More recently, studies have tended to become more intensive and involve masses more data than the earlier studies which often consisted of researchers tracking their own kids' development (or in my case publicly humiliating my children by showing clips of them to my classes or writing about them in the textbook) . The Deb Roy study at MIT which was featured in the BBC Horizon Programme Why Do We Talk (see here and here for links and further reading) showed how huge amounts of data could be collected by filming a child through CCTV for literally years, and then processing that data to see exactly what happened when.
This study shows how another large scale investigation is shedding light on the acquisition of Standard English and African-American Vernacular English (AAVE) in children. The findings, as reported in Science Daily, show that African-American children arrive at school with quite a high incidence of vernacular features in their speech. This then drops off as they go through the first four years of school before spiking again at the end of elementary school and dipping as the children enter high school.
The article suggests that there are implications in these findings for the teaching of Standard English in schools, which might well be the case, but it would also appear that the data tells us a bit more about how young people feel about themselves and their vernacular forms as they get older. Language is of course part of a wider system of social signification as well as a set of words and structures to be acquired, so it's quite likely that as children get older their relationship with their own language will change, and as the research seems to suggest this isn't a regular, smooth process but one that goes in peaks and troughs.
It's interesting to see how children start to pick up slang from their peer group at school as they get older and how this sits with the different varieties of English they hear around them - at school and at home - so it's no surprise that a variety as well-established and widely used in the USA as AAVE will go through cycles as its users negotiate the complex territory of age, ethnicity, individual identity and mainstream, standard language forms.
One of the implications for English teaching might be that studying AAVE (or in the case of the UK, British Black English and other dialects or sociolects) alongside the Standard forms helps to develop a better understanding of both forms and their suitability for different situations. After all, what is being talked about here is quite a positive thing: being fluent in two languages/ codes rather than just one.
Other research into bilingualism in the early years of childhood would seem to suggest that having two or more languages around you is actually a very positive thing, leading to enhanced flexibility in thinking and even health benefits in later years:
Studies of children who grow up as bilingual speakers indicate they are often better at perspective-taking tasks, such as prioritizing, than monolingual children. Experiments with older bilingual speakers indicate that the enhanced mental skills may protect them from problems associated with ageing, such as Alzheimer's disease and dementia.So, as the report puts it, "juggling" two languages helps in many ways, and that doesn't just apply to recognised languages but also varieties of languages.
(If you can get hold of Graeme Trousdale's chapter Variation and Education in Analysing Variation in English (eds.Maguire, Warren & McMahon, April) you can find a clear explanation of how studying non-standard varieties is actually very good for our understanding of grammar in general, and helps with our grasp of standard forms too.)