The Interface Principle posits that morphosyntactic variation does not elicit the same kinds of perceptual reactions as phonetic variables because “members of the speech community evaluate the surface form of language but not more abstract structural features” (Labov, 1993:4). This article examines the effect of linguistic modularity on listeners' social evaluations. Our point of departure is the sociolinguistic monitor, a hypothesized cognitive mechanism that governs frequency-linked perceptual awareness (Labov, Ash, Ravindranath, Weldon, & Nagy, 2011). Results indicate that “higher level” structural variables are available to the sociolinguistic monitor. Moreover, listeners' reactions are conditioned by independent effects of region of provenance and individual cognitive style. Overall, our findings support the claim that sociolinguistic processing is influenced by a range of social and psychological constraints (Campbell-Kibler, 2011; Preston, 2010; Wagner & Hesson, 2014) while also demonstrating the need for models of sociolinguistic cognition to include patterns of grammatical variation (Meyerhoff & Walker, 2013; Walker, 2010).