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Ego depletion was the theory that self-control, or executive function, draws on a finite mental resource that can be exhausted. In 1998 the American social psychologist Roy Baumeister and colleagues proposed a muscle model of willpower and tested it in a room smelling of fresh chocolate chip cookies. Hungry students allowed to eat the cookies worked on an impossible geometric puzzle for about 20 minutes, while those made to eat raw radishes instead gave up after about eight. Follow-up studies found that students who chose to give speeches supporting policies they opposed did worse on later self-control tasks. Researchers then tied the effect to blood glucose, and later found that merely swishing a sugary drink and spitting it out restored performance. The idea spread into explanations of dieting, athletic performance, impulse shopping and even guilt and charity.

Anomalies piled up. A 2011 study found adults over 40 showed no depletion effect, money rewards and comedy videos restored self-control, and Carol Dweck’s research found people who believed willpower was unlimited showed no short-term depletion. A 2010 meta-analysis of 198 tests had called the effect significant, but in 2015 Evan Carter and Michael McCullough found signs of publication bias and the adjusted effect nearly vanished. A 2016 replication across two dozen labs and more than 2,000 participants found no effect, and a 2021 project led by Kathleen Vohs, with 36 labs and over 3,500 participants, found an effect an order of magnitude smaller. Michael Inzlicht and Brandon Schmeichel’s process model recasts depletion as a shift in motivation and attention.

• When students were explicitly forced to read the counter-attitudinal speech rather than choosing to, the depletion effect was much weaker.
• Inzlicht and Gutsell found that suppressing emotions during a distressing film weakened error-related negativity signals from the anterior cingulate cortex.
• Vohs and Heatherton found depleted chronic dieters ate more ice cream in a taste test, while non-dieters did not.
• Clarkson and colleagues showed that false feedback making people feel more depleted hurt their working memory performance.
• A 2020 computational study by Inzlicht’s team found depleted people could still inhibit impulses but were less motivated to do so.