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Social norms are group-held beliefs about how members should behave in a given context.[citation needed] Sociologists describe norms as informal understandings that govern society’s behaviors, while psychologists have adopted a more general definition, recognizing smaller group units, like a team or an office, may also endorse norms separate or in addition to cultural or societal expectations. The psychological definition emphasizes social norms' behavioral component, stating norms have two dimensions: how much behavior is exhibited and how much the group approves of that behavior.
Norms running counter to the behaviours of the overarching society or culture may be transmitted and maintained within small subgroups of society. For example, Crandall (1988) noted that certain groups (e.g., cheerleading squads, dance troupes, sports teams, sororities) have a rate of bulimia, a publicly recognized life-threatening disease, that is much higher than society as a whole. Social norms have a way of maintaining order and organizing groups.

http://en.wikipedia.org/wiki/Social_custom