Infants’ social-cognitive skills develop within the parent-infant relationship. Differences between parents in the way they interact with their infant may have important consequences for infants’ social-cognitive development. Moreover, due to their genetic make-up, some individuals may be more affected by their social environment than others. Indeed, genes that may convey this differential susceptibility have been identified. However, the analysis an use of genetic material is time-consuming, expensive, and surrounded by ethical concerns. Susceptibility may also not be conveyed by a single gene, but rather by many. Obtaining an endophenotype for susceptibility would therefore be worthwhile. Frontal asymmetry (differences in activity of the left and right frontal cortex) may be such an endophenotype. Because it can be measured easily, relatively cheaply, and non-invasively, frontal asymmetry may offer considerable advantages over genetic measures as a differential susceptibility marker.The proposed study investigates frontal asymmetry as an endophenotype for differential susceptibility to effects of maternal sensitivity on infant social cognition. Because infant cognitive development is tightly intertwined with brain development we investigate effects of sensitivity on the neural underpinnings of socio-cognitive development, using fNIRS to image brain activity. We focus on two processes: activation of the mirror neuron system and neural processing of motherese.The proposed study will provide important insights into the neural mechanisms underlying differential susceptibility, highly relevant for our understanding of the processes leading to children’s successful or aberrant development. Insights gained from the study may also inform future studies searching for ways to enhance susceptibility in individually tailored parenting support and interventions. For this purpose a susceptibility marker that can be measured quickly, easily, and non-invasively is vital.