I think comments here don't focus on the most important point of this study, and article:

- citation: But the findings don’t necessarily mean kids speak more because they hear more speech, he says: Instead, “Adults [might] talk back more to kids who produce more language.”

- article show no evidence of socio-economic difference contrary to what was claimed before

- article have a huge database over the world, which makes it a strong study.

- article is a baby step: the next step are to know if it's talking to the child or if it's talking beside the child, for example.