1. Department of Political Science, University of California, San Diego, California 92093, USA
2. Center for Adaptive Behaviour and Cognition, Max Planck Institute for Human Development, Lentzeallee 94, Berlin 14195, Germany
3. Department of Political Science, Stanford University, Palo Alto, California 94305, USA
4. Department of Anthropology, University of California, Davis, California 95616, USA
5. Department of Political Science, University of Miami, Coral Gables, Florida 33124, USA

Correspondence to: James H. Fowler¹ Correspondence and requests for materials should be addressed to J.H.F. (Email: jhfowler@ucsd.edu).

Abstract

Participants in laboratory games are often willing to alter others' incomes at a cost to themselves, and this behaviour has the effect of promoting cooperation^{1, 2, 3}. What motivates this action is unclear: punishment and reward aimed at promoting cooperation cannot be distinguished from attempts to produce equality⁴. To understand costly taking and costly giving, we create an experimental game that isolates egalitarian motives. The results show that subjects reduce and augment others' incomes, at a personal cost, even when there is no cooperative behaviour to be reinforced. Furthermore, the size and frequency of income alterations are strongly influenced by inequality. Emotions towards top earners become increasingly negative as inequality increases, and those who express these emotions spend more to reduce above-average earners' incomes and to increase below-average earners' incomes. The results suggest that egalitarian motives affect income-altering behaviours, and may therefore be an important factor underlying the evolution of strong reciprocity⁵ and, hence, cooperation in humans.

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