STRUCTURAL HOMOPHILY

Homophily, or the fact that similar individuals tend to interact with each other, is a prominent feature of economic and social networks. I show that the equilibrium structure of homophily has empirical power. I build a strategic model of network formation, which produces a unique equilibrium network. Individuals have homophilic preferences and face capacity constraints on the number of links. I develop a novel empirical method, based on the shape of the equilibrium network, which allows for the identification and estimation of the underlying homophilic preferences. I apply this new methodology to the formation of friendship networks.     

Group identity in a network formation game with cost sharing

This paper introduces the choice of group identity, which is comprised of characteristics and commitments to these characteristics, in a network formation model where links costs are shared. Players want to link to the largest group given that linking costs for players of the same (different) characteristic are decreasing (increasing) in their commitments. The equilibrium concepts used are Nash equilibrium as well as one that looks for stability allowing for bilateral negotiations. Conditions are shown under which the endogenous choice of characteristics and commitments allow for multiple groups with segregated or connected networks. When group identity is partly endogenous, such that characteristics are fixed and players only choose commitment, it is shown that the equilibrium population commitment profile dictates whether the resulting network is segregated or connected. When group identity is fully endogenous, it is shown that multiple groups and segregated networks are possible equilibrium outcomes but such outcomes are not stable unless the group size additionally affects the costs of link formation.     

Network formation with heterogeneous players

This paper extends the connections model of network formation by allowing for players who are heterogeneous with respect to values as well as the costs of forming links. Our principal result is that centrality and short average distances between individuals are robust features of equilibrium networks.     

Stackelberg equilibrium with many leaders and followers. The case of zero fixed costs

I study a version of the Stackelberg game with many identical firms in which leaders and followers use a continuous cost function with no fixed cost. Using lattice theoretical methods I provide a set of conditions that guarantee that the game has an equilibrium in pure strategies. With convex costs the model shows the same properties as a quasi-competitive Cournot model. The same happens with concave costs, but only when the number of followers is small. When this number is large the leaders preempt entry. I study the comparative statics and the limit behavior of the equilibrium and I show how the main determinants of market structure interact. More competition between the leaders always displaces the followers. Instead, how a stronger threat of entry affects the equilibrium depends on the technology. With strictly convex costs it is the followers that eventually displace the leaders.     

Financial Innovation and Delegation of Control

The literature on security design has traditionally assumed that control of investment and production rests with the entrepreneur even after flotation. In this article I relax this assumption, which is both unnecessarily strict and unrealistic. The contribution is twofold. First, I show that when control is delegated during flotation, financial innovation is a trade off between agency costs (arising from principal agent relationships) and targeting costs (arising from the inability to split the cash flow). Second, I show that unlevered equity or the use of debt which can be issued without defaulting in equilibrium, are optimal means of financing.     

The circular road revisited: Uniqueness and supermodularity

The model of the circular road has proved to be a popular model of oligopolistic interaction, yet its theoretical properties are not fully explored. In this paper I extend the uniqueness result of price equilibrium in the circular road with equidistant locations from quadratic transportation costs, to a class of convex power transportation costs, i.e. I show uniqueness in oligopoly for a class of utility functions of the consumers. I show that the associated game is supermodular and dominance solvable. The paper also discusses possible extensions and limitations of the model.     

Verifiability and Contract Enforcement: A Model with Judicial Moral Hazard

I model the litigation of a contract containing a variable notobservable by courts, hence nonverifiable, unless the rationaland self-interested judge exerts effort. He values the correctruling but dislikes effort. Judicial effort is discretionary.I show that effort cost is inconsequential—"always breach"is equilibrium for any effort cost. But there exists anotherequilibrium where a small breach rate is achieved even withsignificant effort costs. Maximal remedies for breach are notoptimal. Because effort is discretionary, low effort cost increasesbreach. Pretrial negotiations can have a substantial negativeimpact on verifiability under arbitrarily small deviations fromfull rationality.     

Transaction costs and planner intervention

In this paper, I examine a two-period general equilibrium model in which transaction costs are incurred whenever financial contracts are traded. These transaction costs are real and convex. The presence of these transaction costs results in a Pareto inefficient equilibrium allocation. Attempting to fix this problem, the planner will intervene by scaling the transaction costs either up or down. The intervention must satisfy fiscal balance meaning that the summed value of transaction costs will remain constant. I prove that over a generic subset of household utility functions and endowments and subject to an upper bound on the number of household types, there exists an open set of planner interventions that lead to a Pareto superior allocation.     

Distributive politics of public goods networks through multistage voting

Public goods networks create conflict between districts needing the network and districts that pay the costs. Thus, it is difficult for pork barrel politicians to achieve logrolling with other districts. I show that logrolling is possible for such politicians if they set their agenda properly. To show this, I model the centralized decision‐making process as a multistage vote among representatives from different districts. I assume the districts are better off if and only if the public good “connects” them to the network. Public goods can be provided universally by setting the agenda properly, even when costs exceed the benefits.     

Bargaining and markets: complexity and the competitive outcome

Rubinstein and Wolinsky (Rev. Econ. Stud. 57 (1990) 63-78) consider a simple decentralised market game in which agents meet randomly or voluntarily and bargain over the terms of trade. They show that any individually rational price can be sustained as a sequential equilibrium even though the model has a unique competitive outcome. Here, I consider Rubinstein and Wolinsky's model and show that if complexity costs of implementing strategies enter players’ preferences, together with the standard payoff in the game, then every equilibrium is stationary/Markov and induces the unique competitive price. Thus, I demonstrate that aversion to complexity may provide a justification for the competitive outcome.     

