Why stress and hunger both increase and decrease prosocial behaviour

Humans are regularly in suboptimal psychophysiological states like stressed or hungry. Previous research has made both claims that such impairments should decrease and that they should increase prosocial behaviour. We describe the overarching theoretical reasoning underlying these opposing predictions. Then we discuss empirical research on the two impairments most frequently studied, acute stress and acute hunger, and we find that neither alters prosocial behaviour clearly in one direction. We argue that this is because even under impairments, humans react flexibly to the incentive structure of the specific social situation they are in. Hence, either prosocial or egoistic tendencies get expressed, depending on which strategy can lead to fulfilment of the need the impairment triggered.