Correlated Chronometric and Psychometric Variables

By far the most extensive literature on the relationship between chronometric and psychometric variables is found in the study of mental abilities, particularly general intelligence. Although the earliest empirical studies in this vein date back at least as far as the research of Galton in the late nineteenth century, over 95 percent of the literature on reaction time (RT) and individual differences in mental ability has accumulated over just the past two decades.

The virtual hiatus in this line of research lasted for about 80 years. It is one of the more bizarre and embarrassing episodes in the history of psychology, and one that historians in the field have not adequately explained. A chronology of the bare facts has been outlined elsewhere (Jensen, 1982, pp. 95-98); Deary (2000a, pp. 66-72) provides the fullest account of the misleading secondhand reports of the early studies perpetuated for decades in psychology textbooks. It is a marvelous demonstration of how utterly deficient studies escape criticism when their conclusions favor the prevailing zeitgeist.

The classic example here is the often-cited study by Clark Wissler (1870-1947), a graduate student working under James McKeen Cattell, the first American psychologist to be personally influenced by Galton. The circumstances of this study, overseen by this eminent psychologist and conducted in the prestigious psychology department of Columbia University, could not have been more auspicious. Published in 1901, Wissler's study tested Galton's notion that RT (and various other sensory-motor tests) is correlated with intelligence. The result was a pitifully nonsignificant correlation of - .02 between "intelligence" and RT. The null result of Wissler's test on Galton's idea is what was most emphasized in three generations of psychology textbooks. What their authors seldom pointed out was that all the cards were outrageously (but naively) stacked in favor of the null hypothesis, for example: (1) the severe restriction of the range-of-talent in the subject sample (Columbia University students), which has the statistical effect of limiting the obtained correlation; (2) "intelligence" was not measured psychometrically but merely estimated from students' grade point average, which in selective colleges is correlated with IQ not more than .50; and (3) the reliability of the RT measurements (based on only three trials) could not have been higher than 0.20, as determined with present-day chronometers. Under such conditions, a nonsignificant result was virtually predestined. Yet for decades this study was credited with having dealt the heaviest blow against the Galtonian position! It remained the standard teaching about the relationship between RT and IQ until recently, apparently in total blindness to the fact that in 1904 a now historic classic by the English psychologist Charles Spearman (1863-1945) was published in the American Journal of Psychology, giving detailed notice of the methodological inadequacies of Wissler's study, and also introducing the statistical formula for correcting a correlation coefficient for attenuation (unreliability) due to measurement error.

When I began doing research on the correlation between RT and IQ, in the late 1970s, nearly every psychologist I spoke to about it was at best skeptical or tried to disparage and discourage the idea, in the firm conviction that earlier research had amply proved the effort to be utterly fruitless. Their annoyance with me for questioning this dogma was evident, despite my pointing out that I could find no valid basis for it in the literature. But I did find at least a dozen or so published but generally unrecognized studies (some reviewed by Beck, 1933) that made my venture seem a reasonably good bet. My friends' surprisingly strong conviction and even annoyance that my research program was taking a wrong turn decidedly increased my motivation to pursue the subject further. I was further encouraged by the revival of chronometry for the study of individual differences in the promising research of Earl Hunt and co-workers (1975) at the University of Washington and also that of Robert J. Sternberg (1977) at Yale University. At the time I sensed a changing attitude in the air that perhaps presaged a second chance for the role of mental chronometry in differential psychology.

But I have still often wondered why there was so strong an apparent prejudice against the possibility of finding that RT and mental speed are somehow related to intelligence. Why had this idea been resisted for so long by so many otherwise reasonable psychologists? The most likely explanation, I suspect, is the entrenchment of the following attitudes and implicit beliefs in many psychologists. These attitudes were bedrock in the psychological zeitgeist throughout most of the twentieth century.

(1) Any performance measurable as RT to an elementary task is necessarily much too simple to reflect the marvelously subtle, complex, and multifaceted qualities of the human intellect. A still pervasive legacy from philosophy to psychology is the now largely implicit mind-body dualism, which resists reductionist physical explanations of specifically human psychological phenomena. Any kind of RT was commonly viewed as a merely physical motor reaction rather than as an attribute of mind. Disbelievers in the possibility of an RT-IQ connection pointed out that many lower animals, for instance frogs, lizards, and cats, have much faster RTs than do humans (which in fact is true). And when confronted with good evidence of an RT-IQ correlation, they dismiss it as evidence for the triviality of whatever is measured by the IQ. These obstacles to research on RT are supported by belief systems, not by empirical inquiry.

