Sample characteristics of the full sample (left) and of the subset of children for which longitudinal data was available (right).

MZ  =  monozygotic, DZ  =  dizygotic, FA  =  fractional anisotropy, R = right-handed, WM  =  white matter.
*White matter was segmented reliably in 187 children at baseline, and 117 children at follow-up.
**105 children had reliable white matter segmentations and DTI measurements at both time-points. The last column displays p-values of the differences in sample characteristics for children who participated only once, versus children who participated twice.