We adopt a mechanism design approach to model communication between a principal and a privately informed agent in the context where monetary incentives are not available. We provide a simple condition on the distribution of the agent's type which ensures that the optimal mechanism is continuous. With strict log-concavity of the distribution, there exists a unique optimal mechanism that is characterized.

International audienceWe consider the design of a nonlinear social tariff for residential water in Côte d'Ivoire, which is a case of a monopolistic private operator supplying a population of heterogeneous consumers. The proposed optimal tariff includes an initial “social” block with a low unit price, and higher consumption blocks with a monopoly pricing rule. This optimal nonlinear tariff is calibrated using econometric estimates of a panel-data residential water demand equation. Welfare changes associated with moving from the actual tariff to approximations of the optimal pricing system are computed under different tariff scenarios. We find that gains in consumer welfare would outweigh losses in producer surplus in a majority of Ivorian local communities

Polarized interest groups compete to in ‡uence a decision-maker through monetary contributions. This decision-maker chooses a one-dimensional policy and has private information about his ideal point. Competition between interest groups under asymmetric information yields a rich pattern of equilibrium strategies and payo¤s. Policies are systematically biased towards the decision-maker's ideal point and it may sometimes lead to a "laissez-faire" equilibrium. Either the most extreme decision-makers or the most moderate ones may get information rent depending on the importance of their ideological bias. The market for in ‡uence may exhibit segmentation with interest groups keeping an unchallenged in ‡uence on ideologically close-by decision-makers. Indeed, interest groups stop contributing when there is too much uncertainty on the decision-maker's ideology and when the latter is ideologically too far away.

We use a mechanism design approach to study the organization of interest groups in an informational model of lobbying. Interest groups in‡uence the legislature only by communicating private information on their preferences and not by means of monetary transfers. Interest groups have private information on their ideal points in a one-dimensional policy space and may either compete or adopt more collusive behaviors. Optimal policies result from a trade-o¤ between imposing rules which are non-responsive to the groups'preferences and ‡exibility that pleases groups better. Within a strong coalition, interest groups credibly share information which facilitates communication of their joint interests, helps screening by the legislature and induces ‡exible policies responsive to the groups' joint interests (an informativeness e¤ect). Competing interest groups better transmit information on their individual preferences (a screening e¤ect). The socially and privately optimal organization of lobbying favors competition between groups only when their preferences are not too congruent with those of the legislature. With more congruence, a strong coalition is preferred. Finally, within a weak coalition, interest groups must design incentive compatible collusive mechanisms to share information. Such weak coalitions are always ine¢ cient.Keywords: Communication Mechanisms, Lobbying, Competition, Coalition, Legislative Politics.JEL Classi…cation : D72; D82.1 We thank Jean-Jacques La¤ont, Michel Le Breton, Eric Maskin, Shlomo Weber, François Salanié, Wilfried Sand-Zantman and seminar participants in Toulouse, Stockholm and Madrid for helpful discussions. We are especially grateful to two referees and the editor of this journal for their insightful suggestions and comments.

This paper introduces asymmetric information in a pluralistic model of interest groups competition and analyzes its impact on policy biases. Lobbying groups are uninformed on a decision maker's preferences and use nonlinear contributions not only to compete for the agent's services but also to learn about his preferences in an otherwise standard common agency model of lobbying. Asymmetric information can be either on the decision maker's ideal point (horizontal differentiation) or on the strength of his own preferences for ideology (vertical differentiation). At equilibrium, asymmetric information redistributes bargaining powers between interest groups and the decision maker in non‐trivial ways that may depend on the kind of informational asymmetry which is postulated. Asymmetric information tends to mitigate the influence of interest groups and contributions might be significantly reduced. Interest groups no longer contribute for a change in policy what it is worth to them as under complete information. Contributions incorporate a discount related to the group's ability to solve the asymmetric information problem. (JEL: D72, D82)