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This natural field experiment tests the effects of purely symbolic awards on volunteer retention in a public goods context. The experiment is conducted at Wikipedia, which faces declining editor retention rates, particularly among newcomers. Randomization assures that award receipt is orthogonal to previous performance. The analysis reveals that awards have a sizeable effect on newcomer retention, which persists over the four quarters following the initial intervention. This is noteworthy for indicating that awards for volunteers can be effective even if they have no impact on the volunteers’ future career opportunities. The awards are purely symbolic, and the status increment they produce is limited to the recipients’ pseudonymous online identities in a community they have just recently joined. The results can be explained by enhanced self-identification with the community, but they are also in line with recent findings on the role of status and reputation, recognition, and evaluation potential in online communities. Data, as supplemental material, are available at http://dx.doi.org/10.1287/mnsc.2016.2540 . This paper was accepted by John List, behavioral economics .

Economic theory suggests that market economies are likely to underprovide innovation because of the public good nature of knowledge. Empirical evidence from the United States and other advanced economies supports this idea. We summarize the pros and cons of different policy instruments for promoting innovation and provide a basic \"toolkit\" describing which policies are most effective according to our reading of the evidence. In the short run, R&D tax credits and direct public funding seem the most productive, but in the longer run, increasing the supply of human capital (for example, relaxing immigration rules or expanding university STEM admissions) is likely more effective.

This essay addresses the climate-change externality—its sources, its potential impacts, and the policy tools that are available to stem the rising tides and damages that this externality will likely bring to humans and the natural world. It draws upon my writings in the area, most of which are cited in the references.

Geography and social links shape economic interactions. In industries, schools, and markets, the entire network determines outcomes. This paper analyzes a large class of games and obtains a striking result. Equilibria depend on a single network measure: the lowest eigenvalue. This paper is the first to uncover the importance of the lowest eigenvalue to economic and social outcomes. It captures how much the network amplifies agents' actions. The paper combines new tools—potential games, optimization, and spectral graph theory—to solve for all Nash and stable equilibria and applies the results to R&D, crime, and the econometrics of peer effects.

Cities increasingly address climate change, e.g. by pledging city-level emission reduction targets. This is puzzling for the provision of a global public good: what are city governments’ reasons for doing so, and do pledges actually translate into emission reductions? Empirical studies have found a set of common factors which relate to these questions, but also mixed evidence. What is still pending is a theoretical framework to explain those findings and gaps. This paper thus develops a theoretical public choice model. It features economies of scale and distinguishes urban reduction targets from actual emission reductions. The model is able to explain the presence of targets and public good provision, yet only under specified conditions. It is also able to support some stylized facts from the empirical literature, e.g. on the effect of city size, and resolves some mixed evidence as special cases. Larger cities chose more ambitious targets if marginal net benefits of mitigation rise with city size—if they set targets at all. Whether target setting is more likely for larger cities depends on the city type. Two types are obtained. The first type reduces more emissions than a free-riding city. Those cities are more likely to set a target when they are larger. However, they miss the self-chosen target. Cities of the second type reach their target, but mitigate less than a free-riding city. A third type does not exist. With its special cases, the model can thus guide further empirical and theoretical work.

One lingering puzzle is why voluntary contributions to public goods decline over time in experimental and real-world settings. We show that the decline of cooperation is driven by individual preferences for imperfect conditional cooperation. Many people's desire to contribute less than others, rather than changing beliefs of what others will contribute over time or people's heterogeneity in preferences makes voluntary cooperation fragile. Universal free riding thus eventually emerges, despite the fact that most people are not selfish. (D12, D 83, H41, Z13)

Milton Friedman famously suggested that firms ought not divert profits toward public goods because shareholders can better make these contributions themselves. Despite this, activist shareholders are increasingly successful in persuading firms to be “socially responsible.” We study firm behavior when shareholders care about public goods as well as profits and when managerial contracts reflect these concerns. Under these ideal conditions, managers redirect more profits toward public goods than shareholders would when acting separately—shareholders are poorer but happier. Further, so long as the public good is sufficiently desirable, the manager selects the socially optimal level of output, despite the mismatch between shareholder preferences and those of society at large. This paper was accepted by Joshua Gans, business strategy.

International efforts to provide global public goods often face the challenges of coordinating national contributions and distributing costs equitably in the face of uncertainty, inequality, and free-riding incentives. In an experimental setting, we distribute endowments unequally among a group of people who can reach a fixed target sum through successive money contributions, knowing that if they fail, they will lose all their remaining money with 50% probability. In some treatments, we give players the option to communicate intended contributions. We find that inequality reduces the prospects of reaching the target but that communication increases success dramatically. Successful groups tend to eliminate inequality over the course of the game, with rich players signaling willingness to redistribute early on. Our results suggest that coordination-promoting institutions and early redistribution from richer to poorer nations are both decisive for the avoidance of global calamities, such as disruptive climate change.

Theoretical models suggest that social networks influence the evolution of cooperation, but to date there have been few experimental studies. Observational data suggest that a wide variety of behaviors may spread in human social networks, but subjects in such studies can choose to befriend people with similar behaviors, posing difficulty for causal inference. Here, we exploit a seminal set of laboratory experiments that originally showed that voluntary costly punishment can help sustain cooperation. In these experiments, subjects were randomly assigned to a sequence of different groups to play a series of single-shot public goods games with strangers; this feature allowed us to draw networks of interactions to explore how cooperative and uncooperative behaviors spread from person to person to person. We show that, in both an ordinary public goods game and in a public goods game with punishment, focal individuals are influenced by fellow group members' contribution behavior in future interactions with other individuals who were not a party to the initial interaction. Furthermore, this influence persists for multiple periods and spreads up to three degrees of separation (from person to person to person to person). The results suggest that each additional contribution a subject makes to the public good in the first period is tripled over the course of the experiment by other subjects who are directly or indirectly influenced to contribute more as a consequence. These results show experimentally that cooperative behavior cascades in human social networks.

The literature on the private provision of public goods suggests an inverse relationship between incentives to contribute and group size. We find, however, that after an exogenous reduction of group size at Chinese Wikipedia, the nonblocked contributors decrease their contributions by 42.8 percent on average. We attribute the cause to social effects: contributors receive social benefits that increase with both the amount of their contributions and group size, and the shrinking group size weakens these social benefits. Consistent with our explanation, we find that the more contributors value social benefits, the more they reduce their contributions after the block. (JEL H41, L17, L82)