Self-control is a limited resource; exerting willpower depletes it, reducing subsequent self-control capacity.
In 1998, psychologist Roy Baumeister proposed 'ego depletion' theory: willpower functions like a muscle, and using it exhausts a finite resource, leaving people with depleted self-control. This theory became foundational in psychology, spawning hundreds of studies and influencing popular psychology books like 'Willpower' and self-help programs worldwide. However, beginning around 2010, researchers encountered replication failures. A 2016 meta-analysis of 83 studies by E. J. Finkel and colleagues found evidence for publication bias and suggested the true effect size was dramatically smaller than originally claimed. A 2019 large pre-registered replication attempt with 2,000+ participants found minimal evidence for ego depletion. The original studies had relied on small sample sizes and p-hacking (analysing data multiple ways until finding 'significant' results). Baumeister and colleagues have disputed the meta-analyses and defended the theory, but the burden of evidence now suggests that ego depletion, if real, is far smaller than once believed or may not exist at all. The theory's collapse exemplifies the 'replication crisis' in social psychology and the dangers of building large literatures on weak initial findings.