Bilingual acquisition as the locus of syntactic change

Some grammatical phenomena are more resistant to diachronic change than others. The syntactic core is particularly resilient, raising the question why this is the case and what causes the least vulnerable properties to change. Since fundamental alterations of grammars do not occur across the lifespan of adults, first language acquisition is commonly considered to be the main locus of syntactic change. Under the assumption that language contact leads to cross-linguistic interaction, early bilinguals have been claimed to be the main agents of change. I revisit this debate, focusing on head directionality and V2. Summaries of studies of various acquisition types lead to the conclusion that reanalysis in core syntax does not happen in the course of neither monolingual nor bilingual L1 acquisition. Contrary to hypotheses entertained in diachronic linguistics, neither language contact nor structural ambiguity/complexity has this effect. For core properties to change in L1, the triggering information must be contained in the input. Insufficient exposure, as in heritage language acquisition, can cause morphosyntactic change, though not in the syntactic core. Only second language acquisition exhibits such effects. L2 learners are thus the most likely agents of fundamental syntactic change. I conclude that explanations of the resilience of syntactic phenomena cannot rely exclusively on structural aspects. It results from an interaction of syntactic and developmental factors, defined by grammatical constraint, acquisition principles, and processing demands.