The diversity in vertebrate reproductive patterns provides natural experiments that yield new insights into behavioral endocrinology. Discussed here is the generality of the concept of an organizing sex during sexual differentiation. In its present form the Organizational Concept emphasizes hormonally induced organization of the male phenotype, with the female phenotype being the neutral or default condition. Does this concept extend to vertebrates lacking genotypic sex determining mechanisms? The answer appears to be No. In species with temperature-dependent sex determination, each embryo has an equal probability of developing into either a male or a female; there is no heritable genetic predisposition for sex determination. In species with behavior-dependent sex determination, sex-change occurs during adulthood as a result of perceived alterations in the social environment. In parthenogenetic species, only female individuals exist, yet they display both male-like and female-like 'sexual' behaviors. In contrast to the contemporary view of the Organizational Concept, let us assume that the male pattern is derived and imposed upon the ancestral female pattern. If this perspective is taken, several avenues of study are identified: (i) the importance of sexual similarities; (ii) extending the principle of complementarity of sexual behaviors to the brain; (iii) temperature modulation of sexual differentiation, and (iv) the role of the brain in sex determination.