Thirsty participants in a dictator's game share water more often equally with powerless, anonymous others than they do money, even when they earned both types of rewards previously; so that altruistic behavior is more likely to occur when it comes to sharing primary rewards

How selfish is a thirsty man? A pilot study on comparing sharing behavior with primary and secondary rewards. Astrid Kause, Oliver Vitouch, Judith Glück. PLOS One, August 20, 2018, https://doi.org/10.1371/journal.pone.0201358

Abstract: Human social interactions in daily life involve sharing various types of rewards. Previous research evolving around issues of selfish versus altruistic behavior indicates that when individuals share rewards like money with powerless others, some are purely selfish while a substantial number shares evenly. It is, however, mostly unknown how they share primary rewards like water, compared to secondary rewards like money. We adopt the widely studied Dictator Game for comparing water to be divided among study participants with a monetary reward. We show that thirsty participants share water more often equally with powerless, anonymous others than they do money. This is the case even when they earned both types of rewards in a preceding task. Results indicate that altruistic behavior is more likely to occur when it comes to sharing primary rewards. The ecologically more valid scenario employed in this study provides initial evidence that the concept of a self-interested homo economicus might not apply to everyday social interactions involving rewards other than money.