Exerting effort in a first task can impair self-control performance in a subsequent task. Hundreds of studies have examined this ego depletion effect, but the underlying mechanisms are still unknown. By contrasting the two most prominent models, the strength model and the process model, the following question takes centre stage: Do participants fail to exert self-control because they run short of an unspecified resource or because they lack the motivation to engage in the subsequent task? We contrasted competing predictions (N = 560) from these two models by manipulating monetary incentives to be donated to charity in the first of two tasks. We found evidence of the standard ego depletion effect—self-control performance was impaired after a high- versus a low-demand task in the no-incentive conditions. Incentives had an unexpected effect: Whereas participants in the incentive conditions showed higher intrinsic, autonomous motivation, they did not exert greater effort. This unexpected finding limited the applicability of our registered predictions; thus, we opted to test updated predictions. We discuss the theoretical implications of our understanding of the processes underlying ego depletion effects and their meaning for the ongoing debate about replicability and robustness.