(2) The speed of very complexly determined cognitive behavior is often confused with the sheer speed of information processing. It is noted, for example, that duffers at playing chess seldom take more than a minute or two for their moves, while the greatest chess champions, like Fisher and Kasparov, at times take up to half an hour or more to make a single move. Or it is pointed out that acknowledged geniuses, such as Darwin and Einstein, described themselves as "slow thinkers." Or that Rossini could compose a whole opera in less time than Beethoven would take to compose an overture. "Fast but superficial, slow but profound" is a common notion in folk psychology. But these anecdotes take no account of the amount or the "depth" of mental processing that occurs in a highly complex performance. The few times I have played against a chess master (and always lost), I noticed that all their responses to my moves were exceedingly quick — a second or two. But in tournament competition against others near their own level of skill, these chess masters typically take much more time in making their moves. Obviously they must have to process a lot more complex chess information when competing against their peers.

(3) Applied psychologists have resisted pursuing the RT-IQ relation mainly for practical reasons. There has existed no suitable battery of RT measures shown to have a degree of practical validity for predicting external variables comparable to the validity of psychometric tests (PTs). Nor would RT tasks be as economical, as they require individual testing with a computerized apparatus with special software. So it is unlikely that RT tests could take over the many practical uses of standardized PTs, either individual or group administered. This is presently true. But RT methods have been conceived as serving mainly the purely scientific purpose of testing analytic hypotheses concerning the elemental sources of individual differences in the established factors of mental ability identified by complex PTs.

(4) Psychometricians have downgraded RT as a measure of cognitive ability because RT is mistakenly assumed to measure the same kind of test-taking speed factor that has been identified in PT batteries. This test-speed factor is observed in very simple tasks that virtually all subjects can easily perform. Individual differences in these highly speeded tests can be reliably measured only if the task is scored in terms of how many equally simple items the subject can execute within a short-time limit, such as 1 or 2 min, for example, the Digit Symbol and Coding subtests of the Wechsler Intelligence Scales. The common variance in these speeded tests typically shows up in a large factor analysis of various PTs as a small first-order factor with a weak relation to psychometric g. Its most distinguishing characteristic is its very low correlation with nonspeeded power tests, such as Vocabulary, General Information, and Similarities, or the Raven matrices. Various types of choice RT, on the other hand, have their highest correlations with the most highly g loaded nonspeeded PTs, and they show their lowest correlations with the speeded tests that define the psychometric speed factors, such as coding, clerical checking, and making Xs, which have the lowest g loadings of any PTs. In this respect, the psychometric speed factor is just the opposite of RT measures. So the mistaken equating of mental speed as measured in chronometric paradigms with scores on highly speeded PTs has given the former a bum rap.

The idea that mental speed may be importantly involved in variation in human intelligence was not universally deprecated in American psychology. Early on, one of the pioneers of psychometrics, Edward L. Thorndike (1874-1949), the most famous student of J. McKeen Cattell, attributed a prominent role to mental speed in his set of principles for the measurement of intelligence (Thorndike, Bregmamn, Cobb, & Woodyard, 1927). He also referred to these principles as the "products of intelligence." Within certain specified conditions and limits, all of these hypothetical generalizations have since been proved empirically valid. What is now needed is a unified theory that can explain each of them and the basis of their interrelationships. Because these five principles stated in Thorndike's The Measurement of Intelligence (1927) well summarize some of the most basic phenomena that need to be explained by a theory of individual differences in intelligence, they are worth quoting in full:

1. Other things being equal, the harder the tasks a person can master, the greater is his intelligence (p. 22).

2. Other things being equal, the greater the number of tasks of equal difficulty that a person masters, the greater is his intelligence (p. 24).

3. Other things being equal, the more quickly a person produces the correct response, the greater is his intelligence (p. 24).

4. Other things being equal, if intellect A can do at each level [of difficulty] the same number of tasks as intellect B, but in less time, intellect A is better. To avoid any appearance of assuming that speed is commensurate with level or with extent, we may replace "better" by "quicker" (p. 33). 5. It is important to know the relation between level [difficulty] and speed for two reasons. If the relation is very close, the speed of performing tasks which all can perform would be an admirable practical measure of intellect. The record would be in time, an unimpeachable and most convenient unit (p. 400).

A year before the appearance of Thorndike's 1927 book, two psychologists at Harvard published a small study based on only five subjects. RT was reported to be correlated a phenomenal - .90 and -1.00 with scores on two intelligence tests. In retrospect, these correlations are recognized as obvious outliers — not surprising for such small-sample correlations. It is amazing that the study was not immediately repeated with a much larger sample! Nevertheless, the authors' conclusion was on target in noting the promise suggested by their experiment: "If the relation of intelligence (as the tests have tested it) to RT of any sort can finally be established, great consequences, both practical and scientific, would follow" (Peak & Boring, 1926, p. 94).

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