<jats:title>ABSTRACT</jats:title><jats:p>The mesolimbic dopaminergic system exerts a crucial influence on normal motivated behaviour, but the mechanism of this action in dynamic situations where decisions evolve over time remains unclear. In such circumstances, current (foreground) reward accrual rate needs to be compared continuously with potential rewards that could be obtained elsewhere (background reward rate) in order to determine the opportunity cost of staying or leaving. We hypothesised that dopamine levels specifically modulate the influence of background – but not foreground – reward information in a decision-making task that requires dynamic comparison of these variables for optimal behaviour, and that this effect would be disrupted in individuals with loss of motivation – apathy. We developed a human foraging task based on a normative theory of animal behaviour (marginal value theorem), in which participants decide when to leave locations in which rewards decreased over time in order to pursue greater returns in their environment. People’s decisions to move from current locations conformed closely to foraging principles. Pharmacological manipulation of dopamine D2 receptor activity in healthy individuals using the agonist cabergoline significantly modulated background, but not foreground, reward sensitivity. In a separate study, this same effect was observed in patients with Parkinson’s disease, dependent on presence of apathy. Using an ecologically derived framework we demonstrate a specific mechanism by which dopamine modulates dynamic human decision-making, and how impairment of this mechanism can contribute to pathological loss of motivation.</jats:p>