Intelligence differentiation in adult samples

There is a renewed interest in the so-called differentiation theory. This theory states: the higher the level of g, the less the amount of g variance in any particular cognitive test. The implication of the differentiation theory for the scientific concept of intelligence is noteworthy: g could be more germane for low-ability than for high-ability people. A first battery of cognitive tests was applied to a sample of 3430 participants (mean age = 23.12 years). The sample of the Spanish standardization of the WAIS-III was also analyzed (823 participants with a mean age = 34.41 years). The methodology of Deary et al. [Intelligence 23 (1996) 105] was used for generating fine-grained low- and high-ability groups. The percentage of variance explained by g was computed for low- and high-ability groups, respectively: the mean percentages were 45.85 and 43.96 in the first cognitive battery, and 49.03 and 36.67 in the WAIS-III. These results support the differentiation of intelligence across the range of ability. However, a sample effect is observed: WAIS-III results are more supportive of the differentiation theory. This evidence suggests that ability differentiation could be related to educational differences