This study adjudicates between two opposing accounts of morphological productivity, using English past-tense as its test case. The single-route model posits that both regular and irregular past-tense forms are generated by analogy across stored exemplars in associative memory. In contrast, the dual-route model posits that regular inflection requires use of a formal “add -ed” rule that does not require analogy across regular past-tense forms. Children saw animations of an animal performing a novel action described with a novel verb. Past-tense forms (...) of novel verbs were elicited by prompting the child to describe what the animal “did yesterday.” Collapsing across age group, the likelihood of a verb being produced in regular past-tense form was positively associated with the verb's similarity to existing regular verbs, consistent with the single-route model only. Results indicate that children's acquisition of the English past-tense is best explained by a single-route analogical mechanism that does not incorporate a role for formal rules. (shrink)