It is often asserted that the acquisition of morphosyntax in the first language and in late L2 learning are “fundamentally different” (e.g. Bley-Vroman 1989, 2009): L1 acquisition is often assumed to be uniformly successful, with all learners converging rapidly on the same grammar, while late L2 learners show vast individual differences in attainment and rarely achieve native-like proficiency (e.g. DeKeyser 2000, DeKeyser et al. 2010, Johnson and Newport 1989). This has led many researchers to conclude that first and second language acquisition rely on different learning mechanisms. Specifically, the acquisition of L1 grammar is supposed to rely (almost) exclusively on implicit learning, which is assumed to be diminished in adults. As a result, adults need to compensate by relying on explicit learning, as evidenced by robust correlations between L2 learning achievement and language aptitude.

              In this talk, I argue that the fundamental difference between first and second language acquisition is found mostly in the eyes of beholder. I show that there are considerable individual differences in L1 ultimate attainment. These are particularly striking if we consider speakers with low academic attainment and heritage language learners. Furthermore, I present findings suggesting that L1 ultimate attainment is also strongly correlated with performance on aptitude tests – in fact, this relationship may be even stronger than in the L2. I conclude that L1 and L2 acquisition rely on the same learning mechanisms (albeit not necessarily to the same extent).