We argue that social norms must be measured at the group level of analysis, allow for a range of acceptable behaviors, and be linked to the individual level of analysis to explain social behavior. From a survey of young adults in Wisconsin (1973), we generated measures of family size norms from sibship experience and friends' expected family size. These measures satisfied our primary criterion for a social norm: Those with non-normative family size desires tended to shift expectations toward the norm. The analyses demonstrate the difficulty of estimating normative effects when by its very definition a norm is expected to restrict variation in human behavior.