Many firms adjust employment in a "lumpy" manner -infrequently and in large bursts. This paper shows how workers' incentives can function as an endogenous cost that leads to lumpy firm-level employment adjustments such as mass layoffs.The signaling model features a firm that has private information about idiosyncratic demand shocks and relies on workers' effort for production. When workers observe the firm's employment choice but not the underlying shock, the equilibrium is characterized by partial pooling in which the firm hoards labor to maintain workers' incentives. In response to small negative shocks, the firm initially underadjusts in order to conceal information from workers; it conducts a mass layoff after business conditions deteriorate past a certain point.JEL Classification: D82, J21, J63, M51.