<p>When, how, and why do humans come to distinguish between close, significant others and more casual social contacts? Although a healthy social life encompasses diverse relationship types, close relationships are consistently perceived as qualitatively distinct, characterized by frequent interactions, time spent together, predictability, long-term stability, and reciprocal emotional engagement. Cognitive social neuroscience specifies these features, yet key explanatory domains, including relationship cognition, attachment-based internal working models, and memory processes, remain in relative isolation. The current study proposes a novel perspective of how the foundational distinction between “close” and “casual” relationships may emerge. By integrating perspectives from cognitive social neuroscience, developmental psychology, and memory research, this framework advances a unifying account of relational categorization. We argue that early interactions with primary caregivers in infancy not only establish specific attachment bonds and, consequently, internal working models but also give rise to a highly abstract, domain-general memory schema of relational closeness. We suggest that this generalized schema constitutes a fundamental component of the knowledge systems supporting the formation and maintenance of our most important, close relationships throughout the lifespan.</p>