We assessed a retrospective proxy for during-scan motion based on Multi-Parameter Mapping R2* exponential decay model residual in white matter areas (see Castella et al., 2018, Magn Reson Med; and www.hmri.info for details). The motion proxy (denoted by SDR2*) has been show to accurately reflect absolute motion measures using prospective motion correction. Beyond visual and covariance based quality checks, SDR2* was used to remove 10% of scans with highest motion artefacts before analysis. (a) In resulting clean sample, average SDR2* of each participant correlates with motion parameters during an independent resting-state fMRI scan of same individuals (r = .36, two-sided Pearson’s correlation, p = e-9 from corresponding t-distribution, n = 316 independent subjects) suggesting its validity. (b) Across all scans and individuals the motion proxy was found to be positively associated with increased focal MT in right inferior cortical areas (peak right lingual and fusiform gyrus, z-value = 5.02, p = .016, one-sided Wald-test, voxelwise FDR, n = 497/288 scans/subjects applies for panels b-c). No areas with negative effects or trends of motion were observed. (c) To avoid spurious MT effects induced by motion variability, SDR2* was included as a time-varying (scan by scan) covariate during all presented main analyses. Comparing developmental growth (left column) and sex (right column) effects on myelin-sensitive MT without (top row) and with (bottom row) motion regressor (p < .05, voxelwise FDR). Accounting (linearly) for motion did not affect observed developmental increase of MT (over age or time/visits) and even increased statistical strength of sex effects suggesting females having higher MT in insular and frontal areas (cf Supplementary Fig. 3c, one-sided Wald-test for females over males, accounting for all other covariates such as age and visit etc.) (d) Linear mixed-effects modelling of SDR2* over all scans and subjects revealed no change of motion over visits (left plot, t = 1.47, p = .14, two-sided, df = 564, n = 573/316 scans/subjects, statistics from fixed-effects coefficient) or with age of subject (right plot, t = 0.09, p = .93, df = 564). This supports that presented developmental effects on myelin-sensitive MT (Fig. 1 and Supplementary Fig. 3) are unlikely to be biased by motion. In general, we observed higher motion (t = −5, p = e-6, two-sided, df = 564) in males compared to females. Consequently, accounting for motion induced variability analyses reveals more accurate estimation of sexual dimorphism. (e) Absence of significant compulsivity by time/visit interaction (left panel, t = −0.23, p = .81, two-sided, df = 513, n = 525/273 scans/subjects) and main effects (right panel, t = 1.37, p = .17, two-sided, df = 513) on SDR2* supports independence of main findings in Fig. 3 and motion-related variability. (f) Subjects with higher impulsivity showed more positive change of SDR2* from baseline to follow up scans (t = 1.98, p = .048, two-sided, df = 561, n = 573/316 scans/subjects) but no generally higher motion (t = −0.26, p = .73, two-sided, df = 561). Given the observed positive association of SDR2* and MT above, this suggests that reduced growth of MT over visits in higher impulsive subjects might be regionally underestimated, that is an even more pronounced reduction of growth is likely in some areas. Therefore, motion proxy SDR2* was included in all main analysis and Fig. 4 is likely to represent effects independent from motion-induced artefacts. Plots b & d-f show effects of interest and motion proxy data adjusted for effects of no interest (that is covariates and confounds). Notably, a slightly larger initial sample with 573 native space scans (before spatial processing) with available motion parameters was used to conduct analyses in panels a & d–f.