Unlearning Helps

Abstract

Overregularization seen in child language learning, re verb tense constructs, involves abandoning correct behaviors for incorrect ones and later reverting to correct behaviors. Quite a number of other child development phenomena also follow this U-shaped form of learning, unlearning, and relearning. A decisive learner doesn’t do this and, in general, never abandons an hypothesis H for an inequivalent one where it later conjectures an hypothesis equivalent to H. The present paper shows that decisiveness is a real restriction on Gold’s model of iteratively (or in the limit) learning of grammars for languages from positive data. This suggests that natural U-shaped learning curves may not be a mere accident in the evolution of human learning, but may be necessary for learning. The result also solves an open problem.

Second-time decisive learners conjecture each of their hypotheses for a language at most twice. By contrast, they are shown not to restrict Gold’s model of learning, and correspondingly, there is an apparent lack of reports in child development of the opposite, W-shaped learning curves.