We examine how rewards and penalties under tournament incentives impact price behaviour in experimental asset markets. Adding a penalty to a reward-only contract, or a reward to a penalty-only contract, changes the traders’ behaviour. The experimental markets with adjusted contracts experience less trading, but longer-lived and larger bubbles. This observed effect of penalties is consistent with herd-driven behaviour under relative performance evaluation, while the effect of rewards reflects the influence of the convexity of bonuses. However, these effects dissipate with trader experience. Our findings contribute to the debate attributing market instability to incentive structures in the finance industry.