Performance of virtually every action requires some degree of effort. Thus, it plays a role in motivation and decision-making processes. Recent works suggest that effort-based choice might not always follow the law of less work, i.e., effort negatively influencing evaluation of effort-contingent outcomes. Instead, effort can also have a positive impact on evaluation. To test this proposition, recently termed the effort paradox, we introduce a novel model that allows effort to have both negative

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... and positive effects. Our participants performed a series of choices between money and common objects contingent on exerting different levels of physical effort (stair climbing). Our flexible dual-evaluation model explained participants' choices better compared to previous formal models. Our model also accounts for paradoxical effects of effort on outcome evaluation within an individual, across individuals, and also across different choice situations paradigms, different individuals, and also within the same individual.