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with various problems. The Toronto researchers paid special attention to the brain region known as the anterior cingulate cortex, which watches for mismatches between what you are doing and what you intended to do. It’s commonly known as the conflict-monitoring system or the error-detection system. This is the part of the brain that sounds the alarm if, say, you’re holding a hamburger in one hand and a cell phone in the other, and you start to take a bite out of the cell phone. The alarm inside the brain is a spike in electrical activity (called event-related negativity).
    With their skulls wired, the people in Toronto watched some upsetting clips from documentaries showing animals suffering and dying. Half the people were told to stifle their emotional reactions, thereby putting themselves into a state of ego depletion. The rest simply watched the movies carefully. Then everyone went on to a second, ostensibly unrelated activity: the classic Stroop task (named after psychologist James Stroop), requiring them to say what color some letters are printed in. For example, a row of XXX s might appear in red, and the correct response would be “Red,” which is easy enough. But if the word green is printed in red ink, it takes extra effort. You have to override the first thought occasioned by reading the letters (“Green”) and force yourself to identify the color of the ink, “Red.” Many studies have shown that people are slower to answer under these circumstances. In fact, the Stroop task became a tool for American intelligence officials during the cold war. A covert agent could claim not to speak Russian, but he’d take longer to answer correctly when looking at Russian words for colors.
    Picking the right color proved to be especially difficult for the people in the Toronto experiment who had already depleted their willpower during the sad animal movie. They took longer to respond and made more mistakes. The wires attached to their skulls revealed notably sluggish activity in the conflict-monitoring system of the brain: The alarm signals for mismatches were weaker. The results showed that ego depletion causes a slowdown in the anterior cingulate cortex, the brain area that’s crucial to self-control. As the brain slows down and its error-detection ability deteriorates, people have trouble controlling their reactions. They must struggle to accomplish tasks that would get done much more easily if the ego weren’t depleted.
    That ego depletion results in slower brain circuitry is fascinating to neuroscientists, but for the rest of us it would be more useful to detect ego depletion without covering your skull with wires and electrodes. What are the noticeable symptoms—something to warn you that your brain is not primed for control before you get into a fight with your partner or polish off the quart of Häagen-Dazs? Until recently, researchers couldn’t offer much help. In dozens of studies, they looked unsuccessfully for telltale emotional reactions, turning up either contradictory results or nothing at all. Being depleted didn’t seem to consistently make people feel depressed or angry or discontented. In 2010, when an international team of researchers combed through the results of more than eighty studies, they concluded that ego depletion’s effects on behavior were strong, large, and reliable, but that the effects on subjective feelings were considerably weaker. People in depleted condition reported more fatigue and tiredness and negative emotions, but even those differences weren’t large. The results made ego depletion seem like an illness with no symptoms, a condition that didn’t “feel” like anything.
    But now it turns out that there are signals of ego depletion, thanks to some new experiments by Baumeister and a team headed by his longtime collaborator, Kathleen Vohs, a psychologist at the University of Minnesota. In these experiments, while depleted persons (once again) didn’t show any single