DO INDIVIDUAL-DIFFERENCES IN SPEED REFLECT GLOBAL OR LOCAL-DIFFERENCES IN MENTAL ABILITIES

Two centuries of investigation suggest that decision times on simple l aboratory tasks can sometimes partially reflect speed of elementary ne urophysiological processes. Consequently, findings that decision times on a variety of simple tasks also modestly correlate with scores on i ntelligence tests have been taken as evidence that individual differen ces in general fluid ability also reflect individual differences in ne urophysiological efficiency. A further question is whether these diffe rences are ''global,'' in the sense that they affect ail cognitive sys tems equally, or ''local'' in the sense that they affect some cognitiv e systems more than others. A finding that test scores have constant p roportional effects on decision times for all cognitive tasks, irrespe ctive of their demands, would suggest a ''global'' effect, whereas a f inding that test scores have different effects on different tasks woul d 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 ''glob al'' hypothesis. So does a new analysis of data from 15 different cogn itive 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 perfor mance on some tasks more than on others. This article discusses implic ations of demonstrations of ''local'' rather than ''global'' individua l differences for recent models of the relationship between general ab ility and information processing speed.