Background: Parent-child mutuality is comprised of emotional reciprocity, co-responsiveness, and cooperation, which together represent aspects of co-regulation of emotion and behavior that may be important in the etiology of children's behavior problems. Furthermore, individual differences in children's mutuality and behavior problems involve transactions between genetic and environmental influences. Behavioral genetic designs are useful for elucidating these processes. Methods: The sample included 396 children in adoptive families (53% female, age M = 8.16 years), of whom 244 were siblings (122 pairs). All of the siblings were genetically unrelated. In some families, one child was adopted but the other child was a biological child of the adoptive parents. We observed mother-child dyadic mutuality (a composite score including responsiveness, interaction reciprocity, and cooperation) during videotaped in-home observations. In addition, child behavior problems were assessed and represented as a composite score including observers' ratings of noncompliance, and parents' ratings of aggression and conduct problems. Results: Greater mutuality was associated with lower levels of child behavior problems, both between families and within families (i.e., sibling differences). The sibling intra-class correlation for mutuality with the same parent was near zero in this sample of genetically unrelated siblings. The correlation between child behavior problems and dyadic mutuality did not vary as a function of mother-child genetic similarity. Conclusions: Mother-child mutuality is child specific within families, a finding that is consistent with theories regarding bi-directional parent and child effects in socialization. In addition, there was no evidence of passive gene-environment correlation, suggesting that the link between lower levels of maternal mutuality and higher levels of child behavior problems is not only reflecting overlapping genetic influences on parent and child behavior.