Scant evidence for Spearman's law of diminishing returns in middle childhood

In 1927, Charles Spearman suggested that general cognitive ability, or g, might be stronger at the low end of ability. We explored the manifold of g across the ability distribution in a large sample (range > 800 to > 4000 individuals) of British twins assessed longitudinally at 7, 9 and 10 years old using two verbal and two nonverbal tests at each age, thus testing effects of age on the saturation of g. We rankit-normalized the test scores, then used a median split on the test with the highest factor-loading. We derived the first principal component from the remaining three tests. We performed each analysis for the whole sample (within age) and also separately by sex. The first principal component explains more variance in g in the low ability group at every age and in both sexes separately but the F ratio eigenvalues show that, except at age 7 and principally in females, the difference between the low and high ability groups is not significant.