A pdf document by Dr. Todd R. Risley describing the results of the in-depth research he and his co-researcher, B. Hart, conducted in order to understand what happens in the everyday life of a child with particular focus on the aspects related to language acquisition.
Here is an extended interview with Dr. Risley in which the research is explored in greater depth.
One of the key discoveries was the gulf between families in the degree of talkativeness and how the degree of word exposure correlates so closely to later academic performance. In the taciturn families, children, by the time they enter school, will have been exposed in aggregate to thirteen million words. For the average family, thirty million words. In the most gregarious, forty-eight million words.
Here is another blog article around the research, Word Counts.