Chapters 6, 7 and 8 have shown that in Kate's language production, her two languages form two separate, closed systems which exercise very little (if any) influence on each other on the morphosyntactic level. Thus, the Separate Development Hypothesis has been confirmed (see further chapter 11).

Another major finding is that Kate's third birthday marks a turning point in her linguistic development in general: in both languages, structures start to appear that were absent before, or initially rare patterns start to appear more frequently. It seems as if the child is suddenly much more intensively occupied with the formal aspects of language and their possibilities than before: language as a “formal problem space” (Karmiloff-Smith 1979) is much more at the centre of attention once Kate is into her fourth year. In order to show this more clearly, we have collated all the evidence in chapters 6, 7 and 8 that unequivocally indicates a change in any subsystem that Kate is in the process of acquiring (see Table 9.1).

It is quite striking that the many changes in Kate's two language systems take place at the same time. Furthermore, there are hardly any wide-ranging changes in her language production in the time between the beginning of the study and shortly after Kate's third birthday. So although the actual ‘contents’ of Kate's speech production is quite language-specific, there appears to be a mechanism at work here that concerns both languages at once.