This paper examines how the effort choices of workers within the same firm interact with each other. In contrast to the existing literature, we show that workers can affect the productivity of their co-workers based on income maximization considerations, rather than relying on behavioral considerations such as peer pressure, social norms, and shame. Theoretically, we show that a worker’s effort has a positive effect on the effort of co-workers if they are complements in production, and a negative effect if they are substitutes. The theory is tested using panel data on the performance of baseball players from 1970 to 2003
