The Interface Hypothesis claims that external interfaces are more problematic for learners than internal interfaces/narrow syntax. By analyzing the empirical data from Spanish psych-predicate properties, this article assesses the adequacy of IH. Because the current findings are inconsistent with the IH that target-like pragmatics knowledge can precede syntactic awareness of the same construction, the author suggests the use of Integrative Model of Bilingual Acquisition, which accounts for non-native divergence by resorting to the interplay of a series of factors (i.e. formal complexity, L1-L2 parameter mapping, processing resources, and PLD). This more articulated model is not only able to account for the patterns in these experiments but also constitutes a more integrated explanation for the intricacies of the acquisition process.