Previous work suggests that lifespan developmental differences in cognitive control abilitiesmight be due to maturational and aging-related changes in prefrontal cortex functioning.However, there are other explanations: For example, it could be that children and older adults differ from younger adults in how they balance the effort of engaging in control against its potential benefits. In this work, we assume that the degree of engagement in cognitive effort depends on the opportunity cost of time (average reward rate per unit time). If the average reward rate is high, participants should speed up responding whereas if it is low, they should respond more slowly. Developmental changes in opportunity cost assessments may lead to differences in the sensitivity to changes in reward rate. To examine this hypothesis in children, adolescents, younger, and older adults, we applied a reward rate manipulation in two well-established cognitive control tasks: a modified Erikson Flanker and a task-switching paradigm. We found a significant interaction between age group and average reward rate, such that older adults were more sensitive to the average reward rate than the other age groups. However, as task complexity increased (from the Flanker task to the task-switching paradigm), children also became sensitive to changes in reward rate. This may suggest that when demands on cognitive load reach capacity limitations, participants engage in strategic behaviour to optimize performance: a view we present as the “sweet sport” argument of effort allocation.

In keeping with the view that individuals invest cognitive effort in accordance with its relative costs and benefits, reward incentives typically improve performance in tasks that require cognitive effort. At the same time, increasing effort investment may confer larger or smaller performance benefits—i.e., the marginal value of effort—depending on the situation, or context. On this view, we hypothesize that the magnitude of reward-induced effort modulations should depend critically on the marginal value of effort for the given context, and furthermore, the marginal value of effort of a context should be learned over time as a function of direct experience in the context. Using two well-characterized cognitive control tasks and simple computational models, we demonstrate that individuals appear to learn the marginal value of effort for different contexts. In a task-switching paradigm (Experiment 1), we found that participants initially exhibited reward-induced switch cost reductions across contexts—here, task switch rates—but over time learned to only increase effort in contexts with a comparatively larger marginal utility of effort. Likewise, in a Flanker task (Experiment 2), we observed a similar learning effect across contexts defined by the proportion of incongruent trials. Together, these results enrich theories of cost-benefit effort decision-making by highlighting the importance of the (learned) marginal utility of cognitive effort.