CELF = Clinical Evaluation of Language Fundamentals.
I'm sure Priiak will be on later to expand on this, but for now, my observations:
A few ways to think about how deficits in language may accompany long-term decreases in IQ scores.
1. deficits in reading: over time, a student who has a history of reading challenges, whether they are in word-level decoding, fluency, or comprehension, is exposed to far fewer words, and far less complex language, than a same-ability student without reading deficits. Up until about 3rd grade, when most children are not fluent readers, the gap is insignificant, as the vast majority of learners are acquiring their vocabulary and language exposure (incidental learning) from oral language. Beginning around 3rd grade, when the proverbial shift from "learning to read" to "reading to learn" occurs, struggling readers are at a language-environment disadvantage compared to their same-ability non-disabled peers--a disadvantage that tends to widen over time, especially when the deficits are not quickly remediated.
Even when dyslexia (e.g.) is remediated, there are years of reduced exposure (as well as emotionally-based beliefs and inclinations) that are not easily compensated.
2. The same reasoning behind the impact of reading delays applies to expressive language: Generally, we learn to write well by reading good writing, and by practicing writing. A child with expressive language challenges tends to avoid using language in the challenge areas, which means they get less constructive feedback for developing those language skills, which over time, means they lag against their same-ability non-disabled peers in language development. On formal testing, the earlier items tend to be more concrete-familiar, more likely to be visually or semantically cued, and require less sophisticated verbal responses in order to receive full credit. This provides the kind of scaffolding that a verbally-gifted, but language-impaired, child might be able to leverage in the early years.
3. Developmental curves: While verbal cognition and expressive language obviously are closely related, they are not identical, and may not follow the same developmental curve. Let's imagine (which is not far off the truth) that language development undergoes rapid development in early childhood, and then slows down through middle childhood, and even more so through adolescence, essentially leveling off in late adolescence. At the same time, verbal reasoning/cognition is undergoing its own development, usually in parallel. But possibly not. The complication is, we have to use some expressive language to assess verbal cognition, so if the expressive language curve encounters an obstacle, or is behind the verbal cognition curve, eventually the gap between the two may mean that when we attempt to assess verbal cognition, our assessment will be capped by expressive language.
