>It is common knowledge in psychometrics that a standardized test of reading comprehension is a good proxy for an IQ test. But this is true only if the persons tested are already skilled in word reading. In the psychology of reading, it is important to distinguish between the processes of decoding the symbols that constitute written or printed words (also known as "word reading") and comprehension, or understanding sentences or paragraphs.
The acquisition of decoding skill in young children is highly related to mental age (and to IQ in children of the same chronological age). But after word reading skill is fairly mastered, it is only weakly diagnostic of IQ or g. Children with average or above-average IQ who, with the typical reading instruction in the elementary grades, are still having trouble with word reading by age eight or nine are usually regarded as having specific reading disability and are in need of expert diagnosis and special instruction.
Some 10 to 15 percent of school children are found to have a developmental reading disability. There are two main causes of reading problems, varying in severity and amenability to remediation. One is a slow rate of mental development (manifested as low IQ on nonverbal tests); the other is various forms of dyslexia, in which the reading disability is highly specific and unrelated to g. Children diagnosed as dyslexic may, in fact, obtain very high scores on g-loaded tests if the test does not require reading. Specific reading disabilities show up almost entirely in the decoding aspect of reading, and decoding per se is not highly g-demanding. However, unless the decoding process becomes highly automatized, it occupies working memory (the central information-processing unit) to some extent, thereby hindering full comprehension of the material being read.
People differ much more in reading comprehension than in decoding skill. And it is reading comprehension that is the most unavoidable of the g-loaded activities in the whole educational process. The educational psychologist Edward L. Thorndike, as early as 1917, likened the process of reading comprehension to that of reasoning. He well described the aspects of reading comprehension that demand the full use of working memory and cause it to be highly g loaded: "The mind is assailed as it were by every word in the paragraph. It must select, repress, soften, emphasize, correlate and organize, all under the influence of the right mental set of purpose and demand." Every one of the verbs used here by Thorndike describes a g-related function.
It is probably because of the g demand of reading comprehension that educators have noticed a marked increase in individual differences in scholastic performance, and its increased correlation with IQ, between the third and fourth grades in school. In grades one to three, pupils are learning to read. Beginning in grade four and beyond they are reading to learn. At this latter stage, a deficiency in decoding skills becomes a serious handicap for comprehension. The vast majority of pupils, however, acquires adequate decoding skill by grade four, and from there on, the development of reading comprehension, with its heavy g saturation, closely parallels the pupil's mental age (as measured by IQ tests). Except for the small percentage of persons with specific reading disabilities, the level of reading comprehension of persons who have been exposed to four or more years of schooling is very highly related to their level of g, as measured by both verbal or nonverbal tests.
Unless an individual has made the transition from word reading to reading comprehension of sentences and paragraphs, reading is neither pleasurable nor practically useful. Few adults with an IQ of eighty (the tenth percentile of the overall population norm) ever make the transition from word reading skill to reading comprehension. The problem of adult illiteracy (defined as less than a fourth-grade level of reading comprehension) in a society that provides an elementary school education to virtually its entire population is therefore largely a problem of the lower segment of the population distribution of g. In the vast majority of people with low reading comprehension, the problem is not word reading per se, but lack of comprehension. These individuals score about the same on tests of reading comprehension even if the test paragraphs are read aloud by the examiner. In other words, individual differences in oral comprehension and in reading comprehension are highly correlated.<
Arthur Jensen - 1998
The g Factor;