I had several goals in writing my keynote “Exposure and input in bilingual development”. The first was to emphasize that there are two components to the study of environmental effects on language learning. The first is the stuff ‘out there’ (exposure) that we want to observe and count and whose effects we want to assess; the second is the internal, mentally represented stuff (my input) that is logically related to a particular learning problem. Both exposure and input are indissociable from assumptions about what language acquisition mechanisms do and the nature of linguistic cognition. Accordingly, for example, a decision to count ‘words’ in child-directed speech (CDS) or via a parental questionnaire is not an innocent one. Not only can one find radically different views on what a ‘word’ is (Krause, Bosch & Clahsen, 2015), one can find work that questions the need to postulate such a unit at all (see discussion in MacWhinney, 2000). It follows that adopting a clear position, about which abstract mentally represented elements are crucial cues to learning some phenomenon, is an essential step in deciding what to count in CDS.
