THE IMPORTANCE OF INTRAINDIVIDUAL VARIATION IN REACTION-TIME

Individual differences in the trial-to-trial variability of reaction time (RT), indexed by the standard deviation of the individual's RTs over n trials (RTSD), generally has a larger negative correlation with psychometric g than does the median RT (RTmd) over n trials. Large data sets are brought to bear on the question of whether RTmd and RTSD, which are highly correlated, reflect one and the same source of variance, but with different reliability and validity for predicting g, or represent independent processes. Several lines of evidence consistently lead to the conclusion that RTmd and RTSD, though having a substantial proportion of their variance in common even when measured separately in experimentally independent sets of RT trials, also have significant independent components of variance, each of which is correlated with psychometric g. Hypotheses about the neurophysiological basis of individual differences in the independent components of RTmd and RTSD are discussed.