Regularities in Spearman's Law of Diminishing Returns

Abstract

The lower average correlations among mental tests in groups of individuals of above-average IQ than is found in groups of individuals of below-average IQ implies a smaller general factor, or g, in the higher-IQ group, a phenomenon Spearman called the ‘Law of Diminishing Returns’. This phenomenon was confirmed in six independent data sets based on three different test batteries. It implies that psychometric g is less general in persons of high IQ and more general among those of low IQ. Two heretofore unknown effects were revealed consistently: (1) The g loadings of the various tests in a battery do not differ uniformly or randomly between high- and low-IQ groups; rather, the loadings are much more variable for higher-IQ samples than for lower-IQ samples; (2) Those tests in a battery that are less g-loaded (where g loadings are based on the general population) consistently show greater decrements in their g loadings between low- and high-IQ groups than do the more highly g-loaded tests, which show relatively little difference between low- and high-IQ groups.