A model for adult language learning should integrate theories in language acquisition with theories about learnability and proposals about adult language structures. Two particular problems in language acquisition are examined: (1) establishing what counts as a formal relationship in a particular domain, and (2) retreating from overgeneralizations without negative data with respect to the acquisition of morphology. The "inheritance principle" is proposed as a principle of both adult grammar and language acquisition. This principle is understood as follows: first, a derived form inherits the full subcategorization of its base unless the relation between the two forms is distant; and secondly, if a derived form is distantly related to its base, then only the unmarked portion of the subcategorization is inherited. The "inheritance model" suggests that overgeneralizations and subsequent retreat from overgeneralizations follow in a principled way from the formal relationships that learners posit between items. These two predictions of the inheritance principle were tested in a series of experiments with groups of children aged three to seven. The form of the model derived for the domain of morphology suggests parallel models for other domains of grammar and a programmatic approach to the study of overgeneralization and retreat in language acquisition. (AMH)