This study aimed to examine when children come to distinguish between the private affairs of an individual and the group's matter in relation to group decision making and whether there are any differences between Japan and England in learning the distinction. In total 217 children aged 8, 11, and 13 years, and undergraduates participated in the study. Participants were shown hypothetical situations in which members of a group were going to make a rule restricting behaviors in the personal sphere by majority decision. Participants were asked to judge the legitimacy of group decision making and to give reasons for their judgments. The findings revealed that younger participants from both countries were more likely than older participants to approve of group decision making over individual concerns and to rate transgressions as more serious. It was also found that Japanese participants were more likely to approve of group decision making than the English participants. The younger participants in both countries and the Japanese participants seemed to have an inclination to judge the appropriateness of group decision making according to the perceived degree of benefit of the decision. (Contains 3 tables and 2 figures.)