Incentives are supposed to increase effort, yet individuals react differently to incentives. We examine this heterogeneity by investigating how personal characteristics, preferences, and socio-economic background relate to incentives and performance in a real effort task. We analyze the performance of 1,933 high-school students under a Fixed, Variable, or Tournament payment. Productivity and beliefs about relative performance, but hardly any personal characteristics, play a decisive role for performance when payment schemes are exogenously imposed. Only when given the choice to select the payment scheme, personality traits, economic preferences and socio-economic background matter. Algorithmic assignment of payment schemes could improve performance, earnings, and utility, as we show.