Processing speed and intelligence as predictors of school achievement: Mediation or unique contribution?

The relationships between processing speed, intelligence, and school achievement were analyzed on a sample of 184 Russian 16-year-old students. Two speeded tasks required the discrimination of simple geometrical shapes and the recognition of the presented meaningless figures. Ravenʹs Advanced Progressive Matrices and the verbal subtests of Amthauerʹs Intelligence Structure Test were used as intelligence scales. The teacher-assigned grades in six school subjects that were aggregated into two scales represented real-life school achievement. Latent processing speed and intelligence as individual predictors each accounted for about 18% of the variability in scholastic performance. Taken together, they explained about 28% of the variance of school achievement. Although significantly correlated, each had a unique impact on school achievement; zero-constraining each of the two paths to school achievement resulted in a significantly worsened fit of a model. A mediation effect processing speed → intelligence → school achievement was bootstrapped to obtain an estimate of its statistical significance and was found to be non-distinguishable from zero. The results are inconsistent with the causal hypothesis that states that processing speed is a predictor of real-life scholastic performance because of the impact of processing speed on higher-order cognitive ability, which in turn underlies school achievement.