Ego-depletion stands for a simple idea that self-control is a limited resource, meaning that if you exercise your self-control on certain activities, you will deplete it and thus won’t be able to use for other activities. Ego-depletion has many important implications for problem solving, decision-making, and other adaptive thinking generally; for one, it explains why we may oversimplify decisions and make irrational trade-offs when we feel tired.
This post explains what ego-depletion is and what activities usually lead to it; the next post will provide some suggestions for maintaining and restoring ego-strength thus increasing chances that our thinking and decision-making won’t be impaired by ego-depletion.
Ego-Depletion: Self-Control as a Limited Resource
The basic idea behind ego-depletion is that self-control or willpower is a limited resource. At the risk of oversimplification, you can think of self-control as your willpower tank, by analogy to a fuel tank. When you exhaust your willpower tank, you reach a state called ego-depletion.
To illustrate this idea more concretely, imagine that in the morning your willpower tank has 55 points of self-control. Throughout the day, you will spend your willpower points on various activities that require self-control. For example, you might spend 20 points on making challenging shopping decisions; 7 points resisting chocolate cake your coworker brought to the office; 15 points trying to preserve your good mood after a phone argument with your loved one; 10 points trying to resist checking email so that you could finish an important project you’ve been procrastinating about. All these acts of self-control would leave your willpower tank almost empty. If you hadn’t refilled your willpower tank (for example, with rest or sleep) and now your friend tempts you with a brownie, you will not be able to resist it if you need 10 willpower points to do so. So it is not surprising that we have much harder time resisting temptations in the evening: we usually exhaust our self-control throughout the day, so in the evening we naturally give in to our temptations.
Research on Ego-Depletion
One line of research conducted over the last 15 years has shown that for many people self-control seems to be a limited resource. In a seminal experiment on ego-depletion by Roy Baumeister and his colleagues, all participants had to solve an unsolvable puzzle. However, one group gave up almost twice as fast as the other one. There was no difference in the puzzle, but there was a difference in what these groups were doing before they tried to solve the puzzle. Some participants had freshly baked chocolate cookies in front of them and they could not eat them but had to resist them and satisfy themselves with radishes. Others were allowed to eat the cookies. For the radish-eaters, resisting the cookies depleted their self-control and it led them to give up on the puzzle sooner than they would otherwise.
Yet, this line of research is challenged by others showing that whether willpower is a limited resource depends on how people themselves view their willpower. People who don’t consider willpower a limited resource (i.e. that it cannot be easily depleted by relatively modest mental exertion) may show little or signs of ego-depletion. (This is not to say that for these people willpower is unlimited and that this means that they are immune to very strenuous tasks.) 
Researchers haven’t yet figured out how to directly measure the ego-strength or how much particular activities cause ego-depletion (as in the example above using the willpower points). And of course, everyone is different. Some people have larger willpower tanks than others. Also, different activities deplete different people differently; for example, a glass of red wine may not even tempt you, but a recovering alcoholic may burn his whole willpower tank on resisting that one glass.
What Activities Lead to Ego-Depletion?
Ego-depletion is caused by any activity that requires you to exercise self-control. Among other things, research found the following to be ego depleting:
- Resisting temptations and breaking habits
- Making decisions. Studies show that when people have to make various decisions, such as choosing which items to buy, their first decisions are superior; however, later decisions suffer because of ego-depletion. This phenomenon has been aptly called the decision-fatigue.
- Having conflicting goals and standards. Even small conflicting goals can be depleting. For example, a goal of immediately feeling better often conflicts with the goal of saving money for later.
- Planning and acting on the plan
- Coping with stress. This includes any stressors, including physical ones (try holding a hand in ice water for as long as possible and then see if you’ll be able to resist temptations)
- Physical effort (for example, researchers typically have their participants perform a handgrip stamina exercise: squeeze the handles of a hand exerciser and hold as long as possible)
- Suppressing emotions and preserving a positive mood
- Showing an initiative
- Trying to make a good impression
- Stifling unwanted thoughts (for example, trying not to think about white bears)
What are the Effects of Ego-Depletion?
Ego-depletion above all makes it much harder to fight off temptations. Naturally, it leads us to rely on our default inclinations and desires. Of course, it also means that we’ll have difficulty resisting temptations even if a new activity is different from the one that led to ego-depletion. So whatever depletes you, you’ll find it challenging to resist all sorts of temptations, from eating a tempting food to engaging in illicit sex.
Ego-depletion has very important implications that go beyond our ability to resist a cupcake after the work. Most importantly, it will usually impair our problem solving and decision-making. We might make overly simplified decisions, with irrational trade-offs or put off making important decisions entirely.
 Roy F. Baumeister, Ellen Bratslavsky, Mark Muraven, and Diane M. Tice, Ego-depletion: Is the active self a limited resource? Journal of Personality and Social Psychology, 74, 1252–1265 (1998).
 Veronika Job, Gregory M. Walton, Katharina Bernecker, and Carol S. Dweck, Beliefs About Willpower Determine the Impact of Glucose on Self-Control, Proceedings of the National Academy of Sciences, 110, 14837–14842 (2013). DOI: 10.1073/pnas.131347511.
 Kathleen D. Vohs, Roy F. Baumeister, Brandon J. Schmeichel, Jean M. Twenge, Noelle M. Nelson, Dianne M. Tice, Making Choices Impairs Subsequent Self-control: A Limited Resource Account of Decision Making, Self-regulation, and Active Initiative, 94 Journal of Personality and Social Psychology 883-898 (2008).
 Roy F. Baumeister, Yielding to Temptation: Self-Control Failure, Impulsive Purchasing, and Consumer Behavior.” Journal of Consumer Behavior, 28, 670-676 (2002).