 In short, cooperative relations provide the arena for the emergence of operations, which for Piaget requires the absence of any constraining influence, and is most often illustrated by the relations that form between peers (for more on the importance of this distinction see Duveen & Psaltis, 2008; Psaltis & Duveen, 2006, 2007).This is thus how, according to Piaget, children learn "moral judgement" as opposed to "cultural norms" (or maybe ideological norms).Piaget's research on morality was highly influential in subsequent work on moral development, particularly in the case of Lawrence Kohlberg's highly influential stage theory of moral development which dominated moral psychology research until the end of the twentieth century.