 Through both empirical and scholarly research he has argued that the primary function of sexual behavior, namely the stimulation and coordination of the reproductive physiologies of the interacting individuals (usually male and female), originated with the first unicellular organisms and hence predates the evolution of sexual recombination.As mentioned, Crews has challenged the Organizational/Default doctrine of sex determination, extending it to sexual differentiation of the brain and arguing for its replacement with an Ancestral (female)/Derived (male) paradigm.This concept has led to questions such as why might males be more like females than females are like males?