Van Heusen

What causes bad moods? Why do we sometimes slip into angry fits and melancholy torpors? In general, happy moods have easy explanations – we know why we’re elated. But a bad mood often seems to arrive out of the blue, a gloomy weather pattern that settles in from everywhere all at once. All of a sudden, we find ourselves pissed off without a good reason, which only makes us more pissed off.

The standard theory of bad moods is rooted in a psychological quirk known as ego depletion. Pioneered by Roy Baumeister and Mark Muraven in the 1990s, the basic idea behind ego depletion is that self-control and willpower are limited cognitive resources. As a result, when we overexert ourselves in one domain – say, when we’re on a strict diet, or focused on a difficult task for hours at work – we have fewer resources left over to exert self-control in other domains. This helps explain why, after a long day at the office, we’re more likely to indulge in a pint of ice cream, or eat one too many slices of pizza. A tired brain, preoccupied with its problems, is going to struggle to resist what it wants, even when what it wants isn’t what we need.

A bad mood is no different. When we push our mind too hard, asking it to refrain from carbs and cigarettes, we struggle to avoid the negative thoughts and emotions that lead to sour moods. Consider this 2007 study: The scientists told subjects to refrain from eating a tempting chocolate donut for a few minutes. Then, they insulted these poor (and probably hungry) experimental volunteers. Not surprisingly, those who had successfully resisted the donut were more likely to get aggressive in response to the insult. Or look at the medical literature, in which people on diets are typically “irritable and aggressive.” (This is the so-called cranky dieter effect.) Although we’d like to be happy and polite, those positive moods take cognitive work, and our brain is too tired to care. We lose our temper because we lack the willpower to swallow our angry words.