Developmental psychologists who study children’s reasoning are faced with the task of supplying theoretical models to explain children’s reasoning performance. After the cognitive revolution of the 1960s, these models often took the form of mental structures or rules (Gardner, 1985). For example, in order to explain why an adolescent correctly predicts that a balance will remain level, given a certain configuration of weights, cognitive developmentalists have appealed to structures such as “formal operations” (Inhelder & Piaget, 1958), or to rules such as “If the weights are the same and the distances are the same, it will stay level” (cf. Siegler, 1976). A similar trend occurred in the domain of developmental psycholinguistics in which knowledge of grammatical rules was attributed to children (e.g., Bloom, 1970; Braine, 1963; McNeill, 1966).