# The average does not represent the individual: White matter variability across the brain, across the population, and across the lifespan

## TL;DR

This study shows that brain white matter variability is structured and changes across the lifespan, offering insights into brain development and individual differences.

## Contribution

The study reveals structured patterns of white matter variability across age and its link to individual behavioral differences.

## Key findings

- White matter variability follows a deep-to-superficial spatial gradient and shows convergence-divergence across development and aging.
- Microstructural and macrostructural features have distinct developmental trajectories and hemispheric asymmetries.
- Tract variability is strongly linked to individual behavioral differences, especially during development.

## Abstract

Inter-individual differences in brain anatomy are often treated as noise, yet may encode meaningful principles of organisation. Using >2,800 diffusion MRI scans spanning 0–100 years, we quantified spatial, microstructural and macrostructural variability across 64 major white-matter pathways. Variability is not random: spatial organisation follows a deep-to-superficial gradient, and variability across individuals shows a convergence–divergence profile, decreasing in early development and increasing in ageing. Microstructural and macrostructural features exhibit distinct trajectories and hemispheric asymmetries. Importantly, tract variability relates to individual differences in behaviour, most strongly during development. These results redefine white-matter variability as a structured, developmentally dynamic, lifespan-dependent feature of brain organisation and provide a foundation for normative modelling and precision neuroscience.