Social adversity is linked to poor health outcomes, yet clinically documented social hardship captured by ICD-10 Z55–Z65 codes has rarely been examined in relation to brain imaging markers. We investigated associations between documented social adversity and structural and microstructural brain measures in UK Biobank.

In this cross-sectional study of up to 37,184 participants with linked Hospital Episode Statistics and multimodal MRI data, exposure was defined as any Z55–Z65 code recorded between 2010 and 2020 prior to imaging. Outcomes included standardized structural MRI measures (gray matter, white matter, hippocampal volume, white matter hyperintensity burden) and diffusion MRI markers (fractional anisotropy, mean diffusivity, intracellular and isotropic volume fraction, orientation dispersion). Multivariable linear regression, propensity score matching (1:1–1:3), and inverse probability weighting were applied.

Before matching, exposed participants had lower income, higher comorbidity, poorer self-rated health, and greater medication use; covariate balance was achieved after matching. Z55–Z65 exposure was not associated with volumetric measures (|β| < 0.03 SD). In contrast, exposure was consistently associated with altered white matter microstructure, including lower fractional anisotropy (β ≈ −0.11 to −0.15 SD), higher mean diffusivity (β ≈ 0.08 to 0.11 SD), and lower intracellular volume fraction (β ≈ −0.11 to −0.15 SD), largely driven by psychosocial codes (Z60–Z65).

Clinically documented social adversity was linked to widespread microstructural alterations but not volumetric differences, suggesting diffusion MRI may be a sensitive marker of social stress–related brain aging.