Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Experimental evidence for a pure collaboration effect


What makes us willing to sacrifice our own self-interest for another person? Humans can forgo short-term individual gain to achieve long-term benefits1,2,3,4—but long-run self-interest cannot fully explain unselfish behaviour5. Collaboration in our evolutionary past may have played a role in shaping an innate human sense of distributive justice6, influencing who we consider deserving of our aid or generosity. Previous research has not been able to isolate this response to collaboration as an independent effect, distinct from other motivations to share7,8. Here we present evidence of a pure collaboration effect, distinct from motivations of future reciprocity, in-group favouritism or concern for accountability. We demonstrate this effect among adult subjects in an economic setting, showing that the effect constitutes a psychological phenomenon with relevance for real-world social and political behaviour. This collaboration effect is substantial: it motivates sharing among people otherwise inclined to share nothing and increases the proportion of participants willing to give up half of their allotted money. We find evidence supporting our hypothesis that the collaboration effect operates by creating a sense of debt owed to one’s collaborator.

Your institute does not have access to this article

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Collaboration increases the amount of money given to one’s partner.
Fig. 2: The effect of collaboration is independent of the partner’s relative effort.
Fig. 3: Collaboration increases the probability of reporting indebtedness as one’s primary motivation.
Fig. 4: In-person collaboration increases the amount of money given to one’s partner.

Code availability

The code reproducing the analysis is publicly available in Northwestern University’s Arch Research and Data Repository at:

Data availability

The data sets generated during and analysed for the current study are publicly available in Northwestern University’s Arch Research and Data Repository at: These data sets include data for all figures (Figs. 14).


  1. Ostrom, E. Governing the Commons (Cambridge Univ. Press, New York, 1990).

  2. Andreoni, J. & Miller, J. H. Rational cooperation in the finitely repeated prisoner’s dilemma: experimental evidence. Econ. J. 103, 570–585 (1993).

    Article  Google Scholar 

  3. Nowak, M. A. & Sigmund, K. Evolution of indirect reciprocity by image scoring. Nature 393, 573–577 (1998).

    CAS  Article  Google Scholar 

  4. Nowak, M. A. & Sigmund, K. Evolution of indirect reciprocity. Nature 437, 1291–1298 (2005).

    CAS  Article  Google Scholar 

  5. Fehr, E. & Fischbacher, U. The nature of human altruism. Nature 425, 785–791 (2003).

    CAS  Article  Google Scholar 

  6. Hamann, K., Warneken, F., Greenberg, J. R. & Tomasello, M. Collaboration encourages equal sharing in children but not in chimpanzees. Nature 476, 328–331 (2011).

    CAS  Article  Google Scholar 

  7. Milinski, M., Semmann, D. & Krambeck, H.-J. Reputation helps solve the ‘tragedy of the commons’. Nature 415, 424–426 (2002).

    Article  Google Scholar 

  8. Brewer, M. B. In-group bias in the minimal intergroup situation: a cognitive-motivational analysis. Psychol. Bull. 86, 307–324 (1979).

    Article  Google Scholar 

  9. Konow, J. A positive theory of economic fairness. J. Econ. Behav. Organ. 31, 13–35 (1996).

    Article  Google Scholar 

  10. Alesina, A. & Angeletos, G.-M. Fairness and redistribution. Am. Econ. Rev. 95, 960–980 (2005).

    Article  Google Scholar 

  11. Gilens, M. Racial attitudes and opposition to welfare. J. Polit. 57, 994–1014 (1995).

    Article  Google Scholar 

  12. Shaw, A. & Olson, K. R. Children discard a resource to avoid inequity. J. Exp. Psychol. Gen. 141, 382–395 (2012).

    Article  Google Scholar 

  13. Fehr, E., Fischbacher, U. & Gächter, S. Strong reciprocity, human cooperation, and the enforcement of social norms. Hum. Nat. 13, 1–25 (2002).

    Article  Google Scholar 

  14. Bowles, S. & Gintis, H. The evolution of strong reciprocity: cooperation in heterogeneous populations. Theor. Popul. Biol. 65, 17–28 (2004).

