Sexual differentiation is a cascade of events beginning with the process of sex determination and continuing through different stages of development to establish male or female phenotypes. Following a schema proposed by Phoenix and colleagues in 1959, sex-specific events are often categorized as early occurring organizational effects or later occurring activational effects (McCarthy, Schwarz, Wright, & Dean, 2008; Phoenix, Goy, Gerall, & Young, 1959). Although there are many variations on this basic categorization, including the method of sex determination, timing and number of organizational events, magnitude of differences between the sexes, and which steroid hormones are operative, it continues to serve as a valuable template (McCarthy & Ball, 2008). An important modification to the original hypothesis is that sex steroid exposures during puberty are also associated with organizational effects, and that brain structural modifications in response to changing hormonal levels continue throughout adult life (Sisk & Zehr, 2005).
