Detection of genetic relatedness (i.e. kinship) impacts the social, parental, and sexual behavior of many species. In humans, self-referent phenotype matching based on facial resemblance may indicate kinship. For example, faces that resemble ours are perceived as more trustworthy and attractive. Sex differences in behavioral reactions to facial resemblance among children have also been demonstrated and are consistent with evolutionary theory suggesting that resemblance might serve as a paternity cue. Using event-related fMRI, we show that specific regions of the brain are implicated in processing facial resemblance and a sex difference in cortical response to facial resemblance expressed in children. We found a consistent activation in the fusiform gyrus across all face conditions, which is consistent with previous research on face processing. There were no sex differences in overall response to faces in the fusiform gyrus, and also to faces that did not resemble subjects. When resemblance was not modeled, females showed greater activation to child faces than males. Consistent with parental investment theory and theories of sexual selection, males showed greater cortical activity than females in response to children's faces that resembled them. These data suggest natural selection may have crafted a sexually differentiated neuro-sensory module implicated in detection of facial resemblance that may serve as a kin detection and paternity cue. This process may capitalize on neural substrates involved in self-referent processing and familiarity detection.