Last time, I was talking about the hearing children of deaf parents. They could give us some interesting information on the effect of a verbally impoverished environment in early life , which is the currently fashionable explanation for low IQs and low academic achievement in some non-Parsee minority groups. But since there have been very few decent studies of these kids, we don’t have much info. At least I haven’t found much.
On the other hand, we have quite a bit of info about kids who are themselves deaf, and of course they have an even more impoverished verbal environment.
Deafness has a big impact. Average verbal IQ is 85, a standard deviation below normal. Nonverbal IQ is normal – in the case of deaf children raised by deaf parents, who have the most exposure to sign language, it may be higher than normal. Note that these deaf-of-deaf kids are almost entirely of European ancestry (96%), probably because of the common 35delG connexin-26 mutation, which likely gives heterozygotes some advantage.
Blacks in the US have a similar average verbal IQ, but also score lower on nonverbal IQ tests. In fact, their disadvantage is greater in nonverbal IQ than in verbal IQ.
Seems to me that limited verbal stimulation is not a very plausible primary cause of low test scores and low academic achievement in blacks, because the degree of deprivation needed to cause a 1-standard deviation decline is extreme (deafness), and because there is an even greater depression of nonverbal scores, which, judging from the results in deaf children, should not be affected at all by limited verbal stimulation.