Abstract
Evolutionary explanations of the co-existence of large-scale cooperation and warfare in human societies rest on the hypothesis of parochial altruism, the view that in-group pro-sociality and out-group anti-sociality have co-evolved. We designed an experiment that allows subjects to freely choose between actions that are purely pro-social, purely anti-social, or a combination of the two. We present behavioral evidence on the existence of strong aggression-a pattern of non-strategic behaviors that are welfare-reducing for all individuals (i.e., victims and perpetrators). We also show how strong aggression serves to dynamically stabilize in-group pro-sociality.

Abstract
The presence of workers who reciprocate higher wages with greater effort can have important consequences for firms and labor markets. Knowledge about the extent and determinants of reciprocal effort choices is, however, incomplete. We investigate the role of fairness perceptions and social preferences in a field experiment in which workers were hired for a one-time job. We show that workers who perceive being underpaid at the base wage increase their performance if the hourly wage increases, whereas those who feel adequately paid or overpaid at the base wage do not change their performance. Moreover, we find that only the workers who display reciprocity in a choice experiment show reciprocal effort responses in the field. The workers who lack reciprocity in the choice experiment do not respond to the wage increase, even if they feel underpaid at the base wage. Our findings suggest that fairness perceptions and social preferences are key in workers' performance response to wage increases. In our study, the wage increase affects effort mainly through the removal of perceived unfairness, i.e., the elimination of negative reciprocity toward the firm, rather than positive reciprocity. These results are the first direct evidence of the fair-wage effort hypothesis in the field and also help interpret previous contradictory findings in the literature.

Abstract
We measure the other-regarding behavior in samples from three related populations in the upper Midwest of the United States: college students, non-student adults from the community surrounding the college, and adult trainee truckers in a residential training program. The use of typical experimental economics recruitment procedures made the first two groups substantially self-selected. Because the context reduced the opportunity cost of participating dramatically, 91 % of the adult trainees solicited participated, leaving little scope for self-selection in this sample. We find no differences in the elicited other-regarding preferences between the self-selected adults and the adult trainees, suggesting that selection is unlikely to bias inferences about the prevalence of other-regarding preferences among non-student adult subjects. Our data also reject the more specific hypothesis that approval-seeking subjects are the ones most likely to select into experiments. Finally, we observe a large difference between self-selected college students and self-selected adults: the students appear considerably less pro-social.

Abstract
Evidence from both psychology and economics indicates that individuals give statements that appear to overestimate their ability compared to that of others. We test three theories that predict such relative overconfidence. The first theory argues that overconfidence can be generated by Bayesian updating from a common prior and truthful statements if individuals do not know their true type. The second theory suggests that self-image concerns asymmetrically affect the choice to receive new information about one's abilities, and this asymmetry can produce overconfidence. The third theory is that overconfidence is induced by the desire to send positive signals to others about one's own skill; this suggests either a bias in judgement, strategic lying, or both. We formulate this theory precisely. Using a large data set of relative ability judgements about two cognitive tests, we reject the restrictions imposed by the Bayesian model and also reject a key prediction of the self-image models that individuals with optimistic beliefs will be less likely to search for further information about their skill because this information might shatter their self-image. We provide evidence that personality traits strongly affect relative ability judgements in a pattern that is consistent with the third theory of social signalling. Our results together suggest that overconfidence in statements is more likely to be induced by social concerns than by either of the other two factors.

Abstract
Unprecedented levels of US subprime mortgage defaults precipitated a severe global financial crisis in late 2008, plunging much of the industrialized world into a deep recession. However, the fundamental reasons for why US mortgages defaulted at such spectacular rates remain largely unknown. This paper presents empirical evidence showing that the ability to perform basic mathematical calculations is negatively associated with the propensity to default on one's mortgage. We measure several aspects of financial literacy and cognitive ability in a survey of subprime mortgage borrowers who took out loans in 2006 and 2007, and match them to objective, detailed administrative data on mortgage characteristics and payment histories. The relationship between numerical ability and mortgage default is robust to controlling for a broad set of sociodemographic variables, and is not driven by other aspects of cognitive ability. We find no support for the hypothesis that numerical ability impacts mortgage outcomes through the choice of the mortgage contract. Rather, our results suggest that individuals with limited numerical ability default on their mortgage due to behavior unrelated to the initial choice of their mortgage.

Abstract
Economists and psychologists have devised numerous instruments to measure time preferences and have generated a rich literature examining the extent to which time preferences predict important outcomes; however, we still do not know which measures work best. With the help of a large sample of non-student participants and administrative data on outcomes, we gather four different time preference measures and test the extent to which they predict both on their own and when they are all forced to compete head-to-head. Our results suggest that the now familiar (β, δ) formulation of present bias and exponential discounting predicts best, especially when both parameters are used.

Abstract
Parochial altruism-a preference for altruistic behavior towards ingroup members and mistrust or hostility towards outgroup members-is a pervasive feature in human society and strongly shapes the enforcement of social norms. Since the uniqueness of human society critically depends on the enforcement of norms, the understanding of the neural circuitry of the impact of parochial altruism on social norm enforcement is key, but unexplored. To fill this gap, we measured brain activity with functional magnetic resonance imaging (fMRI) while subjects had the opportunity to punish ingroup members and outgroup members for violating social norms. Findings revealed that subjects' strong punishment of defecting outgroup members is associated with increased activity in a functionally connected network involved in sanction-related decisions (right orbitofrontal gyrus, right lateral prefrontal cortex, right dorsal caudatus). Moreover, the stronger the connectivity in this network, the more outgroup members are punished. In contrast, the much weaker punishment of ingroup members who committed the very same norm violation is associated with increased activity and connectivity in the mentalizing-network (dorsomedial prefrontal cortex, bilateral temporo-parietal junction), as if subjects tried to understand or justify ingroup members' behavior. Finally, connectivity analyses between the two networks suggest that the mentalizing-network modulates punishment by affecting the activity in the right orbitofrontal gyrus and right lateral prefrontal cortex, notably in the same areas showing enhanced activity and connectivity whenever third-parties strongly punished defecting outgroup members.

