The latent organization of white matter microstructure and its relation to fluid intelligence

White matter microstructure is a candidate neurobiological substrate underlying individual differences in fluid intelligence, potentially through differences in neural information transfer. Investigating the association between white matter microstructure and fluid intelligence requires precise modeling of these microstructural properties across the brain. Yet, it remains unclear whether MRI-derived markers of white matter microstructure generalize across tracts to support latent modeling approaches. Therefore, our primary objective was to derive measurement models for markers of white matter integrity (fractional anisotropy, FA), neurite density (intra-neurite volume fraction, INVF), and myelin content (magnetization transfer ratio, MTR) across 52 tracts (HCP-1065 atlas) grouped into 10 functional clusters. We investigated data of N = 365 individuals (age range: 18-74 years) drawn from two independent samples (Dortmund Vital Study: N = 150, Clinicaltrials.gov: NCT05155397; Mainz Network Study: N = 215). Confirmatory factor analyses consistently favored hierarchical bifactor models, capturing both a general factor per marker and orthogonal hemisphere-specific factors, independent of participants' age. The general factors FA and MTR of the favored measurement models were significantly associated with fluid intelligence, assessed with matrix reasoning tests, FA: β = 0.26, p < .001 and MTR: β = 0.25, p = .017. When controlling for age, the association of fluid intelligence with FA remained significant, β = 0.14, p < .043, while the association with MTR was no longer significant, β = 0.11, p = .328. These findings establish anatomically informed measurement models for white matter microstructure and provide a scalable framework for investigating the biological underpinnings of cognitive abilities.