Past research suggests that children who experience multiple transitions in family structure may face worse developmental outcomes than children raised in stable two-parent families and perhaps even children raised in stable, single-parent families. However, multiple transitions and negative child outcomes may be associated because of common causal factors such as parents' antecedent behaviors and attributes. Using a nationally-representative, two-generation longitudinal survey that includes detailed information on children's behavioral and cognitive development, family history, and mother's attributes prior to the child's birth, we examine these alternative hypotheses. Our results suggest that, for white children, the association between the number of family structure transitions and cognitive outcomes is largely explained by mother's prior characteristics but that the association between the number of transitions and behavioral outcomes may be causal in part. We find no robust effects of number of transitions for black children.