While community sport clubs in Japan play an important role in providing children with opportunities to exercise, the aggressive behaviors of parents of club members is a widespread problem. To investigate the relational parameters which contribute to the display of such behaviours, we conducted an online survey among mothers whose child was participating in a community-based junior sport club ( N = 500 ). The survey used a hypothetical situation in which a team member makes a mistake. Variables such as the degree of closeness between the respondents and the mother of the target child and the athletic skill level of the target child were manipulated. Our findings revealed that the respondents tended to have more negative thoughts toward an average child than a child with superior skills. They also tended to display more aggression toward a girl with superior skills than towards an average girl when they were not close to the girl’s mother. The study also found that mothers in sports clubs where parents’ aggressive behaviors were common tended to display more aggression, which implies the existence of pluralistic ignorance.