Public opinion tends to be stable. Once formed, attitudes are persistent and endure over time at both the individual and the aggregate levels. Attitudes toward marriage equality, however, have changed rapidly in recent years. This article posits that this is partly due to people learning that other members of their in-groups are supporters; they then alter their own opinions to be consistent with those of other in-group members. The authors tested this theory using a set of randomized survey experiments that shared identities as fans of professional football. When fans learn—sometimes unexpectedly—that other fans or athletes are supporters of marriage equality, they are motivated to agree in order to further normalize their membership in those sports-fan groups.