In alternations, not all semantic motivation comes from semantic contrast

Functional explanations of alternations often invoke semantic contrast between alternates. In some cases, however, new alternations may arise not to code contrast but simply because the grammar supports multiple roughly equivalent solutions to the same coding problem. Our study illustrates this by exploring the history of English prepositional phrase complements (PPCs) to mental predicates, with a special focus on about and at (as in everyone was enthusiastic about the project). It is shown that about saw a dramatic extension in its use as a PPC marker, resulting in various new alternation contexts. These developments are concomitant to changes in the general semantic structure of about that resulted in stronger grammatical sanction to its use as a PPC marker. Compared to about, at has a far more stable semantic profile overall, contributing to greater stability in its use as a PPC marker as well. In other words, distributional changes in the system of PPCs, and the new alternations that arise as a result, are a side effect of changes in the overall semantic profiles of the prepositions used to mark PPCs.