This study examined potential non-shared environmental processes in middle childhood by estimating statistical associations between monozygotic (MZ) twin differences in externalizing and internalizing problems and positive social engagement, and differential maternal positivity and negativity, over 1 year. Seventy-seven pairs of identical twins participated (M=6.08-years old, 65% male) in two annual home visits. Observers' ratings and maternal reports were gathered. At both assessments, the twin who showed more conduct problems (maternal report and observers' ratings) and less positive social engagement (positive affect, responsiveness) received more maternal negativity and less maternal warmth (self-reports and observers' ratings), relative to his or her genetically identical co-twin. The same patterns held over time, for the associations between change in differential MZ twin conduct problems and social engagement and change in differential maternal behaviour. Effects for child internalizing problems were not consistent within or across raters. Overall, these results indicated that differential maternal warmth and negativity—self-perceived and observed by others—are important aspects of sibling differentiation for both problematic and adaptive behaviours during middle childhood. Copyright © 2009 John Wiley & Sons, Ltd.