Why you might get meaner and more ‘hostile’ when you’re mentally drained, according to a new study

Talk about brain drain.

A new study finds that making several difficult decisions at once can wear down the brain areas responsible for self-control, essentially putting them “to sleep” and leading to aggressive or impulsive behavior.

The research explores the controversial theory of “ego depletion” — the idea that we have limited mental energy for tasks like decision-making or resisting temptations, and depleting that reserve leads to less willpower and more bad choices.

For this study, scientists from the IMT School for Advanced Studies Lucca in Italy wanted to see how “ego depletion” affects brain activity.

They had participants undergo an hourlong series of tasks designed to mentally wear them out before playing games that required varying degrees of aggression and cooperation.

Researchers measured the electrical activity in their brains as they played the games.

They compared the results to a control group, finding that the mentally fatigued participants became significantly more uncooperative as the day wore on.

“Our study shows that mental fatigue has a measurable effect on behavior and that, when a certain degree of fatigue sets in, people are more likely to behave in a hostile manner,” said Erica Ordali, a research fellow at the IMT School and first author of the paper.

Ordali’s team found that spending just 45 minutes on executive function tasks like making plans and solving problems increases the likelihood you’ll engage in aggressive acts in social situations.

They linked the behavioral changes to sleep-like activity within frontal brain areas related to decision-making and impulse control. The disengagement of these areas explains the reduced ability to self-control, the researchers said.

The findings were published Monday in the journal PNAS.

“Overall, these findings have important implications for multiple situations in everyday life, including economic transactions and legal agreements, as they demonstrate that when the brain is ‘tired’ we may make choices that go even opposite to our own interest,” said Pietro Pietrini, coauthor of the paper and director of the Molecular Mind Lab at the IMT School.

But not everyone subscribes to the “ego depletion” theory.

One large-scale study from 2020 found no evidence of a depletion effect after asking participants to complete a task that involved crossing off the letter “e” on a sheet of text in certain instances or writing a story about a recent trip without using words containing the letters “a” or “n.”

“We tested an ego depletion hypothesis on more than 3500 participants in 36 independent laboratories, which used one of two experimental protocols,” the authors of that study wrote. “The results lead us to conclude that depletion is not as reliable or robust as previously assumed.”