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In this study we were primarily concerned with paternal behavior with a focus on its proximal (immediate causation) and distal (function and evolution) levels of analysis, in the context of maternal depression. Several authors point out the flexibility of paternal caretaking in nature and argue that, in humans, it is facultative and, thus, more open to calculations of maximizing inclusive fitness than maternal behavior. In addition, evolutionary approaches of depression offer functional analyses highlighting its adaptive function. From an evolutionary perspective, postpartum depression can have a signal function communicating mother’s need for extra support when she is unable to cope, especially from the partner. Our main objective was to examine the associations between paternal behavior, maternal depression and family relationships. We hypothesized that when mothers are depressed, fathers assume a more active role within the family system, increasing investment to compensate for mother’s « partial functioning » due to depression, promoting their child’s well-being and, thus, potentially increasing their inclusive fitness. Forty-six father-child dyads were observed in a free- play situation and assessments of the degree of father-child interactional synchrony (interpersonal distance, visual and body orientation and dyadic involvement) were made. Greater fatherchild proximity and higher visual attunement, when mothers presented postpartum depression and later depression were found. These results were discussed in relation to the evolutionary hypothesis of depression, Trivers’ Parental Investment Theory and Family System Theory.