After decades of collaborating to increase child language vocabulary, Betty Hart and Todd Risley spent 2 1/2 years intensely observing the language of 42 families throughout Kansas City. Specifically, they looked at household language use in three different settings:
1) professional families;
2) working class;
3) welfare families.
Hart and Risley gathered an enormous amount of data during the study and subsequent longitudinal follow-ups to come up with an often cited 30 million word gap between the vocabularies of welfare and professional families by age three. This number came from the data that showed welfare children heard, on average, 616 words per hour, while children from professional families (essentially children with college educated parents) heard 2153 words per hour.
The longitudinal research in the following years demonstrated a high correlation between vocabulary size at age three and language test scores at ages nine and ten in areas of vocabulary, listening, syntax, and reading comprehension. This study was subsequently used to fuel the fire of arguments for early childhood programs such as Head Start.