General cognitive function and the brain's structural connectome

General cognitive function ('g') reflects a broad capacity for flexible information processing, yet how it is supported by brain-wide structural connectivity remains unclear. We mapped this relationship in 38,824 individuals (26-84 years) across three cohorts, showing that g is supported by a widely distributed white matter network whose macroscopic wiring capacity, microstructural organisation, and age sensitivity contribute in distinct ways. Across streamline count (SC), fractional anisotropy (FA), and mean diffusivity (MD) weightings, meta-analytic associations with g were widespread at global, nodal and edge levels, spanning all cerebral lobes and key subcortical structures and dependent on both inter- and intra-hemispheric connectivity, particularly ipsilateral long-range inter-lobar connections. White matter node-g associations spatially mirrored independent cortical morphometry-g associations, indicating convergence of grey and white matter contributions to cognitive performance. Effect sizes increased with age: MD associations became more negative and FA more positive, particularly in frontal regions. Edge-level findings replicated across cohorts and predicted g in a hold-out sample. Together, these findings indicate that g reflects a distributed structural communication backbone whose integrity becomes increasingly relevant across adulthood.