Multiple interfaces and incomplete acquisition

In recent years, explanations and predictions of developmental delays or inability to reach native-like attainment in specific areas of grammatical knowledge have been linked to the architecture of the language faculty. A recurrent claim is that linguistic properties at interfaces are inherently more “complex” than linguistic properties internal to a specific domain (syntax, phonology, semantics) due to the integration of different levels of linguistic knowledge/analysis. Even within interfaces, not all interfaces are created equal; some have been claimed to be even more complex, vulnerable, or more problematic than others. Slabakova (2008) claims that properties at the syntax–semantics interface are largely unproblematic in adult L2 acquisition, but functional morphology (the morphology–syntax interface), by contrast, is the bottleneck of acquisition. Drawing on the distinction between internal and external interfaces, Tsimpli and Sorace (2006) and Sorace and Serratrice (2009) have also argued that the syntax–semantics interface (an internal interface) is eventually acquired at the near-native level, while, the syntax–discourse interface (an external interface) presents prolonged difficulty in both L2 acquisition and L1 attrition. In this article I question these recent claims about vulnerability of specific interfaces based on empirical evidence from recent studies on incomplete acquisition in L2 and heritage language acquisition. I discuss examples of grammatical properties involving multiple interfaces and show how the concept of internal and external interfaces is problematic.