Comparative studies of various aspects of human development indicate that, during evolution, it has slowed down. From multiple areas of knowledge and at separate times, several authors have related this feature to some notions that parallel it to behavioral plasticity. Here, I reflect on behavioral plasticity and its place in cognitive evolution. Throughout the paper, I maintain a comparative perspective that emphasizes the crucial role that juvenile behavioral selection has played in shaping cognitive behavior in various animal species. From its anchoring in the positive correlative evolution between cognitive behavior and developmental modes, this perspective leads to an argument in favor of replacing the concept of behavioral plasticity with that of cognitive differentiation. Behind the concept of behavioral plasticity (i.e., the ability of a genotype to produce different behaviors in response to various environmental conditions), there is a link between the purpose of the behavioral phenotype and the evolutionary shaping of the genotype that supports it; also, behind this concept is the idea that there are distinct but finite phenotypic possibilities, each a function of a prior evolutionary interaction between genotype and environment. In conceptual contrast, the purpose of a behavioral phenotype developed from a process of cognitive differentiation is evolutionarily independent of prior environmental interaction (i.e., it has no pre-existing purpose).