This dissertation consists of three self-contained essays on the economics of social interactions. The first chapter is coauthored with Lorenzo Verstraeten. Knowing that Individuals interact with their peers, we study how a social planner can intervene, changing these interactions, in order to achieve a particular objective. When the objective is welfare maximization, we describe the interventions for games of strategic complements and strategic substitutes. We show that, for strategic complements, the planner uses resources to target central players; while she divides individuals into separated communities in the case of strategic substitutes. We study which connections she targets in order to achieve these goals. The second chapter is coauthored with Lorenzo Verstraeten and analyzes a model of contagion on social network. We ask how a social planner should intervene to prevent contagion. We characterize the optimal intervention and the cost associated. We discuss the intuition behind the choice of the planner and we provide comparative static on the cost of intervention for different type of network. In the third chapter I develop a theoretical study about groups relationship and ask whether intragroup cooperation crowd-out intergroup cooperation. I consider a gift-giving game where cooperation endogenously arises, within and across groups. Cooperation is sustained through peer punishment with the help of a group specific monitoring technology. I specify under which conditions cooperation crowding-out occur. I identify two classes of equilibrium: a Sorting equilibrium where guilty players prefer to be matched outside their group due to a less efficient Out-Group monitoring technology, and a Non Sorting equilibrium where the higher level of In-Group cooperation makes it more attractive for everybody. I then compare their welfare properties and draw conclusions on optimal punishment levels.

This is a collection of essays in public economics, focusing on credit constraints in higher education and peer effects in high school. The first chapter explores importance of credit constraints for college choice through a dynamic life-cycle model. The second chapter studies the effect of exposure to foreign-born students in the U.S. high-schools on educational, social and health outcomes of their peers.

When John Harsanyi came to Stanford University as a candidate for the Ph.D., I asked him why he was bothering, since it was most un likely that he had anything to learn from us. He was already a known scho lar; in addition to some papers in economics, the first two papers in this vol ume had already been published and had dazzled me by their originality and their combination of philosophical insight and technical competence. However, I am very glad I did not discourage him; whether he learned any thing worthwhile I don't know, but we all learned much from him on the foundations of the theory of games and specifically on the outcome of bar gaining. The central focus of Harsanyi's work has continued to be in the theory of games, but especially on the foundations and conceptual problems. The theory of games, properly understood, is a very broad approach to social interaction based on individually rational behavior, and it connects closely with fundamental methodological and substantive issues in social science and in ethics. An indication of the range of Harsanyi's interest in game the ory can be found in the first paper of Part B -though in fact his owncontri butions are much broader-and in the second paper the applications to the methodology of social science. The remaining papers in that section show more specifically the richness of game theory in specific applications.

This collection of essays presents a variety of approaches to understanding the dynamics of human interaction.

Since his pioneering application of economic analysis to racial discrimination, Gary S. Becker has shown that an economic approach can provide a unified framework for understanding all human behavior. In a highly readable selection of essays Becker applies this approach to various aspects of human activity, including social interactions; crime and punishment; marriage, fertility, and the family; and "irrational" behavior. "Becker's highly regarded work in economics is most notable in the imaginative application of 'the economic approach' to a surprising breadth of human activity. Becker's essays over the years have inevitably inspired a surge of research activity in testimony to the richness of his insights into human activities lying 'outside' the traditionally conceived economic markets. Perhaps no economist in our time has contributed more to expanding the area of interest to economists than Becker, and a number of these thought-provoking essays are collected in this book."—Choice Gary Becker was awarded the Nobel Prize in Economic Science in 1992.