Spearman’s Hypothesis in the Vietnam Experience Study and National Longitudinal Survey of Youth ‘79

There are few empirically derived theories explaining group differences in cognitive ability. Spearman's hypothesis is one such theory which holds that group differences are a function of a given test's relationship to general intelligence, g. Research into this hypothesis has generally been limited to the application of a single method lacking sensitivity, specificity, and the ability to assess test bias: Jensen’s method of correlated vectors. In order to overcome the resulting empirical gap, we applied three different psychometrically sound methods to examine the hypothesis among American blacks and whites in the Vietnam Experience Study (VES) and the National Longitudinal Survey of Youth 1979 (NLSY ‘79). We first used multi-group confirmatory factor analysis to assess bias and evaluate the hypothesis directly; we found that strict factorial invariance was tenable in both samples and either the strong or the weak form of the hypothesis was supported, with 87 and 78% of the group differences attributable to g in the VES and NLSY ’79 respectively. Using item response theory metrics to avoid pass rate confounding, a strong relationship between g loadings and group differences (r = 0.80 and 0.79) was observed. Finally, assessing differential item functioning with item level data revealed that a handful of items functioned differently, but their removal did not affect gap sizes much beyond what would be expected from shortening tests, and assessing the effect this had on scores using an anchoring method, the differential functioning was found to be negligible in size. In aggregate, results supported Spearman's hypothesis but not test bias as an explanation for the cognitive differences between the groups we studied.