In natural language, evidence suggests that, while some rules are productive (regular), applying broadly to new words, others are restricted to a specific set of lexical items (irregular). Further, the literature suggests that children make a categorical distinction between regular and irregular rules, applying only regular rules productively during acquisition. This strong distinction has led to the central question explored in this dissertation: what governs the acquisition of productive rules in children? In the literature, a number of approaches have been proposed to account for the productivity of some rules, but most fail to capture this acquisition process adequately. This dissertation focuses on one model of productivity, the Tolerance Principle, which has been shown to accurately predict productive rule formation on a number of rigorous measures. The goal of this dissertation is to test the Tolerance Principle as a model of productive rule acquisition using artificial grammar learning experiments in children and adults. To this end, we conduct three experiments to assess whether the Tolerance Principle can predict productive rule formation in children and adults. Across these three experiments, we find that the behavior of children is well predicted by the Tolerance Principle model, but the behavior of adults is not. Thus, a secondary goal of the dissertation is to argue that the Tolerance Principle is a model of productive rule formation that is exclusive to children. We hypothesize that cognitive differences between children and adults, particularly memory and cognitive control differences, may explain why the behavior of children but not adults is well predicted by the Tolerance Principle. We then demonstrate how these hypotheses can be tested in two further experiments with adults. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]