Transactional models of analysis can examine both moment-to-moment interactions within a dyad and dyadic patterns of influence across time. This study used data from a prospective adoption study to test a transactional model of parental depressive symptoms and mutual negativity between mother and child over time, utilizing contingency analysis of second-by-second behavioral data. To consider both genetic and environmental influences on mutual negativity, depressive symptoms were examined in both adoptive and birth mothers. Adoptive mother depressive symptoms at 9?months increased the likelihood that, at 18?months, children reacted negatively to their mothers' negative behavior, which in turn predicted higher levels of adoptive mother depressive symptoms at 27?months, suggesting that over time, mothers' depressive symptoms influence and are influenced by moment-to-moment mutual negativity with their toddlers. Birth mother depressive symptoms moderated the association between mutual negativity at 18?months and adoptive mother depressive symptoms at 27?months, suggesting a child-driven contribution to maternal depressive symptoms that can be measured by a genetic sensitivity.