This paper discusses the role of cognitive factors in language change; specifically, it investigates the potential impact of argument ambiguity avoidance on the emergence of one of the most well-studied syntactic alternations in English, viz. the dative alternation. Linking this development to other major changes in the history of English like the loss of case marking, I propose that morphological as well as semantic-pragmatic ambiguity between prototypical agents and prototypical recipients in ditransitive clauses plausibly gave a processing advantage to patterns (...) with higher cue reliability such as prepositional marking, but also fixed clause-level order. The main hypotheses are tested through a quantitative analysis of ditransitives in a corpus of Middle English, which confirms that the spread of the PP-construction is impacted by argument ambiguity and demonstrates that this change reflects a complex restructuring of disambiguation strategies. (shrink)