[8–10]. Our results suggest that positive feelings arising from sharing one’s resources with strong social ties may be one mechanism by which such behaviour was reinforced, and thus may serve an evolutionarily adaptive function. The present findings also provide empirical support for the value of relying on participants’ definition of what constitutes a strong or weak social tie. While it may seem intuitive to classify spending targets into categories based on relationship type – such that friends and family would be classified as ‘‘strong ties’’ – our data suggest that this distinction lacks the same predictive power as participants’ self-categorization of the people in their lives as strong or weak ties. Future research on the emotional benefits of prosocial behaviour could benefit from considering the level of intimacy in a relationship rather than categorizing individuals by relationship type. These findings should not be taken to suggest that people should avoid spending on weak social ties. Indeed, treating an acquaintance from yoga to a coffee after class might help to build a new strong tie. Thus, spending money on a weak social tie might help facilitate the development of new strong ties in the longer term. This study is not without limitations. One simultaneous strength and shortcoming of this design is that participants were asked to recall a previous spending experience, rather than engage in a new spending behaviour for the purposes of this study. While this methodology may leave open questions of whether the same emotions would occur immediately after spending, this reminis- cence-based methodology has been used successfully in previous research [11] and captures the kind of remembered utility that is an important component of the overall utility of experiences [12,13]. References 1. Dunn EW, Aknin LB, Norton MI (2008) Spending money on others promotes happiness. Science 319: 1687–1688. 2. VanLear CA, Koerner A, Allen DM (2006) Relationship Typologies. In Vangelisti AL, Perlman D, eds. The Cambridge handbook of personal relationships. Cambridge: Cambridge University Press. pp 91–112. 3. Granovetter MS (1973) The strength of weak ties. Amer J Soc 78(6): 1360. 4. Lyubomirsky S, King L, Diener E (2005) The benefits of frequent positive affect: Does happiness lead to success? Psych Blltn 131(6): 803–851. 5. Fowler JH, Christakis (2009) Dynamic spread of happiness in a large social network: Longitudinal analysis over 20 years in the Framingham heart study. Br Med J 337: a2338. 6. Hill AL, Rand DG, Nowak MA, Christakis (2010) Emotions as infectious diseases in a large social network: the SISa model. Proc R Soc B 277: 3827–3835. 7. Watson D, Clark LA, Tellegen A (1988) Development and validation of brief measures of positive and negative affect: The PANAS scales. J Pers Soc Psych 54: 1063–1070. Spending Target Matters For Happiness Past research in our lab has repeatedly shown that people are happier when they use financial resources to benefit others rather than themselves [Aknin, Dunn, Sandstrom & Norton, submitted, 1,14]. Given that our aim in the current study was to answer the question of whether the happiness we gain from spending on others depends on who, specifically, the money is spent on, we did not include a condition in which participants were asked to recall spending on themselves. Finally, this work adds to the growing body of empirical research suggesting how people might best spend money on others in order to reap emotional benefits and, more broadly, determining the conditions under which prosocial behaviour might lead to happiness. Knowing that individuals are happier after engaging in prosocial behaviour directed toward strong rather than weak social ties allows for simple and straight-forward applications. Of course, further research should examine why engaging in prosocial behaviour directed towards strong social ties leads to greater happiness, how long the mood benefits last, and whether doing kind deeds for an acquaintance helps transform a shallow relationship into a deep friendship. The current results, however, shed novel insight into translating spending choices into happiness: the next time you find a few spare dollars in your pocket, you will be happiest if you treat your best friend. Author Contributions Conceived and designed the experiments: GMS LBA EWD MIN. Performed the experiments: GMS LBA. Analyzed the data: GMS LBA. Contributed reagents/materials/analysis tools. Wrote the manuscript: GMS LBA EWD MIN. 8. Dal Bo, P (2005) Cooperation under the shadow of the future: Experimental evidence from infinitely repeated games. Am Econ Rev 95(5): 1591–1604. 9. Nowak, MA, Sigmund, K (1992) Tit for tat in heterogeneous populations. Nature 355: 250–253. 10. Trivers, RL (1971) The evolution of reciprocal altruism. Q Rev Biol 46: 35–57. 11. Van Boven L, Gilovich T (2003) To do or to have? That is the question. J Pers Soc Psych 85(6): 1193–1202. 12. Kahneman D, Wakker P, Sarin R (1997) Back to Bentham? Explorations of experienced utility. Q J Econ 112: 375–405. 13. Zauberman G, Ratner R, Kim B (2009) Memories as assets: Strategic memory protection in choice over time. J Con Rsrch 35(5): 715–728. 14. Dunn EW, Ashton-James CE, Hanson MD, Aknin LB (2010) On the costs of self-interested economic behavior: How does stinginess get under the skin? J Hlth Psych 15: 627–633. PLoS ONE | www.plosone.org 3 February 2011 | Volume 6 | Issue 2 | e17018