Little is known by economists about how emotions affect productivity.  To make 

persuasive progress, some way has to be found to assign people exogenously to 

different feelings.  We design a randomized trial.  In it, some subjects have their 

happiness levels increased, while others in a control group do not.  We show that a 

rise in happiness leads to greater productivity in a paid piece-rate task.  The effect is 

large; it can be replicated; it is not a reciprocity effect; and it is found equally among 

males and females.  We discuss the implications for economics.