In their studies, the psychologists who created this theory -- Roy Baumeister and his colleagues -- make people do annoying things, like sit hungry in front of chocolate and not eat it, and then they test how long they're willing to exert their will -- say by forcing themselves to work at a puzzle.
There's a good article by John Tierney about it here, focusing more on decision-making. And there's a good review of Baumeister's new book, co-authored with John Tierney, here, that's more of an overview.
The metaphor of ego-depletion is that of a muscle that gets worn out. If you don't want to use up your self-control, don't force yourself to do too many difficult things at one time. But if want to have more self-control, you can strengthen it with exercises.
Supposedly, if you use your willpower to keep your room tidy, keep a diary of what you eat, or speak in complete sentences, and you'll have more of it when it comes time to quitting smoking or studying your German or whatever.
Like muscles, self-control needs food. Ironically (as we say nowadays), self-control feeds on sugar, and an influx of glucose will help you make better decisions. Sugary soft drinks? Yes. Cake, yes. Diet coke? No. Tuna plate with lettuce, hold the carbs? Not so much.
They connection between self-control and glucose is so powerful that, as Tierney says, "The mere expectation of having to exert self-control makes people hunger for sweets."
The jokes kind of write themselves, don't they?
Anyway, there's one big thing at the center of ego depletion theory that doesn't seem right to me. Sure, it's plausible that when you have to exert self-control, your ability to exert self-control goes down. And sure, it's also easy to believe that when you have your act together in small domains like keeping things tidy you're more likely to have your act together in bigger ways.
But the muscle metaphor fails in one crucial way: there's nothing like "falling off the wagon" for your muscles.
With self-control, it seems like you can be going along in life with everything in order when a wave of difficulty just knocks everything out of whack. People who've returned from war have been massively exercising their self-control, but it does not always seem easy to transport that "strength" back home.
Muscles aren't like that. When they get weak, they get weak slowly.
I have another theory. Isn't it possible that what depletes your self control, really, is being harassed, annoyed, and unhappy?
Then using your self-control can make you harassed, annoyed, and unhappy -- and if I had to sit hungry in front of chocolate I'd be harassed, annoyed and unhappy. But other things can make you feel this way too. Don't you think a person who just received bad news, or got yelled at, or was contemplating death, would also find it harder to work on the puzzle?
Why not just say, being in a bad mood makes you have less self-control, and using your self-control is one thing that can put you in a bad mood? And that sweets put you in a good mood? Nothing surprising there.
That would at least explain the falling off the wagon problem. As we all know, the causes of bad moods are vast and varied.
And let me just close by observing that if the ego-depletion theory, or anything like it, is right, then the situation we have is this: people whose self-control is worn down by having to select good options from among bad, together with massive industry forces devoted to wearing down our self-control to make us select bad options instead of good ones.
From that point of view, it's a miracle we're not even less healthy and more in debt than we are.