Abstract
Economists are increasingly interested in how group membership affects individual behavior. The standard method assigns individuals to "minimal" groups, i.e. arbitrary labels, in a lab. But real group often involve social interactions leading to social ties between group members. Our experiments compare randomly assigned minimal groups to randomly assigned groups involving real social interactions. While adding social ties leads to qualitatively similar, although stronger, in-group favoritism in cooperation, altruistic norm enforcement patterns are qualitatively different between treatments. Our findings contribute to the micro-foundation of theories of group preferences, and caution against generalizations from "minimal" groups to groups with social context.

Abstract
Firms are often organized into groups. Group membership has been shown empirically to have positive effects, in the form of increased prosocial behavior toward in-group members. This includes an enhanced willingness to engage in altruistic punishment of inefficient defection. Our paper provides evidence of a dark side of group membership. In the presence of cues of competition between groups, a taste for harming the out-group emerges: punishment ceases to serve a norm enforcement function, and instead, out-group members are punished harder and regardless of whether they cooperate or defect. Our results point to a mechanism that might help explain previous mixed results on the social value of punishment, and they contribute to understanding the sources of conflict between groups. They also point to an important trade-off for firms: introducing competition enhances within-group efficiency but also generates costly between-group conflict.

Abstract
Civil wars between ethnic or religious groups have cost millions of lives in recent history (1). In Bosnia and Herzegovina alone, tens of thousands were killed and millions displaced in the war between 1992 and 1995. However, although intense hatred against other groups can lead to tragedy, it often seems to come in tandem with stronger altruism and cooperation within one's own group (2, 3). This combination may have played a key role in early human development (4). Are there ways to mitigate group conflict while at the same time harvesting the potential benefits from stronger cooperation within groups? A report by Alexander and Christia on page 1392 of this issue (5) suggests that the answer is yes, under the right circumstances.

Abstract
Assigning a subjective value to a contribution to a public good often requires reflection. For many reasons, this reflection may be put off, reinforcing the underprovision of public goods. We hypothesise that nudging individuals to reflect on whether to contribute to a public good leads to the formation of issue-specific altruistic preferences. The hypothesis is tested in a large-scale field experiment on blood donations. We find that an active-decision intervention substantially increases donations among subjects who had not previously thought about the importance of donating blood. By contrast, contributions of individuals who had previously engaged in such reflection are unchanged.

Abstract
Recent shortages in the supply of blood donations have renewed the interest in how blood donations can be increased temporarily. We survey the evidence on the role of financial and other incentives in eliciting blood donations among donors who are normally willing to donate pro bono. We present the predictions from different empirical/psychological-based theories, with some predicting that incentives are effective while others predict that incentives may undermine prosocial motivation. The evidence suggests that incentives work relatively well in settings in which donors are relatively anonymous, but evidence indicates also that when image concerns become important, incentives may be counterproductive as donors do not want to be seen as greedy.

Abstract
In this paper, we argue that important labor market phenomena can be better understood if one takes (a) the inherent incompleteness and relational nature of most employment contracts and (b) the existence of reference-dependent fairness concerns among a substantial share of the population into account. Theory shows and experiments confirm that, even if fairness concerns were to exert only weak effects in one-shot interactions, repeated interactions greatly magnify the relevance of such concerns on economic outcomes. We also review evidence from laboratory and field experiments examining the role of wages and fairness on effort, derive predictions from our approach for entry-level wages and incumbent workers' wages, confront these predictions with the evidence, and show that reference-dependent fairness concerns may have important consequences for the effects of economic policies such as minimum wage laws.

Abstract
The retention of previous donors and the recruitment of new donors is a¦serious challenge for many blood donation services in their effort to¦prevent blood shortages. More and more services make use of some sort of¦donation incentives. However, the use of (material) incentives to motivate¦blood donors is fiercely controversial and there is a longstanding (ethical)¦debate about whether it should be allowed that donors receive material¦rewards. Interestingly, this debate is dealt with in almost complete absence¦of systematic empirical evidence on the effectiveness of material incentives¦in encouraging people to donate.¦In this paper, we argue that the discussion on what is ethical in motivating¦blood donors should be enriched with empirical evidence based on field¦experiments. We confront the Titmuss controversy with recent results from¦an experiment administering lottery tickets as a motivation device.¦Moreover, we take up a neglected phenomenon in the study of blood¦donors: many non-donors are not principally against donating blood they¦have just never made up their mind about becoming active blood donors.¦We propose active decisions as a mechanism to transform latent prosocial¦preferences into actual prosocial behavior.

Abstract
Spillovers of prosocial motivation can enhance the provision of public goods, and have implications for the cost-benefit analysis of policy interventions. We analyze a large-scale intervention among dyads of pre- registered blood donors. A quasi-random phone call provides the instrument for identifying endogenous and exogenous social interaction. The phone call has a strong effect on the recipient's propensity to donate. Between 40% and 44% of that behavioral impulse is transmitted within dyads due to motivational spillovers. This creates a significant social multiplier to policy interventions, with estimates ranging between 1.6 and 1.8. There is no evidence for exogenous social interaction.