In recent years research on language acquisition (both L1 and L2) has gradually turned away from the domain of pure description towards dealing with problems of explanation. Most students of language development feel that it is somewhat insufficient to merely state the linguistic regularities and verbal strategies that according to the available production data appear to govern processes of language learning; rather, it seems desirable that the regularities observable in the learner's verbal behavior over time be in some wayexplained. Frequently, the termexplanationis—implicitly rather than explicitly—understood as an attempt to reduce the specific linguistic regularities found in language learning processes to a set of some more general (i.e. non-linguistic) principles that will not only control other learning tasks as well, but will also govern man's mental activities as such (Slobin 1973, Schlesinger 1977, Larsen-Freeman 1976, Bever 1970, Wong-Fillmore 1976). Among those mental domains which appear to provide principles general enough to be applicable to problems of language, learning cognition or rather cognitive development turned out to be the most promising. As a consequence, a large number of studies focused on the attempt toexplainformal linguistic properties of language learning processes in terms of more general regularities in man's cognitive abilities as they develop (Seliger 1980, Gass 1980, Felix 1976, Sinclair-de-Zwart 1973, Meisel 1979, Clahsen 1979). In fact, the termexplanationfrequently means little more than reducing language acquisition to aspects of cognitive development.
