(4 of 6)
Of course, the brain is not actually a muscle, apt as the analogy seems, so what makes it behave like one? One thing may be glucose--the brain's fuel of choice--with willpower rising and falling along with our glucose levels. In a 2010 study at the University of South Dakota, investigators recruited 65 undergraduates and had them participate in a classic delayed-gratification game, offering them the chance to roll dice and win either $120 that day or $450 that they couldn't have for 31 days. Many of the typically cash-strapped students decided an immediate payout was more valuable than a larger one later. But one thing helped them defer the reward: subjects who had consumed sugary sodas before the experiment were likelier to pick the later $450 than those who had drunk artificially sweetened sodas.
"The brain is always monitoring its resource levels," says McGonigal. "If sugar is rising, we feel like we can defer indulging ourselves. In other studies, investigators control people's willpower as if with a joystick by putting them on a glucose infusion and regulating it up and down."
Not everyone agrees that this how neural metabolism works. The brain does need a lot of glucose, but like a computer, it can run many programs at once, and willpower is not a very costly one--it requires the sugar equivalent of less than half a Tic Tac per minute, says Kurzban. "The glucose model is metabolically implausible," he argues. "The brain isn't a hydraulic system that needs a constant pressure; it's an information-processing system. If your browser's running slowly, you don't check your battery."
Getting in Your Own Way
Even as the glucose model is being debated, psychologists agree on a few other, less technical phenomena that can sabotage willpower. Take the what-the-hell effect--which is exactly what it sounds like. You're on a diet, you have a bit of ice cream, and then--what the hell, the day's a loss anyway--you might as well finish the whole pint. There's a lot of what passes for thinking in this, which makes it hard not to blame yourself after a binge is done. But you may be less responsible than you think.
In a 2010 study, Heatherton and two colleagues recruited 100 subjects, half of whom were chronic dieters and the rest of whom had little history of having to control their weight. They were slid into an fMRI scanner to see how their brains reacted to images of food. The nondieters showed activity in the nucleus accumbens, one place appealing cues are processed, and little activity in the amygdala, which would have indicated an aversion to food. The dieters showed just the opposite, suggesting that they were trying--successfully--to control their appetites. All of the subjects were then taken out of the scanners and given 15-oz. (444 ml) milk shakes to drink. They then went back in and were shown the same images.