Intelligence and the "personal equation"

Over two centuries ago astronomers, studying star transit time, noted individual differences in reaction times. This observation came to be known as the "personal equation." Despite early efforts to establish a connection between RT and general intelligence, it is only fairly recently that speed of information processing has again become a major focus as a critical element of intelligence. Much of the systematic work on this association has been conducted by Jensen. In addition to firmly establishing the relationship between RT and intelligence, Jensen has explored the role of intraindividual variability. Several generalizations can now be made: 1) average RT and standard deviation of RT are correlated, but each has a unique connection with intelligence; 2) RT and standard deviation of RT reflect the operation of different processes; 3) the more complex the RT task, the greater the influence of intraindividual variability, 4) the contribution of intraindividual variability increases as IQ decreases; and 5) there are strong neurological and genetic influences on intraindividual variability. Jensen proposed a "neural oscillation" model to explain various features of intraindividual variability. His theory blends facts uncovered through behavioral research on RT with neurological events.