Do individual differences in speed reflect “global” or “local” differences in mental abilities?

Abstract

Two centuries of investigation suggest that decision times on simple laboratory tasks can sometimes partially reflect speed of elementary neurophysiological processes. Consequently, findings that decision times on a variety of simple tasks also modestly correlate with scores on intelligence tests have been taken as evidence that individual differences in general fluid ability also reflect individual differences in neurophysiological efficiency. A further questions is whether these differences are “global,” in the sense that they affect all cognitive systems equally, or “local” in the sense that they affect some cognitive systems more than others. A finding that test scores have constant proportional effects on decision times for all cognitive tasks, irrespective of their demands, would suggest a “global” effect, whereas a finding that test scores have different effects on different tasks would suggest local effects. Comparisons by many eminent psychometricians have suggested local specificity of effects. However, when replotted, their published data seem to offer equally strong support for a “global” hypothesis. So does a new analysis of data from 15 different cognitive tasks taken by 93 volunteers with scores on the Cattell Culture Fair test ranging from 11 to 40 points. However, further inspection of these results shows that rather than having equal, scalar effects on all tasks differences in Cattell scores systematically affected performance on some tasks more than on others. This article discusses implications of demonstrations of “local” rather than “global” individual differences for recent models of the relationship between general ability and information processing speed.