This has been a fantastic thread so far.
Cricket, I agree with you about the flaws inherent to retrospective studies, but I wonder about this:
I know too many parents, for instance, who have told me that their kids were speaking in sentences at 18 months whose kids were barely saying a few words at that age (and not combining any of them).
No doubt some of these parents were overrepresenting their kids, or being lax on the meaning of the word "sentence", but how do you know their kids were "barely saying a few words at that age". Pretty much every kid I know does way more in the comfort of their house than they do in public. Now this is perhaps an extreme case (and it's not about age; just about the fact that the evidence in front of a person may not be all the evidence there is), but my dd spent her 2nd year in a day care with three fantastic "teachers" who were very loving to her, and whom she was fond of and at least one of them never heard her say a SINGLE word for over half the year. DD is not 2E, she is not particularly shy, and she talks (and at that time was talking) like a parrot at home. Her teacher didn't know she could say two words (except for the fact that I kept on reassuring her that she talked, and relating anecdotes from day care that dd would tell me about), much less would she have guessed that dd is gifted. So... MMV?
