The use of dialect and often 'ungrammatical' English in the speech of children called disadvantaged lends plausibility to the popular belief that these children's generally lower IQs and scholastic progress are attributable to environmentally caused verbal and linguistic deficits. We read that
Children from low socioeconomic groups develop deficits in intellectual functioning because they lack adequate intellectual, particularly verbal, stimulation ... children in these groups receive less verbal stimulation from parents - through being talked to, read to, taken on trips, etc. - than children in middleclass groups, and the parents are usually not very good examples for children to follow in learning language. (Furfey & Harte, 1970, p. 313) It is to be expected that children from homes where certain words are used will do better on a vocabulary test involving those words than will children from homes where the words are never heard . . . most intelligence tests are loaded with middle-class content that is found to be more familiar to white children than to Negro children. (Brown, 1965, p. 186)
Such statements do indeed appear very plausible, even self-evident. But is linguistic deprivation actually an adequate explanation of
intelligence differences? The point is not at issue that learning good English is an advantage to upward social mobility. We are not concerned here with these secondary social consequences of grammar and dialect, but rather with the effect of language on
intelligence and intelligence test scores. Several lines of evidence are highly relevant in evaluating the linguistic deprivation hypothesis of intellectual deficit.