RESUMEN

Social media has the potential to encourage prosocial behaviors at scale, yet very little causal evidence exists on the impact of related efforts. Blood donation is a particularly difficult, but essential prosocial behavior that is often critically undersupplied. We examine the effect of Facebook's blood donation tool on voluntary blood donation. We partnered with four major blood banks in the United States covering 363 collection facilities in 46 states and Washington, D.C. We tracked the tool's impact on blood donations during its staggered rollout on a sample of more than 47,000 facility-date observations from March 2019 to September 2019. The tool caused an increase of 0.55 total donations per facility per day (+4.0% [95% CI: 0.04%-8.0%]), and an increase of 0.15 donations from first-time donors per facility per day (+18.9% [95% CI: 4.7%-33.1%]). Longitudinal evidence from Brazil and India suggests the share of donors who both received a message from the tool and stated they were influenced by Facebook to donate increased from 0% to 14.1% [95% CI: 12.1%-16.2%] in the first year of the tool's deployment (i.e., September 2018 to August 2019). These meaningful increases, especially from first-time donors, demonstrate that social media platforms can play an important role in fostering offline prosocial behaviors that benefit the health and well-being of societies around the world.

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Peaceful coexistence and trade among human groups can be fragile and intergroup relations frequently transition to violent exchange and conflict. Here we specify how exogenous changes in groups' environment and ensuing carrying-capacity stress can increase individual participation in intergroup conflict, and out-group aggression in particular. In two intergroup contest experiments, individuals could contribute private resources to out-group aggression (versus in-group defense). Environmental unpredictability, induced by making non-invested resources subject to risk of destruction (versus not), created psychological stress and increased participation in and coordination of out-group attacks. Archival analyses of interstate conflicts showed, likewise, that sovereign states engage in revisionist warfare more when their pre-conflict economic and climatic environment were more volatile and unpredictable. Given that participation in conflict is wasteful, environmental unpredictability not only made groups more often victorious but also less wealthy. Macro-level changes in the natural and economic environment can be a root cause of out-group aggression and turn benign intergroup relations violent. This article is part of the theme issue 'Intergroup conflict across taxa'.

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RESUMEN

In an experiment we investigate preferences for allocation of a public good among group members who contributed unequally in providing the public good. Inducing the group goal of productivity resulted in preferences for equitable allocations, whereas inducing the group goals of harmony and social concern resulted in preferences for equal final outcomes. The study makes a contribution by simultaneously treating provision and allocation of a public good, thus viewing these as related processes. Another contribution is that a new paradigm is introduced that bears closer resemblance to real life public good dilemmas than previous research paradigms do.