This paper explains involuntary unemployment in terms of the response of firms to workers' group behavior. Workers' effort depends upon the norms determining a fair day's work. In order to affect those norms, firms may pay more than the market-clearing wage. Industries that pay consistently more than the market-clearing wage are primary, and those that pay only the market-clearing wage are secondary. Thus, this paper also gives a theory for division of labor markets between primary and secondary.