Aptitude Testing

Hull argues that aptitude testing, like medicine and engineering, is ceasing to be a job for amateurs and becoming the work of technically trained professionals. The book's purpose is twofold: to present the fundamental principles of aptitude testing and to describe the most effective and economical methods of constructing batteries of aptitude tests. Hull contends that prediction of human performance requires the systematic combination of multiple measures, weighted by their empirical contributions, rather than reliance on single tests or subjective judgment. The book is designed both as a textbook for university classes and as a handbook for practitioners in vocational guidance, personnel work, and employment selection.

Hull identifies the multiple regression equation as the only really adequate procedure for combining raw test data into prognostic scores. He surveys alternative methods—letter scales, percentiles, IQ, standard deviation units, T-scores—and finds each insufficient for the task of predicting success from multiple predictors. The multiple regression equation allows each test's contribution to be weighted according to its correlation with the criterion, producing a composite score that maximizes predictive accuracy. Hull provides a brief and simple explanation of the technique, making it accessible to readers without advanced statistical training. This advocacy of multiple regression was part of his broader effort to mathematize applied psychology.

Hull distinguishes sharply between aptitude signs and aptitude tests. An aptitude sign is a more or less reliable indicator of aptitude behavior, but not a sample of it; an aptitude test, by contrast, directly samples the behavior in question. The book devotes substantial attention to discredited signs: physiognomy, chiromancy (palm-reading), chiropnamy (hand-reading), and handwriting analysis. Hull presents empirical evidence against the validity of character judgments based on photographs, showing that viewing subjects in three dimensions offers no advantage over photographs. Certain head-measures do offer slight correlations with university grades, but Hull treats these as weak and unreliable. This skeptical treatment of anatomical signs was a significant contribution to professionalizing the field.

Hull estimates that the respective contributions toward success of capacity or ability, industry or willingness, and chance or accident are approximately 50 percent, 35 percent, and 15 percent, in that order. He presents this as a mere estimate and does not claim precision for the figures. A contemporary reviewer noted that the meaning of each factor and the accuracy of the estimate require further consideration. This estimate is significant as an early attempt to decompose the determinants of achievement into measurable components, though it rests on assumptions about the stability and independence of the factors that later research would question.

Reviews were generally positive about the book's practical utility and readability. The Journal of Applied Psychology called it scientific, readable, and an able contribution to a field where good texts are few, noting that it was seldom that humor entered into a scientific work of this sort. A review in the American Journal of Psychology praised the clarity of Hull's exposition of intelligence theories and the multiple regression method but raised methodological concerns. The reviewer noted that the normality of distribution of mental traits was assumed too uncritically, that no allowance was made for the fact that obtained distributions are too wide and must be corrected for chance factors, and that reliability measures of individual aptitude scores were not adequately considered.

Aptitude Testing represents Hull's early applied work in psychometrics, before he turned to the systematic development of behavior theory. The statistical and methodological commitments evident in the 1928 book—particularly the preference for formal, quantitative models and the use of multiple regression to combine predictors—anticipate the hypothetico-deductive approach he would later apply to learning. However, the substantive content differs: the later books (Principles of Behavior, Essentials of Behavior, A Behavior System) address the fundamental laws of learning and motivation, while Aptitude Testing addresses the practical problem of predicting occupational success. The book is best understood as a transitional work that established Hull's reputation as a rigorous methodologist before he became known as the architect of neo-behaviorism.