A generally agreed upon definition of a native speaker takes the circumstances of
acquisition as the major criterion for defining a native speaker; that is, a native speaker is
seen as someone who acquires a language in early childhood (Piller, 2001; Davies, 2003).
Cook (1999) also sees ‘the language learnt first’ as a crucial element in defining what
constitutes a native speaker. However, in the case of balanced bilinguals who acquire two
or more languages simultaneously since the earliest days of language development, the
term ‘native speaker’ becomes more problematic.