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Common agency and public good provision under asymmetric information

The provision of public goods under asymmetric information has most often been viewed as a mechanism design problem under the aegis of an uninformed mediator. This paper focuses on institutional contexts without such a mediator. Contributors privately informed on their willingness to pay non-cooperatively offer contribution schedules to an agent who produces the public good on their behalf. In any separating and informative equilibrium of this common agency game under asymmetric information, instead of reducing marginal contributions to freeride on others, principals do so to screen the agent’s endogenous private information obtained from privately observing other principals’ offers. Under weak conditions, the existence of a differentiable equilibrium is shown. Equilibria are always ex post inefficient and interim efficient if and only if the type distribution has a linear inverse hazard rate. This points to the major inefficiency of contribution games under asymmetric information and stands in contrast to the more positive efficiency result that the common agency literature has unveiled when assuming complete information. Extensions of the model address direct contracting between principals, the existence of pooling uninformative equilibria, and the robustness of our findings to the possibility that principals entertain more complex communication with their agent.


Introduction
Since Green and Laffont (1979), the provision of public goods under asymmetric information has most often been viewed as a mechanism design problem under the aegis of an uninformed mediator having a full commitment ability. This paper relaxes this assumption and focuses on cases without such a mediator. Contributors who are privately informed on their willingness to pay non-cooperatively offer contribution schedules to an agent who produces the public good on their behalf.
Our first motivation for undertaking such analysis comes from observing that, in many real-world settings, centralized mechanisms and uninformed mediators with a strong ability to commit to those mechanisms are not available. Health, environment, multilateral foreign aid, and other transnational public goods are all examples of public goods with voluntary provision by sovereign countries. There is no mediator to design the mechanisms that those countries should play to reveal their preferences. Politics and games of influence among interest groups offer other important examples. Key decision-makers might not have much commitment power to organize and ex ante design competition between interest groups. Instead, they only react ex post to the lobbying contributions they receive from those groups. 1 In those contexts, it is important to know whether a game of voluntary contributions fares well under asymmetric information, i.e., what are the positive and normative properties of the corresponding Bayesian-Nash equilibria.
Our second motivation is theoretical. Although earlier works on asymmetric information (Clarke 1971, Groves 1973) studied specific mechanisms for the provision of public goods, the bulk of the literature has departed from the analysis of real-world institutions to characterize instead properties of the whole set of incentive-feasible allocations. 2 In the standard framework (sometimes referred to as the centralized mechanism approach in what follows), an uninformed mediator, moving first, designs a mechanism for informed players. This mechanism induces an equilibrium allocation that is Bayesian incentive compatible, feasible (i.e., contributions cover the cost of the public good), and might respect the agents' veto constraints. No other institutional constraint on the kind of mechanisms or on the communication devices that can be used is considered. In this paper, we impose that such allocation is an equilibrium outcome of a game of voluntary contributions taking place under asymmetric information. In such a game, a privately informed contributor might want to offer a contribution schedule that is flexible enough to cope with different realizations of others' preferences. An agent collects contributions, endogenously learns something about the contributors' preferences from observing their mere offers, and chooses the level of public good accordingly.
This institutional setting is thus viewed as a common agency game under asymmetric information with privately informed contributors non-cooperatively designing contribution schedules. 3 We are interested in the general properties of such games both in terms of how information is aggregated, and in terms of ex post and interim efficiency.
Our first important results are related to the process by which the equilibrium outcome aggregates contributors' private information. At a best response to what others offer, a given principal designs his own contribution not only to signal his preferences 1 Grossman and Helpman (1994). 2 This research strategy of the public good literature stands in sharp contrast with the way the literature on auctions has evolved. There, equal efforts have been devoted to the study of particular auction formats and to the characterization of the general properties of unrestricted auction mechanisms. 3 See Martimort (2006) for a survey of the literature.
to the common agent, but also to extract the endogenous private information that this agent may have learned from observing others' offers. Signaling turns out to be costless in our environment because of private values (the principals' private information does not enter directly into the agent's utility function) and risk neutrality. Focusing on informative outcomes that aggregate information efficiently, we study separating Bayesian equilibria, i.e., contributors with different valuations offer different contribution schedules. In our private values environment, those contributions are the same as if the agent was perfectly informed on principals' valuations and out-of-equilibrium beliefs following unexpected offers are irrelevant in characterizing the equilibrium. Instead, screening is costly. Each principal has to learn what the agent has endogenously learned from observing others' contributions. 4 Standard mechanism design techniques can nevertheless be used to compute best responses. When choosing how much to contribute, each principal behaves actually as a monopsonist in front of an agent who is endogenously privately informed on the preferences of other contributors. By a standard screening argument, 5 this principal contributes less at the margin than his marginal valuation to decrease the agent's information rent. Intuitively, the agent can always ask for more from a given principal by pretending that others have not contributed enough. Each principal has then to reduce his own contribution to make that strategy less attractive to the agent. As far as existence is concerned, we show that the marginal contribution in any equilibrium solves a complex functional equation with rather stringent boundary conditions. This equation links the equilibrium's marginal contribution, its inverse, and its derivative. It is thus nonlocal by nature. Boundary conditions come from characterizing the bidding behavior of the two principals who have the highest and the lowest valuations, and who altogether implement a given output. We show that there always exists a differentiable equilibrium of the game under weak conditions on distributions. The idea is to analyze best responses in terms of the distribution of marginal contributions that a principal offers and to provide conditions under which that best-response mapping is monotonically decreasing: If principal 2's distribution of marginal contributions increases in the sense of first-order stochastic dominance, principal 1's own distribution decreases. This monotonicity helps to define a set of distributions that is stable by the best-response mapping and from which a fixed point can be found using Schauder's Second Theorem. Finally, we also show uniqueness when the distribution of types is uniform.
