Mental fatigue may make rewards more desirable, according to a study in rats and humans. Exerting cognitive effort has been linked with making unhealthy choices. In the past, the link has been explained via a weakening of inhibitory control or will power.

Marcello Solinas and colleagues explore the possibility that cognitive effort may also make unhealthy choices more tempting by increasing the perceived reward. The work is published in PNAS Nexus.

Rats who completed a cognitively demanding task self-administered more cocaine than rats who did not complete a cognitive demanding task -- or rats who were allowed to rest to 2-4 hours after completing the complex task. Humans who were given a task that requires significant cognitive effort -- suppressing the thought of a white bear while listing other thoughts -- ate more potato chips and rated the chips as better-tasting than controls who had not completed an effortful task, suggesting that cognitive effort intensified participants' hedonic experience of snacking on the salty and fatty snack.

To rule out the possibility that cognitive effort increases the likelihood that humans make extreme judgments in general, a follow-up study using difficult and easy writing tasks found that ratings of chocolate increased after cognitive effort but ratings of the length of a pen or the brightness of a yellow post-it note did not. The authors suggest that this sequence is not a mere byproduct of evolution but could be adaptive in some contexts.

According to the authors, the results have implications for the management of addiction and other unhealthy behaviors.