Agreement configurations in language development: A movement-based complexity metric

•A new metric of complexity based on movement relative to L1 language acquisition.•The experimental study of morphological agreement in a forced choice paradigm.•Empirical support for the selective delay of complex agreement configurations.

Three different agreement configurations in Italian (Determiner–Noun, Subject–Verb, Clitic–Past Participle) can be naturally ranked from a minimum to a maximum of complexity in terms of the movement operations they necessarily involve, and of the derived representations at the interfaces. We put forth the hypothesis that this complexity ranking has predictive capacities with respect to the timing of full mastery of the different configurations in acquisition: a more complex configuration is expected to be fully mastered later than a less complex configuration. We check the consistency of the predicted sequence with the available data from corpus studies. Then, we test the prediction experimentally through the Forced Choice of Grammatical Form paradigm with children of age three, four and five acquiring Italian.