It is well accepted that parent-child interactions are bidirectional by nature, yet not much is known about the psychophysiological activity underlying these interactions. This study examined, during a parent-child interaction, how a child’s negativity statistically predicted maternal frontal electroencephalograph (EEG) asymmetry and how a mother’s negativity statistically predicted child frontal EEG asymmetry. Thirty-four mother-child dyads participated in the study. Maternal and child behaviors and physiology were measured during a puzzle task. Results indicated that mothers whose children exhibited more challenging behaviors during the dyadic interaction displayed more right (relative to left) asymmetry, as did children whose mothers were high in negativity during the interaction. These findings suggest that mothers and children react to each other’s signals not only behaviorally but also physiologically.