    Article  Google Scholar 

  15. Gintis, H., Henrich, J., Bowles, S., Boyd, R. & Fehr, E. Strong reciprocity and the roots of human morality. Soc. Justice Res. 21, 241–253 (2008).

    Article  Google Scholar 

  16. Fong, C. M., Bowles, S. & Gintis, H. in Handbook of the Economics of Giving, Altruism and Reciprocity Vol. 2 (eds Kolm, S.-C. & Ythier, J. M.) 1439–1464 (Elsevier, London, 2006).

  17. Aristotle. Nicomachean Ethics (Harvard Univ. Press, Cambridge, MA, 1926).

  18. McCall, L. The Undeserving Rich (Cambridge Univ. Press, New York, 2013).

  19. Corbit, J., McAuliffe, K., Callaghan, T. C., Blake, P. R. & Warneken, F. Children’s collaboration induces fairness rather than generosity. Cognition 168, 344–356 (2017).

    Article  Google Scholar 

  20. Locke, J. Two Treatises of Government (1689) (Millar, London, 1764).

  21. Guala, F. Reciprocity: weak or strong? What punishment experiments do (and do not) demonstrate. Behav. Brain Sci. 35, 1–15 (2012).

    Article  Google Scholar 

  22. Axelrod, R. & Hamilton, W. D. The evolution of cooperation. Science 211, 1390–1396 (1981).

    CAS  Article  Google Scholar 

  23. Otten, S. & Moskowitz, G. B. Evidence for implicit evaluative in-group bias: affect-biased spontaneous trait inference in a minimal group paradigm. J. Exp. Soc. Psychol. 36, 77–89 (2000).

    Article  Google Scholar 

  24. Dasgupta, N. Implicit ingroup favoritism, outgroup favoritism, and their behavioral manifestations. Soc. Justice Res. 17, 143–169 (2004).

    Article  Google Scholar 

  25. Leyens, J.-P. et al. The emotional side of prejudice: the attribution of secondary emotions to ingroups and outgroups. Personal. Soc. Psychol. Rev. 4, 186–197 (2000).

    Article  Google Scholar 

  26. Chatman, C. M. & von Hippel, W. Attributional mediation of in-group bias. J. Exp. Soc. Psychol. 37, 267–272 (2001).

    Article  Google Scholar 

  27. Brewer, M. B. The psychology of prejudice: ingroup love or outgroup hate? J. Soc. Issues 55, 429–444 (1999).

    Article  Google Scholar 

  28. Hoffman, E., McCabe, K. & Smith, V. L. Social distance and other-regarding behavior in dictator games. Am. Econ. Rev. 86, 653–660 (1996).

    Google Scholar 

  29. Bohnet, I. & Frey, B. S. Social distance and other-regarding behavior in dictator games: comment. Am. Econ. Rev. 89, 335–339 (1999).

    Article  Google Scholar 

  30. Henrich, J. et al. ‘Economic man’ in cross-cultural perspective: behavioral experiments in 15 small-scale societies. Behav. Brain Sci. 28, 795–855 (2005).

    PubMed  Google Scholar 

  31. Amir, O., Rand, D. G. & Gal, Y. K. Economic games on the internet: the effect of $1 stakes. PLoS One 7, e31461 (2012).

    CAS  Article  Google Scholar 

  32. Rand, D. G. et al. Social heuristics shape intuitive cooperation. Nat. Commun. 5, 3677 (2014).

    CAS  Article  Google Scholar 

Download references


Funding for experiment 4 was provided through Northwestern University’s Institute for Policy Research. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We thank G. Huber, J. Druckman, D. Botti, A. Fang and P. Tucker for important feedback.

Author information

Authors and Affiliations



M.C.M. conducted experiment 4. M.C.M. and A.S.G. otherwise contributed equally.

Corresponding author

Correspondence to Mary C. McGrath.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Methods and Supplementary Tables 1 and 2.

Reporting Summary

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

McGrath, M.C., Gerber, A.S. Experimental evidence for a pure collaboration effect. Nat Hum Behav 3, 354–360 (2019).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing