The first scholars to emerge with a specific culprit in hand were Betty Hart and Todd R. Risley, child psychologists at the University of Kansas, who in 1995 published the results of an intensive research project on language acquisition. Ten years earlier, they recruited 42 families with newborn children in Kansas City, and for the following three years they visited each family once a month, recording absolutely everything that occurred between the child and the parent or parents. The researchers then transcribed each encounter and analyzed each child's language development and each parent's communication style. They found, first, that vocabulary growth differed sharply by class and that the gap between the classes opened early. By age 3, children whose parents were professionals had vocabularies of about 1,100 words, and children whose parents were on welfare had vocabularies of about 525 words. The children's I.Q.'s correlated closely to their vocabularies. The average I.Q. among the professional children was 117, and the welfare children had an average I.Q. of 79.
When Hart and Risley then addressed the question of just what caused those variations, the answer they arrived at was startling. By comparing the vocabulary scores with their observations of each child's home life, they were able to conclude that the size of each child's vocabulary correlated most closely to one simple factor: the number of words the parents spoke to the child. That varied greatly across the homes they visited, and again, it varied by class. In the professional homes, parents directed an average of 487 “utterances” — anything from a one-word command to a full soliloquy — to their children each hour. In welfare homes, the children heard 178 utterances per hour.