We study the effects on performance of incentives framed as gains or losses, as well as the effort channels through which individuals increase performance. We also explore potential spill-over effects on a non-incentivised activity. Subjects participated in a medically framed real-effort task under one of the three contracts, varying the type of performance incentive received: (1) no incentive; (2) incentive framed as a gain; or (3) incentive framed as a loss. We find that performance improved similarly with incentives framed as losses or gains. However, individuals increase performance differently under the two frames: potential losses increase participants’ performance through a greater attention (fewer mistakes), while bonuses increase the time spent on the rewarded activity. There is no spill-over effect, either negative or positive, on the non-incentivised activity. We discuss the meaning and implications of our results for the design of performance contracts.