Turning now to the normative properties of those equilibria, any equilibrium is necessarily ex post inefficient. For screening purposes, each principal always contributes less at the margin than what it is worth to him and "free-riding" arises. This is not to cipals environments. Those authors derive general Revelation Principles for multiprincipals games where each principal should try to learn from the agent whatever information he has on his own preferences, but also on what he privately learns from observing others' offers. In a pure strategy Bayesian equilibrium as analyzed below, principals perfectly conjecture the strategy followed by others. They are a priori unaware of their exact types, but may try to learn those types from asking the common agent about what he learns from observing offers made by other principals. 5  hide his type to the common agent as the centralized mechanism design approach predicts. 6 Instead, a principal induces less production to reduce the information rent that the agent gets from learning the preferences of others. Downward distortions below the first-best necessarily follow from this new source of distortion. The absence of a mediator forces privately informed contributors to communicate through a self-interested agent. Communication occurs via the offer of a contribution schedule that reveals the corresponding principal's type in any informative equilibrium. Given that ex post efficiency fails and interim efficiency is a more relevant efficiency concept under asymmetric information, we ask whether equilibria are nevertheless interim efficient and under which circumstances if any. 7 The additional screening costs from having communication take place through the agent explain why the Bayesian equilibria of the voluntary contribution game generally fail to be interim efficient. Interim efficiency is obtained if and only if the type distribution has a linear inverse hazard rate. We derive the symmetric equilibrium marginal contribution in that case. Beyond that non-generic case, public intervention under the aegis of an uninformed mediator is helpful in coordinating contributions. This points at the major inefficiency of contribution games under asymmetric information and stands in sharp contrast to the striking positive efficiency result that the common agency literature has unveiled when assuming complete information. 8 Extensions of the model address direct contracting and communication between principals, the existence of pooling uninformative equilibria, and the robustness of our findings to the possibility that principals entertain more complex communication with their agent.
Section 2 reviews the literature. Section 3 presents the model. Section 4 shows how to derive symmetric differentiable equilibria of the common agency game under asymmetric information. We present there also the Lindahl-Samuelson conditions satisfied at equilibrium and provide tractable examples. Section 5 discusses existence and uniqueness. Section 6 analyzes welfare properties of equilibria. Section 7 discusses several extensions of our model. Section 8 concludes. Proofs are relegated to the Appendix.

Review of the literature
Following Wilson (1979) and Bernheim and Whinston (1986), the common agency literature has developed an analytical framework to tackle a variety of important problems such as menu auctions, public goods provision through voluntary contributions, 9 or policy formation with competing lobbying groups in complete information environments. 10 Imposing that contributions are "truthful," i.e., reflect the relative preferences 6 Laffont and Maskin (1979), Güth and Hellwig (1987), Rob (1989), and Mailath and Postlewaite (1990). 7 Holmström and Myerson (1983) and Ledyard and Palfrey (1999). Because the common agent might get a positive rent in equilibrium from his endogenous private information, he might also receive a positive weight in the social welfare function maximized by the uninformed mediator offering the centralized mechanism designed to achieve a given interim efficient allocation. 8 Bernheim and Whinston (1986). 9 Laussel and Le Breton (1998). 10 Grossman and Helpman (1994).
