I've been pondering a question this morning largely out of ignorance, so I'm submitting this to you the reader. That is, if a child is born into a family from one linguistic culture but is immediately adopted into a family from another linguistic culture, should there be any effect on language development?
I've always assumed - really not even thinking much about it - that the human linguistic structural map is universal, a biological condition of being human. Particular languages are then simply plotted onto that map as a child's experience teaches her a certain vocabulary, grammatical structure, and cultural and social norms connected to that linguistic culture. So biological parenthood doesn't matter for language development. It does for genetic predispositions, perhaps for learning any language at all, but at least not in terms of the biological parents' culture and language. What does matter for language development is who the actual parents are, what language(s) they speak, where they're from, their culture, the experiences they go on to share with their child, etc.
Is this assumption wrong? Is it rather that cultural/ethnic background plays some role in shaping the pre-linguistic structure of language for any given individual? If so, then it could be the case that children from one culture raised in another might exhibit slower language development than the norm. If so, I imagine that this is not necessarily bad or good and actually may have many more benefits than drawbacks (such as a later ability to tack more easily between different interpretive frameworks). But this is all just guessing on my part.
To make this more concrete... friends of mine - English speakers living in the US who also speak a couple of other languages - adopted a child from Central America who has lagged in developing spoken English, the language spoken at home and by most people around him. He's not developmentally delayed in any other way. In fact, he's remarkably coordinated for a child of his age (around 2 1/2 years old). No one considers this a real problem at this point - kids all develop at different rates and there's nothing too far out of the norm for this little guy. The case just gives rise to this question.
Like I said, I haven't looked into this more carefully. There may be a simple answer or some renowned study of which I'm ignorant. Thoughts are welcome.