1 Are motivational biases adaptive? An agent-based model of human judgement under uncertainty Dylan Evans 1 , Annerieke Heuvelink 1 and 2 and Daniel Nettle 3 1 Corresponding author: Biomimetics Group Department of Mechanical Engineering University of Bath Bath BA2 7AY UK Email: d.evans@bath.ac.uk 2 Department of Philosophy University of Utrecht Heidelberglaan 8 3584 CS Utrecht Netherlands 3 Departments of Psychology and Biological Sciences The Open University Walton Hall Milton Keynes MK7 6AA UK Abstract Human judgement has been shown to involve a number of biases, under which people’s assessment of the likelihood of a state of affairs is affected by their judgement of how desirable that state of affairs is. It has been suggested that such biases are adaptive, since in an uncertain world they lead people to make the least costly and most beneficial decisions. However, it is has never been shown formally that making biased decision-making could be an evolutionarily adaptive strategy. We approach this question by constructing an agent-based computer model, in which agents with different decision rules are allowed to compete in a variety of environments. We find that under certain environmental conditions unbiased agents using classicial decision theory are outperformed by agents with biases similar to those observed in humans in empirical studies. Our model supports the view that the biases people display in judgement under uncertainty may be design features rather than design flaws of the human mind.