When linguistic input provides inconsistent evidence for grammatical structures, children tend to regularize. For example, children learning languages from parents who are imperfect users of the language regularize their parents' inconsistent usages (Singleton & Newport, 2004). Previous studies (Hudson Kam & Newport, 2005, 2009) have examined this phenomenon using artificial languages and have found that children, but not adults, regularized this type of variable input. However, little is known about precisely how and when such regularization occurs. In a series of 5 experiments, we investigate how various aspects of the distribution of the input probabilities affect such behavior, as well as whether this behavior varies by age. Adults and children were exposed for 5 days to one of five artificial languages. Languages differed in the probability with which determiners were used. After exposure, subjects produced novel sentences; determiner use was scored. The first two experiments investigated the role of the type of variability present in the input. When taught a language with unpredictable, "inconsistent" variability, five- and six- year old children regularized the variability in their own language productions, always using the most frequent determiner more often than occurred during exposure. When taught a language with predictable, "lexically-consistent" variability, children did not regularize, but instead learned the pattern responsible for the variability, and used this pattern in their own productions. In both cases, adult's productions matched those present in their input. The second set of experiments investigated which feature of the distribution children are sensitive to, as well as whether this effect varies by age. Adults and 5-8 year old children were exposed to one of three artificial languages, differing in the distribution of determiners. Each language had a dominant determiner that was used more frequently than any of the other determiners, and a minority determiner that was used less often. The probability with which these determiners were used, was systematically varied such that a given probability of usage (40%) was the dominant determiner in one language, but the minority determiner in the other. In line with previous results, the 5-6 year old children in these experiments regularized the dominant determiner in their productions; adult productions reflected the probabilities present in their input. Children ages 7-8 regularized less than younger children, but more than adults. This suggests that the tendency to regularize begins to drop off at an early age, but does so gradually. Moreover, though a determiner is used equally frequently across two languages (40%), children regularize it only when it is the dominant determiner. It is thus the relationships between alternating forms--and not simple frequency--that affects the youngest learners. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]