AbstractPrevious research has indicated that children perceive social category members as having intrinsic obligations toward each other, which shape their expectations for social interactions. However, it is unclear whether teenagers (aged 13 to 15) and young adults (aged 19 to 21) continue to hold such beliefs, given their increased experience with group dynamics and external social rules. To explore this question, three experiments were conducted with a total of 360 participants (N = 180 for each age group). Experiment 1 examined negative social interactions using different methods in two sub‐experiments, while Experiment 2 focused on positive social interactions to examine whether participants viewed social category members as intrinsically obligated to avoid harming each other and to offer assistance. Results revealed that teenagers evaluated within‐group harm and non‐help as unacceptable, regardless of external rules, whereas they viewed between‐group harm and non‐help as both acceptable and unacceptable, depending on the presence of external rules. Conversely, young adults considered both within‐group and between‐group harm/non‐help as more acceptable if an external rule permitted such behavior. These findings suggest that teenagers believe that members of a social category are intrinsically obligated to help and not harm each other, whereas young adults believe that individual social interactions are constrained mainly by external rules. That is, teenagers hold stronger beliefs than young adults about intrinsic interpersonal obligations to group members. Thus, in‐group moral obligations and external rules contribute differently to the evaluation and interpretation of social interactions at different developmental stages.