IQ predicts achievement – but not the other way around

Are cognitive g and academic achievement g one and the same g? An exploration on the Woodcock–Johnson and Kaufman tests.

Scott Barry Kaufman, Matthew R. Reynolds  , Xin Liu  , Alan S. Kaufman  , Kevin S. McGrew   Intelligence (2012)

http://scottbarrykaufman.com/wp-content/uploads/2012/02/Kaufman-et-al.-2012.pdf

In a previous post I wrote that IQ scores correlate 0.4-0.7 with grades in school.  School grade level in influenced by other factors beyond intelligence (personality factors, quality of instruction, home and school environment etc.).

It turns out that the correlations between "cognitive g" (general intelligence which is estimated by the IQ score) and "achievement g" (scores in tests measuring mainly reading, writing and math that have national norms) – are higher.

Some of the modern intelligence tests (like the later versions of the Wechsler, Kaufman and Woodcock Johnson) have an achievement battery in addition to the cognitive battery.  The achievement battery measures mainly reading, writing and math.

The authors of this study assessed the correlations between children's test scores in the national American sample of the KABC2 and KTEA2 (Kaufman cognitive and achievement batteries) and between children's test scores in the national American sample of the WJ3COG and WJ3ACH (Woodcock Johnson cognitive and achievement batteries).

They found that the average correlation across ages (4-19) between scores in KABC2 and KTEA2 is 0.86!

The average correlation across ages between scores in WJ3COG and WJ3ACH is 0.80!

What does it mean?

Intelligence tests are very good predictors of academic achievement.  One of the leading intelligence scholars in the world, Deary, found that the correlation between g measured at age 11 and achievement (in national examinations across 25 different subjects) measured at age 16 was 0.81!

 But the other way around may not be true:  in a research done with special education students, intelligence was predictive of future achievement whereas achievement was  not  predictive of future intelligence.  Current achievement may reflect lack of motivation, lack of opportunity to learn, bad instruction, learning disability (that lowers achievement but is reflected in only a few low cognitive abilities).

The high correlation between intelligence and achievement scores is also a result of mutual influences and interactions between achievement and cognition: cognitive abilities influence learning, and the learning process in school improves cognitive processes.