(a) Reduced frontal MT growth rates with higher compulsivity (C, t = −2.494, p = .013, two-sided, df = 427) and impulsivity (I, t = −3.268, p = .0012, two-sided, df = 474) scores observed (illustrated with coloured median split group trajectories in left and right panel) using linear-mixed effects modelling testing time/visit by trait interaction (accounting for covariates such as subject’s mean age, sex, interactions etc., n = 497/288 scans/subjects in a-b, t (p) values from corresponding fixed-effects coefficients). Frontal, development-independent MT (across gray and adjacent white matter) at baseline distinguishes compulsivity (left panel) and impulsivity (right panel) with a ‘hypo-myelination’ in the latter dimension (t = 2.298, p = .022, two-sided, df = 474), but no baseline-effect in the former trait (t = 1.025, p = .30, two-sided, df = 474). Voxel-based analyses reveal similar effects: while impulsivity differences were linked to quantitative MT deficits (Fig. 4c) that further widens with development, compulsivity suggests a local decrease of a subtle myelin-related head-start (Fig. 3c). X-axis: time of scan in years relative to each subject’s mean age over all visits (cf. supplemental methods). Both dimensions showed a reduction in myelin-related longitudinal growth (impulsivity by time interactions: t = −2.795, p = .005, two-sided, df = 421; compulsivity by time: t = −1.99, p = .047, two-sided, df = 421). (b) Global growth rate was found to be associated with patterns of both compulsivity and impulsivity. Linear-mixed effects modelling of global frontal MT trajectories with continuous trait scores showed additive effects of both risk scores on MT growth (no interaction, p > .31). Panel shows brain trajectories with post-hoc median splits to illustrate these associations. High-risk subjects (red) scoring high on both scores (+: above median) express a cessation of MT development, while low-risk subjects (blue, -: below median) show the most pronounce MT growth. Subjects with mixed patterns (purple: high compulsive & low impulsive, green: low compulsive & high impulsive) exhibit intermediate MT growth rates. This suggests that frontal developmental myelin-related growth patterns are significant indicator of traits in terms of expressing higher values on the considered dimensions. (c) All presented results in Fig. 3 & 4 were focused on brain correlates of individual variability of trait compulsivity and impulsivity as defined in methods section. Here we focus on brain correlates of developmental changes of impulsivity in terms of the longitudinal progression of BIS total scores over study visits/time. As suggested by Guillaume et al. (2014, NeuroImage) time-varying behavioural scores were decomposed in purely within- and between subjects components and entered as regressors in voxel-wise modelling of myelin sensitive MT (in addition to covariates time/visits, age_mean, sex and confounds, n = 376/188 scans/subjects). All statistical maps were obtained from longitudinal sandwich estimator for VBQ analysis. Here, surface projections for Z-maps (p < .01 unc., one-sided) are shown, testing for negative effect of within-subject changes of BIS. We observed focal tendencies for individual BIS growth (impulsivity) being inversely related to MT growth over visits (peak in anterior inferior frontal gyrus, z-value = 3.67, p = .00013 unc.). That suggests that the reduced growth rate is more strongly expressed in subjects who manifest an accentuated impulsivity increase over study visits, such that subjects who manifest an even more restricted growth in myelin become more impulsive.