Periods of stress and mental fatigue are often when people feel least equipped to resist temptation. But why? A new study published in PNAS Nexus has discovered that after intense mental effort, not only is self-control depleted, but rewards also feel more satisfying. The study, which examined both rats and humans, provides a clearer picture of why self-regulation often fails under stress and strain.
The researchers sought to explore the link between mental fatigue and self-regulation failures, particularly during periods of stress or cognitive strain. Previous studies had shown that mental exhaustion weakens self-control, but these focused primarily on a reduced ability to resist temptation. Less was known about whether fatigue also intensifies the appeal of rewards themselves. By examining this underexplored mechanism, the researchers aimed to better understand the processes that lead to unhealthy behaviors, such as addiction and overeating, and provide new insights into managing self-regulation.
“I have spent many years doing research on self-control. It’s one of the most important human traits and a major key to success in life, personally, socially, and professionally,” said study author Roy F. Baumeister, a visiting professor at Harvard University and author of Willpower: Rediscovering the Greatest Human Strength.
“Many studies have found that self-control is like a muscle. After exercise, it is ‘tired’ – does not perform as well. But this is a major extension. Mental fatigue not only weakens self-control – it intensifies feelings and desires. Thus, there is a ‘doubly whammy’ – people have less ability to resist plus stronger impulses and desires.”
“I met the lead author (Marcello Solinas) by accident at a conference, where he was presenting his research on rats,” Baumeister explained. “I told him we had similar findings with people. We decided to put our data together. To find the same effect with rats and cocaine, and with humans and other desires, is very exciting.”
Solinas, CNRS director of research at the University of Poitiers, told PsyPost: “I was interested in the effects of cognitive exercise on addiction with the expectation that cognitive exercise would decrease cocaine intake. The results went in the opposite direction, which actually was a finding predicted by the ego-depletion theory proposed by Baumeister.”
In the first set of experiments with rats, the researchers manipulated cognitive effort by having the animals engage in a demanding task. Rats in the high-effort group were trained to perform an attentional set-shifting task, requiring them to adapt their responses to changing rules to receive food rewards. This task was cognitively taxing, as it required attention, flexibility, and inhibitory control.
In contrast, rats in the low-effort group performed a simpler version of the task, where food rewards were provided without requiring them to exert significant cognitive effort. After completing their tasks, the rats were allowed to self-administer cocaine or saline. To investigate whether rest altered the effect of cognitive effort, some rats were allowed to rest for a few hours before the cocaine session, while others were given immediate access.
The researchers measured how much cocaine the rats consumed and monitored their locomotor activity, a marker of reward sensitivity. The findings revealed that rats in the high-effort group consumed more cocaine when given immediate access compared to those in the low-effort group. This suggested that cognitive effort heightened the rewarding effect of the drug.
“The results in rats were completely surprising because we expected a decrease in cocaine-intake after a cognitive exercise,” Solinas said. “In addition, the most parsimonious explanation of increased cocaine consumption in rats was not the usual depletion of self-control capacity but simply that cocaine was more rewarding after a cognitive effort.”
However, this effect diminished when the rats were allowed to rest before accessing cocaine, with high-effort rats consuming less cocaine than even the low-effort group. Furthermore, rats in the high-effort, no-rest group displayed increased locomotor activity after cocaine administration, supporting the idea that cognitive effort intensified reward sensitivity.
The human experiments mirrored these findings in the context of food rewards. In one experiment, participants were assigned to either a high-effort or low-effort cognitive task. The high-effort group was instructed to suppress thoughts of a “white bear” while completing a thought-listing exercise—a well-established method for inducing cognitive strain. Meanwhile, the low-effort group performed a similar task but without any restriction on their thoughts. Afterward, participants were given potato chips under the pretense of evaluating their taste and texture. The researchers measured both the quantity of chips consumed and participants’ reported enjoyment.
The results showed that participants in the high-effort group ate more chips and rated them as more enjoyable compared to those in the low-effort group. Statistical analyses indicated that the increased consumption was driven by heightened enjoyment of the chips, demonstrating that cognitive effort intensified the reward value of the food. A mediation analysis confirmed that this heightened enjoyment accounted for the increased consumption.
In a second experiment with human participants, the researchers sought to determine whether cognitive effort selectively heightened the appeal of rewarding stimuli or broadly amplified all sensory perceptions. Participants completed a writing task that required varying levels of cognitive effort: the high-effort group wrote essays while avoiding common letters (such as “A” and “N”), while the low-effort group avoided uncommon letters (such as “X” and “Z”).
Afterward, participants tasted a piece of chocolate and rated its appeal, then evaluated neutral items like pens and sticky notes. The results showed that participants in the high-effort group found the chocolate more enjoyable and expressed a greater desire for more. However, their evaluations of neutral objects were unaffected, suggesting that cognitive effort specifically heightened the appeal of rewards rather than broadly altering sensory perceptions.
“The convergence between the human and rat data is surprising to a scientist, though there are many similarities between human and rat brains,” Baumeister told PsyPost. “(But also huge differences, of course.) It means that the pattern of feeling things more strongly when fatigued is common across multiple species. It is not just human beings.”
The findings provide evidence that “intense cognitive effort can not only reduce self-control (as predicted by the ego-depletion theory) but it can also increase the desire for strong rewards, drugs and palatable food, whose consumption could lead to unhealthy consequences,” Solinas said. “These are the immediate after- effect of cognitive effort. However, cognitive effort in the long run could be beneficial. The fact that this mechanism is conserved in rodents and humans suggest an evolutionary role of the increase of reward sensitivity induced by cognitive effort.”
While the study provides compelling evidence of the link between mental fatigue and heightened reward sensitivity, it has some limitations. For instance, the rat experiments involved only male subjects, which limits the generalizability of the findings to female rats. Additionally, the cognitive tasks used for rats and humans differed, making direct comparisons challenging. The researchers also noted that while cocaine served as a clear reward stimulus in rats, ethical considerations prevented its use in human participants, necessitating reliance on food rewards for the human studies.
“One should always be careful, and future research might change the picture, though usually not completely,” Baumeister said. “But we do have more studies with the same finding.”
Future research could explore how these effects vary across sexes, age groups, and different types of rewards. Investigating the neural mechanisms underlying this phenomenon could also clarify how fatigue alters brain activity in reward-related regions, such as the prefrontal cortex and nucleus accumbens. Understanding these processes may inform interventions for addiction and other behaviors associated with self-control failures.
“We would like to understand the neurobiological mechanisms underlying these effects, which is possible in rats, in order to design novel strategies to reduce the negative effects of cognitive effort,” Solinas said.
“Understanding self-control is a key to success in life, also a key to understanding the self,” Baumeister added. “For me, the big picture is to understand the human self. My understanding after almost a half a century of research is covered in my 2022 book, The Self Explained: How And Why We Become Who We Are.
The study, “Cognitive effort increases the intensity of rewards,” was authored by Mejda Wahab, Nicole L Mead, Stevenson Desmercieres, Virginie Lardeux, Emilie Dugast, Roy F Baumeister, and Marcello Solinas.
