Inconsistent Flynn effect patterns may be due to a decreasing positive manifold: Cohort-based measurement-invariant IQ test score changes from 2005 to 2024

Abstract

Generational IQ test score changes in the general population (i.e., the Flynn effect, typically reported as increases of 2–4 IQ points per decade) have recently been observed to behave inconsistently. It has been speculated that these inconsistencies may be attributable to the well-established negative relation of test score gains with psychometric g. Here, we provide the first direct empirical investigation of cross-temporal changes in the positive manifold of intelligence. In this cohort-comparison study, we examined performance changes in two population-representative Germanophone samples (N = 1267) across six measurement-invariant intelligence subscales from 2005 to 2024. Our analyses revealed substantial declines in single-factor analysis-based g assessments (ΔR² range: ‐.037 to -.066) from 2005 to 2024. Despite this decrease in the positive manifold strength, we observed meaningful test score increases in all domains (d range: 0.18 to 1.24), with the largest gains in the lower tail of the intelligence distribution (i.e., conforming to Rodgers', 1998, idea of narrowing ability distributions). Our findings provide direct evidence for a decreasing strength of the positive manifold of intelligence as a noticeable driver of the accumulating evidence for negative Flynn effects, which may be a consequence of increasing ability differentiation in the general population.