Social norms have the potential to alter the functioning of economic markets. We test whether norms shape the aggregate labor supply curve by preventing workers from supplying labor at wage cuts—leading decentralized individuals to implicitly behave as a cartel to maintain wage floors in their local labor markets. We partner with 183 existing employers, who offer jobs to 502 workers in informal spot labor markets in India. Unemployed workers are privately willing to accept jobs below the prevailing wage, but rarely do so when this choice is observable to other workers. In contrast, social observability does not affect labor supply at the prevailing wage. Workers give up 49% of average weekly earnings to avoid being seen as breaking the social norm. In addition, workers pay to punish anonymous laborers who have accepted wage cuts—indicating that cartel behavior is reinforced through the threat of social sanctions. Punishment occurs for workers in one’s own labor market and for those in distant regions, suggesting the internalization of norms in moral terms. Finally, consistent with the idea that norms could have aggregate implications, measures of social cohesion correlate with downward wage rigidity and business cycle volatility across India.