In the landmark study published in their book Meaningful Differences in the Everyday Experience of Young American Children, Betty Hart & Todd Risley recorded parent-child spoken interactions in 42 families. They discovered that some children heard 30 million fewer words by their fourth birthdays than others—and that this “word gap” has long-term effects on later child outcomes. Meaningful Differences has had a huge impact in the twenty years since Brookes published it, prompting followup studies on the word gap and major projects like the Thirty Million Words® Initiative.

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