Young children understand how social connections affect what people know about each other.

An unwritten expectation in our everyday social interactions is that intimate personal information about someone-"insider knowledge"-is usually confined within close relationships. For example, it would be odd, or even unsettling, if a stranger knew about your favorite movie. Such expectations about who knows what about whom constitute a cornerstone of complex social behavior, but much remains unknown about their cognitive underpinnings and developmental origins. Drawing on parental report (Study 1) as well as a novel experimental approach using controlled but naturalistic videochat conversations (Study 2 & 3), we find that 4- to 5-y-old children have an abstract, theory-like understanding of how social connections give rise to interpersonal knowledge. Self-report, facial expressions, and memory errors provide converging evidence that children were surprised when someone possessed insider knowledge that is misaligned with their relationships, such as a stranger knowing their favorite food (Study 2a) or their own parent knowing a stranger's favorite movie (Study 2b). Children also generated coherent ad-hoc explanations about how someone might have acquired that knowledge, appealing to either first-hand observations or second-hand sources (Study 3). These findings demonstrate an early-emerging understanding of how individual minds are shaped in the context of their social networks, supporting a precocious ability to detect and explain anomalies in what people know about each other in real-time conversations. The current work also opens possibilities for leveraging open-ended online interactions to study social cognition without compromising experimental control.