/He Brain, Nadeau demonstrates that the sex/gender system is not some arcane bit of academic jargon that has no impact on our daily lives. It is the greatest source of conflict in the politics of our sexual lives for a now obvious reason: the brains of men and women are not the same, and the differences can have behavioral consequences. Yet the intent of the book is to serve the cause of full sexual equality and not to escalate the gender war.

Nadeau argues that an improved understanding of the relationship between sex and gender can not only enlarge the bases for meaningful communication between men and women, it could also serve as the basis for an improved standard of sexual equality that eliminates the grossly unfair treatment of women sanctioned by the current standard.