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Abstract
The 1959 publication of the paper by Phoenix et al. was a major turning point in the
study of sexual differentiation of the brain. That study showed that sex differences
in behavior, and by extension in the brain, were permanently sexually differentiated
by testosterone, a testicular secretion, during an early critical period of development.
The study placed the brain together in a class with other major sexually dimorphic
tissues (external genitalia and genital tracts), and proposed an integrated hormonal
theory of sexual differentiation for all of these non-gonadal tissues. Since 1959,
the organizational-activational theory has been amended but survives as a central
concept that explains many sex differences in phenotype, in diverse tissues and at
all levels of analysis from the molecular to the behavioral. In the last two decades,
however, sex differences have been found that are not explained by such gonadal hormonal
effects, but rather because of the primary action of genes encoded on the sex chromosomes.
To integrate the classic organizational and activational effects with the more recently
discovered sex chromosome effects, we propose a unified theory of sexual differentiation
that applies to all mammalian tissues.