Why Is Reaction Time Correlated With Psychometric g?

It seems almost incredible that in dividual differences in reaction time (RT) in simple tasks that involve no intellectual content and are so easy as to be performed by most persons in less than 1 s should be correlated with scores on nonspeeded, com plex tests of reasoning ability, vo cabulary, and general knowledge? the kinds of content that compose IQ tests. Nevertheless, in recent years, the correlation between RT and IQ has become an empirically well es tablished fact, based on thousands of subjects in scores of studies con ducted in many laboratories around the world.1 What might all these studies tell us about the nature of hu man intelligence? Before trying to answer this question, it is necessary to summarize some concepts and empirical generalizations about the key phenomena.2