In the field of child language acquisition studies, the emphasis has so far mainly been on monolingual children's language development. In their quest for explanations of this development researchers have increasingly turned to cross-linguistic studies, comparing with one another monolingual children acquiring different languages (see e.g. the recent collection of papers edited by Slobin 1985b). Such cross-linguistic research has, amongst others, been motivated by the question of the relative importance of language-universal vs. language-specific factors in acquisition (see e.g. Slobin 1985c, Berman 1986, Mills 1986c). It can be argued, however, that comparisons between monolingual children acquiring different languages are not the ideal empirical basis for addressing this issue. After all, in such comparisons most psycho-social variables cannot be held constant, and thus one can never be certain what the precise reasons are for any differences or similarities found in the acquisition patterns of children learning different languages: these may be due to purely linguistic factors, but also to other factors having to do, amongst others, with differences in cognitive development, cultural environment or socialization patterns.

A child growing up with two languages from birth, on the other hand, offers a unique opportunity for investigating general theoretical issues in the language acquisition field, since here the number of possibly influential variables is reduced to a minimum: the bilingual child comes the closest to being the ‘perfect matched pair’.