Challenging the ''law of diminishing returns''

The law of diminishing returns (Spearman, 1927) states that the size of the average correlation between cognitive tasks tends to be relatively small in high-ability groups and relatively high in low-ability groups. Studies supporting this finding have tended to contrast very low-ability subjects (IQ < 78) with subjects from higher ability ranges and to use tests that have poor discriminatory power among the higher ability levels. In the first study described in this article, tasks that provide good discrimination among the higher ability levels were used. A sample of high-ability (N = 25) and of low-ability (N = 20) 15-year-old boys completed four single tests, two with low and two with high g saturations, and two competing tasks formed from these single tests. The results indicated that, contrary to the predictions of the law of diminishing returns, the amount of common variance was greater in the high-ability group. It is suggested that the law of diminishing returns does not take into account the factor of task difficulty and that there are situations in which the exact reverse of this law holds. A second study again compared correlations obtained with extreme groups (N = 28 and N = 29), this time on measures of perceptual speed, which are easy for all ability levels. Results indicated that correlations among the perceptual speed measures were the same for both groups. In neither of these studies was there any support for the law, which seems to be dependent on the very high correlations obtained from samples at the extreme lower end of the ability continuum.