Humans possess explicit, rule-based, and culturally determined systems for identifying kin, but kinship inferences are also influenced implicitly by cue-based mechanisms found commonly across the animal kingdom. These mechanisms are fallible. An evolutionarily informed signal-detection analysis suggests that (a) cue-based kin recognition may sometimes be biased in favor of false-positive errors, resulting in implicit kinship inferences even in the presence of nonkin, and (b) the tendency toward this inferential error may vary predictably in response to specific developmental and contextual circumstances. This analysis has important implications for a wide variety of psychological phenomena (especially in the realms of person perception, interpersonal attraction, and prosocial behavior) and leads to the deduction of many novel hypotheses.