Willpower

That seemingly small exercise of self-control was associated with a big drop in the brain’s fuel of glucose.
    To establish cause and effect, the researchers tried refueling the brain in a series of experiments involving lemonade mixed either with sugar or with a diet sweetener. The strong taste of the lemon made it hard for the tasters to know whether real sugar or diet sweetener was used. The sugar gave them a quick burst of glucose (though not for long, so the experimenters needed to get to the point pretty soon). The diet sweetener didn’t furnish any glucose or, indeed, any nutrition at all.
    The effects of the drinks showed up clearly in a study of aggression among people playing a computer game. At first, the game seemed reasonable, but it soon became impossibly difficult. Everyone got frustrated as the game went on, but the one who got a sugar-filled drink managed to grumble quietly and keep playing. The others started cursing aloud and banging the computer. And when by prearranged script the experimenter made an insulting remark about their performance, the glucose-deprived people were much more likely to get angry.
    No glucose, no willpower: The pattern showed up time and again as researchers tested more people in more situations. They even tested dogs. While self-control is a distinctively human trait, in the sense that we’ve developed it so extensively in the process of becoming cultural animals, it’s not unique to our species. Other social animals require at least some degree of self-control to get along with one another. And dogs, because they live with humans, must often learn to bring their behavior into line with what must seem to them to be absurd and arbitrary rules, like the ban on sniffing the crotches of houseguests (at least the human ones).
    To mimic the human studies, the experimenters first depleted the willpower of one group of dogs by having each dog obey “sit” and “stay” commands from its owner for ten minutes. A control group of dogs was simply left alone for ten minutes in cages, where they had no choice but to remain and therefore didn’t have to exercise any self-control. Then all the dogs were given a familiar toy with a sausage treat inside it. All the dogs had played with this toy in the past and successfully extracted the treat, but for the experiment the toy was rigged so that the sausage could not be extracted. The control group of dogs spent several minutes trying to extract it, but the dogs who’d had to obey the commands gave up in less than a minute. It was the familiar ego-depletion effect, and the canine cure turned out to be familiar, too. In a follow-up study, when the dogs were given different drinks, the drinks with sugar restored the willpower of the dogs who’d had to obey the commands. Newly fortified, they persisted with the toy just as long as the dogs who’d been in cages. The artificially sweetened drink had no effect, as usual.
    Despite all these findings, the growing community of brain researchers still had some reservations about the glucose connection. Some skeptics pointed out that the brain’s overall use of energy remains about the same regardless of what one is doing, which doesn’t square easily with the notion of depleted energy. Among the skeptics was Todd Heatherton, who had worked with Baumeister early in his career and eventually wound up at Dartmouth, where he became a pioneer of what is called social neuroscience: the study of links between brain processes and social behavior. He believed in ego depletion, but the glucose findings just didn’t seem to add up.
    Heatherton decided on an ambitious test of the theory. He and his colleagues recruited dieters and measured their reactions to pictures of food. Then ego depletion was induced by asking everyone to refrain from laughing while watching a comedy video. After that, the researchers again tested how their brains reacted to pictures of food (as compared with nonfood pictures). Earlier work by Heatherton and Kate Demos had shown that these pictures produce various reactions in key brain sites, such as the nucleus accumbens and the amygdala. These same reactions were found again. Among dieters, depletion caused an increase in activity in the nucleus accumbens and a corresponding decrease in the amygdala. The crucial change in this experiment involved a manipulation of glucose. Some people drank lemonade sweetened with sugar, which sent glucose flooding through the bloodstream