ABSTRACT The Shallow Structure Hypothesis (SSH) has been highly influential in the study of adult second language (L2) acquisition. With respect to L2 sentence processing, the essential claim is that L2 speakers rely less on syntactic information and more on alternative sources of information (e.g., lexical semantics and discourse) than adult first‐language (L1) speakers. This idea, however, has been controversial, and the ample body of empirical research on it has produced a pattern of indeterminate, mixed, and contested results, potentially due to important theoretical and methodological differences in the studies. This article reviews the SSH as a theory of L2 sentence processing and considers the ongoing story that prominent work seems to be telling about it. Whatever amount of truth the SSH may turn out to capture, we find overall that the hypothesis and extant research on it have played a beneficial role in driving forward the field's understanding of L2 sentence processing in general. We are optimistic that future work will as well—especially if it continues to trend towards greater theoretical and methodological acuity—and we provide recommendations for advancing the state of the science.
Swanson et al. (2026) studied this question.