Do developmental changes in inhibitory ability underpin developmental changes in intelligence

Abstract Intelligence in children increases with age until adult levels of performance are achieved. Dempster (1991) proposed that developmental changes in inhibitory processes underpin these changes in the development of intelligence. The evidence Dempster presented to support this thesis typically takes the form of noting changes in inhibitory performance that occur in the same time frame as changes in psychometric intelligence (Dempster, 1991, 1992, 1993; Dempster & Corkill, 1999). He also provides correlational evidence from studies in which intelligence scores are correlated with various inhibitory measures. One problem with much of the evidence presented by Dempster is that it does not distinguish between developmental and individual differences in inhibition and/or intelligence. Developmental differences are differences in performance between children at different ages. Individual differences are differences in performance between children of the same age. The majority of evidence Dempster provides concerns individual differences in inhibition and the relationship of these differences to intelligence rather than the relationship of any developmental differences to intelligence. Anderson (1987) suggests that the processes underpinning these two types of differences are not necessarily the same. For example, individual differences may be related to speed of processing, while developmental differences may be related to changes in inhibitory ability. Therefore, a more accurate test of Dempster’s thesis is to assess whether developmental changes in inhibition are related to developmental changes in intelligence, rather than whether individual differences in inhibition are related to intelligence. This was the primary goal of this thesis. A secondary goal was to address whether or not any developmental changes seen were primarily due to changes in inhibition or could be accounted for by changes in speed of processing. Measures which utilise difference score reaction time (RT) measures as inhibitory indices such as the stroop task do not typically account for this potential confound. A number of researchers have addressed this problem of difference score measures and proposed alternative analytic techniques (Christ, White, Mandernach, & Keys, 2001; Christ, White, Brunstrom, & Abrams, 2003; Faust, Balota, Spieler & Ferraro, 1999). Each inhibitory measure used in the current study will attempt to control for group and individual speed differences, either by utilising one of these alternative techniques or i Abstract using regression analysis to identify the contribution of speed to the developmental shift in intelligence.using regression analysis to identify the contribution of speed to the developmental shift in intelligence. To assess the relationship of developmental changes in inhibition and the relationship of these changes to changes in intelligence seven inhibitory tasks and three psychometric intelligence tasks were completed by 360 children aged 7 to 11 and 100 young adults aged 17 to 21. The inhibitory measures were the Stop Signal task, the go-nogo task, 2 versions of the Simon task, the spatial Stroop task, inhibition of return, and the antisaccade task. The three intelligence measures were the Cattell Culture Fair, Ravens Progressive Matrices and the Wechsler Intelligence Scale for Children (WISC-III). Developmental trends in inhibitory measures were found for the Stop Signal task, the go-nogo task, the two Simon tasks and the antisaccade task. However, the developmental trends evident for the difference scores for the Simon tasks and one of the difference scores for the antisaccade task disappeared once the scores were transformed to account for group and individual differences in speed, indicating that these changes should not be considered developmental changes in inhibition. Performance on the different inhibitory indices was in general not correlated. Three measures showed a relationship with age and fluid intelligence: Stop Signal RT; go-nogo error rate and antisaccade error rate. The relative contribution of each of these inhibition measures to fluid intelligence (as measured by Ravens scores) was found to be relatively small with R-squared values of 1.8%, 5.2% and 13% respectively. For the go-nogo task the age-related contribution of speed was larger than the age-related contribution of inhibition. This is counter to Dempster’s thesis. For the Stop Signal RT and antisaccade error rate the age-related contribution was larger than the speed-related contribution. These findings provide minimal support for Dempster’s thesis. However, the major proportion of developmental variance in fluid intelligence scores between the ages of 7 and 11 remains unaccounted for. In conclusion, Dempster’s thesis that developmental changes in inhibition underpin the developmental changes in intelligence is not supported by the findings from the present study.