Something’s Gotta Give: Reward-Induced Effort Modulation in Task-Switching Reveals a Flexibility-Stability Trade-off

Everyday behaviour requires balancing two control modes: flexibility, for switching between tasks, and stability, for maintaining goals in the face of interference. One influential account posits that these two modes trade off: increased flexibility comes at the cost of reduced stability, and vice-versa. However, empirical support for this trade-off remains equivocal. We propose that this trade-off arises from reward-induced effortful control modulations during tasks that simultaneously require flexible and stable control. Across two experiments (N=80 and N=87), participants completed a task-switching paradigm in which cued reward incentives were manipulated over varying timescales. Large rewards increased flexibility, evinced by reduced task switch costs, but reduced stability, evinced by increased incongruence costs, indicating a reward-induced trade-off. In Experiment 1, such control adjustments— both at the block-to- block and individual differences level—followed this trade-off. Overall, these findings provide evidence for a reward-driven flexibility-stability trade-off; higher incentives promote flexibility at the cost of stability.