Markets, Bargaining, and Networks with Heterogeneous Agents 
Fernando Vega-Redondo (European University Institute)

The paper proposes an intertemporal model of bargaining among heterogeneous buyers and sellers placed on a bipartite network. First, it characterizes conditions on the network under which its trading restrictions are inessential and the outcome is arbitragefree. Instead, when the system is segmented in di erent trading components, we show how these come about and how prices are determined in each of them. Second, we turn to the issue of network endogeneity, focusing on those networks that are Pairwise Stable. Such networks are shown to always exist and be arbitrage-free. In the latter respect, therefore, they satisfy one of the key properties displayed by frictionless markets. We identify, however, a sharp contrast regarding another key feature: Pairwise-Stable networks are generically ine?cient if the matching process is genuinely decentralized. This uncovers a fundamental incompatibility between individual incentives and social welfare in endogenous trading networks. We explain that such incompatibility is not only due to buyer/seller heterogeneity but is also caused by the incentives underlying network formation in a trading context. (joint with Arnold Polanski)

***********

Behaviour in Social Networks: Direct Complements and Indirect Substitutes
Sergio Currarini (Università di Venezia)

Many types of economic and social activities involve significant behavioral complementarities (peer effects) with neighbors in the social network. The same activities often exert externalities, that cumulates in “stocks” affecting agents’ welfare and incentives. For instance, smoking is subject to peer effects, and the stock of passive smoke increases the marginal risks of bad health, decreasing the incentives to smoke. In the linear quadratic framework studied by Ballester et al. (2006), we consider contexts where agents’ incentives decrease with the “stock” to which neighbors are exposed (agents may, for instance,  care about their friends’ health). In such contexts, the patterns of strategic interaction differ from the network of social relations, as agents display strategic substitution with distance-two neighbors. We show that behavior is predicted by a weighted Bonacich centrality index, with weights accounting for distance-two relations. We find that both maximal behavior and key-players tend to move to the periphery of the network, and we discuss the effect of close-knit communities and segregated groups on aggregate behavior. We finally discuss the implications for peer effects identification and for the emergence of potential biases in the estimation of social effects.