Abstract
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.
Highlights
The positive manifold of correlations among scores on diverse cognitive tests was discovered, and named as g, by Charles Spearman (Spearman, 1904)
We experimented with various cut-off points to slice the distribution into ability groups with closely similar standard deviations, but we found that skewness in the observed data contributed to distortions
Two clear results emerge from our study: first, the differences we found between the low and high ability groups were, with the exception of the 7 year olds, not significant and second, the low > high effect in the absolute strength of the manifold was systematic despite the differences in tests and administration, across age and across sex
Summary
The positive manifold of correlations among scores on diverse cognitive tests was discovered, and named as g, by Charles Spearman (Spearman, 1904). The reliability of g is greater than the reliability of height and weight measured in a doctor’s office (Jensen, 1998, p50), its predictive power leaves rival psychometric constructs in the dust yet, despite a century of research, certain properties of g are still unresolved
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