Are the effects of g on achievement smaller at higher ability levels?




Carrigan, Jamison Elder

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Spearman’s Law of Diminishing Returns (SLODR) posits that the contribution of a general intelligence factor (g) is lower at higher ability levels in explaining individual differences in cognitive ability test scores. This phenomenon has been supported across a variety of methodologies. The present study sought first to model SLODR using factor mixture modeling wherein high- and low-ability classes were created using latent modeling instead of through a priori group selection. The linking sample of the WISC-V and WIAT-III was used (n = 181). This approach was supported in the present study.

Known classes were then generated based on this latent class model; these known classes were used to model the relation between intelligence and various achievement domains. As an extension of the SLODR phenomenon, it was hypothesized that the loadings of each achievement domain onto g would be lower in the high-ability group. This hypothesis was based on the premise that SLODR extends to domains that are highly correlated with intelligence such as achievement. Because SLODR posits that a given battery’s subtests are more intercorrelated in lower-ability groups, it would therefore follow that the correlation (and therefore the factor loading) between g and achievement domains in reading, mathematics, and writing similarly increase in a lower-ability group. In most cases, this hypothesis was supported; the standardized loading of the achievement domain onto g was lower in the high-ability class in seven of the nine measured achievement domains. These findings suggest that the relation between intelligence and achievement is not static across the ability spectrum as has previously been assumed. Further, this study suggests that psychologists conducting psychoeducational assessment should use a three-stratum model of intelligence in which broad abilities are also analyzed, especially for individuals with higher cognitive ability. Future research is needed to assess this hypothesis across batteries and methodologies.


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