Finite population noncooperative games with linear-quadratic utilities, where each player decides how much action she exerts, can be interpreted as a network game with local payoff complementarities, together with a globally uniform payoff substitutability component and an own-concavity effect. For these games, the Nash equilibrium action of each player is proportional to her Bonacich centrality in the network of local complementarities, thus establishing a bridge with the sociology literature on social networks. This Bonacich–Nash linkage implies that aggregate equilibrium increases with network size and density. We then analyze a policy that consists of targeting the key player, that is, the player who, once removed, leads to the optimal change in aggregate activity. We provide a geometric characterization of the key player identified with an intercentrality measure, which takes into account both a player's centrality and her contribution to the centrality of the others.

This article investigates whether virtual contact, initiated through a documentary film, can promote interethnic harmony. We carried out a cluster-randomized field experiment involving over 3,300 households across 121 multiethnic villages in Bangladesh. We find that a documentary film, designed to humanize the ethnic-minority Santals and evoke empathy among the ethnic majority Bengalis, increased the ethnic majority’s prosociality toward minorities, though the strength of the evidence varies by treatment arm and outcome. Using emotion-detecting software to analyze facial expressions during the film viewing suggests that the documentary elicited emotional responses consistent with empathy. We do not find evidence that the intervention reduced the prevalence of negative stereotypes and discriminatory opinions toward minorities. In villages assigned to target network-central people, we find positive behavioral effects on untreated individuals, including Santals, and village-level administrative data suggest a reduction in police complaints in those villages. About five months after the intervention, we conducted a casual-work field experiment involving 720 participants from the main intervention. In this task, pairs of ethnic-majority and minority participants jointly produced paper bags for a local supplier under a piece-rate compensation scheme. We find positive treatment effects on productivity for both ethnic groups, with effects concentrated in villages where network-central people were treated. For the ethnic majority, increased prosociality, and for the ethnic minority, reciprocity or peer pressure may have contributed to increased productivity. Overall, our findings suggest that virtual contact and social networks may help promote harmony in multiethnic communities.

We analyze a model of R&D alliance networks where firms are engaged in R&D collaborations that lower their production costs while competing on the product market. We provide a complete characterization of the Nash equilibrium and determine the optimal R&D subsidy program that maximizes total welfare. We then structurally estimate this model using a unique panel of R&D collaborations and annual company reports. We use our estimates to study the impact of targeted versus nondiscriminatory R&D subsidy policies and empirically rank firms according to the welfare-maximizing subsidies they should receive.

We develop a model of market entry under social learning through word of mouth (WOM). The success of an entrant depends on consumer awareness generated via WOM, modeled as a percolation process on a random graph. The likelihood of an entrant gaining significant awareness depends on network structure, characterized by the first three factorial moments of the degree distribution. We identify three pricing equilibria: blockaded, deterred, and accommodated entry. The model demonstrates that increased network density can shift equilibria from blockaded to deterred and eventually to accommodated entry. Numerical simulations suggest that consumer surplus may be nonmonotonic with respect to network density. Additionally, if incumbents can charge personalized prices based on consumers' connectivity, they may optimally set lower prices for highly connected consumers.

We propose a model of intergenerational transmission of education wherein children belong to either highly educated or low-educated families. Children choose the intensity of their social activities, while parents decide how much educational effort to exert. Using Add Health data, we find that, on average, children’s homophily acts as a complement to the educational effort of highly educated parents but as a substitute for the educational effort of low-educated parents. We also find that policies that subsidize kids’ socialization efforts can backfire for low-educated students because they tend to increase their interactions with other low-educated students.

There is substantial empirical evidence showing that peer effects matter in many activities. The workhorse model in empirical work on peer effects is the linear‐in‐means (LIM) model, whereby it is assumed that agents are linearly affected by the mean action of their peers. We develop a new general model of peer effects that relaxes the linear assumption of the best‐reply functions and the mean peer behavior and that encompasses the spillover, conformist model, and LIM model as special cases. Then, using data on adolescent activities in the United States, we structurally estimate this model. We find that for many activities, individuals do not behave according to the LIM model. We run some counterfactual policies and show that imposing the mean action as an individual social norm is misleading and leads to incorrect policy implications.

We examine how social interactions and friendships shape students' political opinions in a natural experiment at Sciences Po, a leading French university specializing in social and political sciences. The quasi-random assignment of students into short-term integration groups before their academic curriculum reduces political opinion gaps and fosters friendship formation. Using same-group membership as an instrumental variable for friendship, we find that friendship reduces opinion differences by 40% of a standard deviation in the opinion gap. Our evidence supports a homophily-enforced mechanism: friendships form among initially politically similar students, leading them to join political associations together, reinforcing their similarity. However, friendship does not significantly influence politically dissimilar pairs. Instead, it reduces opinion divergence without enforcing ideological convergence.