...when children learn two languages from birth each language proceeds on its own independent course, at a rate that reflects the quality of the children's exposure to each language
One well established fact about monolingual development is that the size of children's vocabularies and the grammatical complexity of their speech are strongly related. It turns out that this is true for each language in bilingual children. But vocabulary and grammar in one language are not related to vocabulary or grammar in the other language.
Turns out, the data were consistent only with the final possibility -- that the rate of vocabulary and grammar development are a function of something external to the child and that exerts separate influences on growth in English and Spanish;
...the key takeaway from our study is that it's not the quantity of what the children are hearing; it's the quality of their language exposure that matters. They need to experience a rich environment.
In young bilingual children 2 languages develop simultaneously but independently