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.

中文翻译：

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双语习得是句法变化的根源

有些语法现象比其他语法现象更能抵抗历时变化。句法核心特别有弹性，提出了为什么会出现这种情况以及是什么导致最不脆弱的属性发生变化的问题。由于语法的根本改变不会在成年人的一生中发生，因此第一语言习得通常被认为是句法变化的主要场所。在语言接触导致跨语言互动的假设下，早期双语者被认为是变革的主要推动者。我重新审视这场辩论，重点关注头部方向性和 V2。对各种习得类型的研究总结得出这样的结论：无论是单语还是双语母语习得过程中都不会发生核心句法的重新分析。与历时语言学中的假设相反，语言接触和结构歧义/复杂性都没有这种效果。对于要在 L1 中更改的核心属性，触发信息必须包含在输入中。接触不足，就像在传统语言习得中一样，可能会导致形态句法的变化，尽管不是句法核心的变化。只有第二语言习得才能表现出这样的效果。因此，第二语言学习者最有可能是基本句法变化的推动者。我的结论是，对句法现象弹性的解释不能完全依赖于结构方面。它是句法和发展因素相互作用的结果，由语法约束、习得原则和处理要求定义。