On the existence of pure strategy Bertrand equilibrium

Summary The paper analyses the existence of pure strategy Nash equilibrium in price competition (or Bertrand equilibrium) in a homogeneous product market when costs are strictly convex and proves that if output is demand determined such equilibrium always exists. This paper also characterises such equilibria and shows that if firms are identical such equilibria are necessarily non-unique. However for firms with asymmetric costs it can be unique or non-unique.I am greatly indebted to Anjan Mukherji and Kunal Sengupta for this paper. I also express my profound gratitude to a referee of this journal for a very helpful set of suggestions.     

A geometric approach to the price of anarchy in nonatomic congestion games

We present a short, geometric proof for the price-of-anarchy results that have recently been established in a series of papers on selfish routing in multicommodity flow networks and on nonatomic congestion games. This novel proof also facilitates two new types of theoretical results: On the one hand, we give pseudo-approximation results that depend on the class of allowable cost functions. On the other hand, we derive stronger bounds on the inefficiency of equilibria for situations in which the equilibrium costs are within reasonable limits of the fixed costs. These tighter bounds help to explain empirical observations in vehicular traffic networks. Our analysis holds in the more general context of nonatomic congestion games, which provide the framework in which we describe this work.     

A difficulty with the address models of product differentiation

Summary. In models of product differentiation and location models it is implicitly assumed that consumers can afford to buy the differentiated goods in the market. I show that with income heterogeneity there are severe existence problems of a price equilibrium in models of horizontal product differentiation with unit demand because some consumers are income-constrained. The result generalizes to other models of product differentiation, search, and switching costs. I present an alternative specification of variable individual demand in which this kind of existence problem cannot arise. Received: October 17, 1997; revised version: February 20, 1998     

Does and Should a Commodity Medium of Exchange Have Relatively Low Storage Costs?

I provide new existence and welfare results for a version of the Kiyotaki-Wright model. I construct an equilibrium where all agents use mixed strategies. Consequently, an object with a higher storage cost must have a higher acceptability. Therefore, the endogenous transaction pattern corresponds to the observation that money is dominated in rate of return by other assets (e.g., bonds), something that is a central issue in monetary economics. Furthermore, at least in a neighborhood of equal storage costs, the equilibrium that I construct Pareto dominates alternative equilibria in which better objects are widely accepted.     

Strategic default and optimal audit resources with costly state verification

I develop a model in which the ability to repay a loan is private information that can only be verified by the bank at some costs, which can be recovered from the borrower if it has reported untruthfully. The bank will optimize the resources it spends on this auditing of borrowers and the resulting equilibrium is then characterized. It is shown that in equilibrium, a significant fraction of companies default strategically, but most are captured via auditing. The failure rates of banks are also small. Finally extensions are discussed to include limited liability to banks and the partial recovery of auditing costs as well as punitive costs to borrowers.     

Information acquisition during a descending auction

We compare the effects of information acquisition during a descending auction with its static counterpart, the first-price sealed-bid auction. In a framework with heterogeneous prior information, we show that an equilibrium with information acquisition exists in both auction formats. We show that everything else equal information acquisition is more desirable in the dynamic auction. Moreover, we characterize a set of parameter values where more information is acquired in the dynamic auction in equilibrium. If the costs of information acquisition are sufficiently low, the sealed-bid auction generates more revenue although the descending auction is more efficient.     

Prices Versus Quantities: Technology Choice,Uncertainty and Welfare

Technological improvements have proven essential in mitigating environmental problems such as climate change, depletion of the ozone layer and acid rain. While it is well-known that price- and quantity-based regulatory instruments provide different investment levels, the effects on the choice between different technologies have received scant attention. This paper expands on the prices versus quantities literature by investigating firms’ technology choice in the face of demand and supply side uncertainty. I show that the regulator can not design tradable emissions permits and an emissions tax such that the two regimes are equivalent, even in terms of expected values. Moreover, a tax encourages the most flexible abatement technology if and only if stochastic costs and the equilibrium permit price have sufficiently strong positive covariance, compared with the variance in consumer demand for the good produced. Finally, the firms’ technology choices are socially optimal under tradable emissions permits, but not under an emissions tax.     

MARKET ACCESS,INVESTMENT, AND HETEROGENEOUS FIRMS*

This article presents a model of international trade in which heterogeneous firms can expand through capital acquisitions. I show that demand elasticities are a crucial element in predicting which firms invest, in what location, and for what reason. High‐productivity firms, who tend to sell goods at a low elasticity, invest for market access (tariff jumping). Middle productivity firms, who tend to sell at a higher elasticity, invest for productivity improvement. The relative value of trade costs dictates which incentive is larger. In equilibrium, trade liberalization can reduce aggregate productivity by reducing an important source of investment demand: foreign firms.     

Natural monopoly and differential pricing

Under uniform pricing a monopolist cannot make a positive profit in equilibrium. I analyze how differential pricing can be exploited by a natural monopolist to deter entry when entry is costless. In a two-stage game with price competition before quantity competition I show that the incumbent firm can deter entry and make a positive profit in equilibrium. The incumbent sets two different prices, the low price to deter entry and the high price to generate profit. Entry is not possible because of scale effects. If dumping is allowed for all firms no positive profits are realizable, but welfare is reduced. I show that for some parameter values the incumbent is forced to engage in a stunt (i.e., set a negative low price) to keep entrants out.     

Farsighted network formation

This paper studies a model of dynamic network formation when individuals are farsighted: players evaluate the desirability of a “current” move in terms of its consequences on the entire discounted stream of payoffs. We define a concept of equilibrium which takes into account farsighted behavior of agents and allows for limited cooperation amongst agents. We show that an equilibrium process of network formation exists. We also show that there are valuation structures in which no equilibrium strategy profile can sustain efficient networks. We then provide sufficient conditions under which the equilibrium process will yield efficient outcomes.