A study reveals that prolonged mental exertion leads to changes in the brain’s frontal cortex, resembling sleep activity, which heightens aggressive behaviors.
This finding supports the idea that mental fatigue compromises decision-making abilities, influencing how individuals react in competitive or cooperative settings.
Understanding Mental Fatigue and Aggression
Prolonged mental fatigue can weaken brain regions essential for self-control, leading people to behave more aggressively.
In a new multidisciplinary study published today (November 11) in PNAS, researchers in neuroscience and economics at the IMT School of Advanced Studies Lucca link the debated concept of “ego depletion”—the idea that willpower diminishes with use—to physical changes in brain areas that manage executive functions. Specifically, mental fatigue in an awake brain appears to correlate with an increase in EEG waves typically seen during sleep in the frontal cortex, the region responsible for decision-making.
Theories and Critiques of Ego Depletion
Theories of ego depletion emerged in the scientific literature in the early 2000s, centering on the idea that self-control is a limited cognitive resource. This theory suggests that exercising self-control can deplete it over time, with implications for behavior. Studies in behavioral economics have used cognitive manipulations in economic games to demonstrate the effects of ego depletion, showing that people experiencing mental fatigue exhibit less empathy, a reduced inclination toward altruism, and an increased tendency toward aggression.
In more recent years, however, this theory has been criticized: subsequent studies have not always managed to replicate the effect of “consumption” of willpower for individuals engaged in strenuous cognitive tasks or, if they have succeeded, they have found a much smaller effect than initially estimated. Moreover, the brain correlates of such an effect remained obscure.
Innovative Study Links Brain Waves and Behavioral Changes
The new study addresses the classic problem by adding the neuroscientific perspective. The research on sleep has identified a phenomenon called “local sleep”: it happens when some brain areas in the awake individual begin to show on the EEG typical neural activity seen during sleep, namely delta waves. It has been shown that this happens particularly in cases of mental fatigue. “Our starting hypothesis was that local sleep would be the neuronal manifestation of the phenomenon of ego depletion known to psychology,” observes Erica Ordali, research fellow at the IMT School and first author of the paper.
To test this hypothesis, the researchers subjected a group of individuals to some fatigue tasks lasting one hour — instead of the classic fifteen minutes usually used in this type of study — in order to make the potential effect, if present, more evident. Next, individuals played economic games that required varying degrees of aggression and cooperation, including the so-called hawk and dove game. In this game, limited resources are to be shared in a hostile environment situation, with people having the choice between collaborating or overbearing behavior, which may result in the loss of resources for both parties. Compared to a control group not subjected to cognitive fatigue, the individuals who had undergone it proved to be significantly more uncooperative and hostile. Specifically, the peaceful cooperation rate dropped from 86 percent in the “No Fatigue” to 41 percent in the “Fatigue” group (p>0.001, for a total of 447 subjects).
Implications for Decision-Making and Social Behavior
All study participants in the experiment (n=44) underwent electroencephalograms while playing economics games. In line with the study hypothesis, fatigued individuals showed the emergence of areas with typical sleep waves in some areas of the frontal cortex, which was completely absent in the others. “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,” says Ordali.
“These results provide a scientific basis to popular wisdom that suggests to ‘sleep on it’ before making a decision, by showing that metabolic exhaustion within certain brain areas does affect our decision-making processes,” says Pietro Pietrini, coauthor of the paper and Director of the Molecular Mind Lab at the IMT School, where the study was designed. “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. As a matter of fact, this is what people do also in most criminal acts” concludes Pietrini.
Reference: “Prolonged exertion of self-control causes increased sleep-like frontal brain activity and changes in aggressivity and punishment” by Erica Ordali, Pablo Marcos-Prieto, Giulia Avvenuti, Emiliano Ricciardi, Leonardo Boncinelli, Pietro Pietrini, Giulio Bernardi and Ennio Bilancini, 11 November 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2404213121
The study was conducted by the IMT School for Advanced Studies Lucca in collaboration also with researchers at the University of Florence.
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