The L1 morpheme studies do not allow us to say whether inflectional morphemes act as triggers as Clahsen [Clahsen, H., 1991, Constraints on parameter setting: A grammatical analysis of some acquisition stages in German child language, “Language acquisition: A Journal of Developmental Linguistics” 1(4), pp. 361-391] has claimed for L1 acquisition. According to this position, referred to as the "continuity hypothesis, principles and categories of UG are available from the beginning but need to be activated or triggered by properties of the input".
- Zobl & Liceras (2004), Pag. 243