This study examined whether the differential maternal treatment of twins affected the twins' on-task behaviors in a teaching interaction. A total of 175 mothers and their same-sex 14-month-old twins were videotaped in their homes in triads. This was repeated for 170 of the families when the children were 24 months, and for 146 of the families when their children were 36 months old. Mothers were told to teach their children a difficult sorting task, and the first 2.5 minutes of each mother-child interaction was coded for warmth and control on the part of mothers, and, for the children, for time spent watching mother and time spent working on the task. Analyses indicated that maternal control was related positively to children's on-task behavior at 24 months and negatively at 36 months. Maternal warmth correlated positively with children's on-task and attention behaviors. When mothers were warmer with one child, that child worked on the task more than the other child. However, when mothers spent more time with one child, that child spent less time working on the task than the other child. (MDM)