Willpower is a limited resource that gets 'used up' during the day (ego depletion).
Verdict: contested
A famous early effect largely failed a big pre-registered replication; the resource model is now seriously doubted.
What the evidence shows
Baumeister and colleagues (1998) proposed that self-control draws on a shared, depletable resource: resist cookies now and you will give up sooner on a puzzle later. The idea shaped a huge literature and popular advice about decision fatigue and glucose.
A pre-registered replication across 23 labs (Hagger et al., 2016) found the depletion effect was essentially zero using an agreed protocol. The result did not settle the debate — proponents argue the chosen task was weak, and some newer studies still report effects — but it removed the sense of a large, robust phenomenon. Today the strong 'willpower is a fuel tank' model is contested; motivation, beliefs about willpower, and task interest may explain much of what looked like depletion.
Sources
Baumeister, R. F., Bratslavsky, E., Muraven, M., & Tice, D. M. (1998). Ego depletion: Is the active self a limited resource?.
Journal of Personality and Social Psychology, 74(5), 1252–1265
After exerting self-control on one task, participants performed worse on a subsequent self-control task — interpreted as a depleted shared resource.
DOI: 10.1037/0022-3514.74.5.1252 →Hagger, M. S., Chatzisarantis, N. L. D., et al. (2016). A multilab preregistered replication of the ego-depletion effect.
Perspectives on Psychological Science, 11(4), 546–573
Across 23 labs using a common protocol, the ego-depletion effect was near zero.
DOI: 10.1177/1745691616652873 →