of the principals among alternatives, Bernheim and Whinston (1986) reduce the equilibrium indeterminacy of those games and select efficient equilibria. 11 With such truthful schedules, what a principal pays at the margin for inducing a change in the agent's decision is exactly what it is worth to him and the "free-riding" problem in public good provision cannot arise. Modulo truthfulness, common agency aggregates preferences efficiently under complete information. 12 Modeling private information on the principals' side justifies the use of nonlinear contributions for screening purposes in the first place. The "truthfulness" requirement is then replaced by incentive compatibility constraints. The cost of putting on firmer foundations the use of schedules is that ex post efficiency is lost and the conditions for interim efficiency become severe. In sharp contrast to complete information models, contribution games under asymmetric information are most often inefficient even in the interim sense. This gives a less optimistic view of decentralized bargaining. Paralleling those complete information papers, Stole (1990), Martimort (1992Martimort ( , 1996aMartimort ( , 1996b, Mezzetti (1997), Biais et al. (2000), and Martimort and Stole (2002 among others analyze oligopolistic screening environments where different principals elicit information privately known by the common agent at the contracting stage. These papers stress the impact of oligopolistic screening on the standard rent/efficiency tradeoff. We focus instead on asymmetric information on the principals' side. Like in this earlier literature, the presence of competing principals introduces an additional distortion. In the standard common agency literature, a given principal needs to worry that the mechanism he offers affects the agent's choices of the contracting variables controled by other principals. The distortion channel in our paper is different. Since principals cannot coordinate by communicating their types to a mediator, they do so through the agent and endow the latter with private information that the agent can exploit to obtain an information rent. The agent's private information vis-à-vis each principal is endogenous: it is what the agent may have learned from observing the other principals' offers. 13 The additional distortion due to the principals' non-cooperative behavior can thus be explained by their desire to extract the information rent associated to such endogenous information. Contrasting with the use of schedules stressed by Bernheim and Whinston (1986), the complete information literature on voluntary provision of public goods highlights inefficiency and free-riding in models where contributors are restricted to offer fixed 11 Multiplicity might still come from the flexibility in sharing the aggregate surplus among the contributing principals and their common agent (Bernheim and Whinston 1986). 12 These results have been extended in many different directions. Dixit et al. (1997) introduce redistributive concerns by relaxing the quasi-linearity assumption. Laussel and Le Breton (1998) study incomplete information on the preferences of the common agent, but focus on ex ante contracting when agency costs are null. Other extensions less directly relevant for the analysis of this paper include Prat and Rustichini (2003), who study competition among principals trying to influence multiple agents, and Bergemann and Välimäki (2003), who consider dynamic issues. 13 Bond and Gresik (1997)  contributions (Bergstrom et al. 1986). Other solutions to this inefficiency problem include refunds (Bagnoli and Lipman 1989) and multistage mechanisms in environments with partially verifiable information (Jackson and Moulin 1992).
There exists a tiny literature on voluntary contributions for a 0-1 public good by privately informed agents. These works derive equilibrium strategy using techniques from the auction literature (Alboth et al. 2001, Menezes et al. 2001. Menezes et al. (2001) stress the strong ex post inefficiency of equilibria, whereas Laussel and Palfrey (2003) and Malueg (2008a, 2008b) find more positive results using interim efficiency. Our assumption that the level of public good is continuous invites the use of a differentiable approach. Interim efficiency is now much more stringent since it should apply not only on a line in the type space as in the 0-1 case (namely the set of types for which there is indifference between producing or not producing the public good), but on the whole type space. This is too demanding beyond the case of linear inverse hazard rates.
Finally, it is also useful to situate our contribution within the existing mechanism design literature on public goods. Since Clarke (1971) and Groves (1973), it is well known that ex post efficiency is possible under dominant strategy implementation. d 'Aspremont and Gerard-Varet (1979) show that one can maintain budget balance and efficiency under Bayesian implementation. Laffont and Maskin (1979), Güth and Hellwig (1987), Rob (1989), and Mailath and Postlewaite (1990) stress the role of participation constraints to generate inefficiency. A game of voluntary contributions ensures participation by principals, relies on Bayesian strategies, and finally generates a positive surplus for the agent. Hence, ex post inefficiency necessarily arises. When a centralized mechanism is offered by an uninformed mediator, inefficiencies are due to the contributors' incentives to hide their own types to this mediator: the so-called free-riding problem. Under common agency, as we will see below, contributors reveal instead their types by offering contracts to the agent but want to screen this agent according to what he has learned from others. This is no longer contributors who underestimate their valuations but their common agent who wants to claim to each principal that others have a lower willingness to pay: a different source of inefficiency in public good provision.

The model
Consider two risk-neutral principals P i (i = 1 2) who derive utility from consuming a public good that is produced in nonnegative quantity q. 14 15 This public good may be an infrastructure of variable size or a charitable activity, or it may also have a more abstract interpretation as a policy variable in some lobbying games. The public good is excludable so that noncontributors do not enjoy the public good. Principals P i get a utility V i (θ i q t i ) = θ i q − t i from consuming q units of the good and paying an amount t i . 14 Extending our analysis to the case of more than two principals significantly increases complexity. Indeed, we will see below that each principal designs his contribution to screen others' types. Having more than two principals leads thus to a difficult multidimensional screening problem when computing each principal's best response. We leave those issues for further research. 15 The public good can also be produced in quantity 0 or 1 and q is then viewed as its variable quality.
Principals are privately informed on their respective valuations θ i . Types are independently drawn from the same common knowledge and atomless distribution on = [θ θ ] (we denote θ =θ −θ > 0) with cumulative distribution function F(·) and everywhere positive and differentiable density f = F . Unless specified otherwise, we assume thatθ > 0 andθ < ∞ with |f (θ)| being bounded. 16 The inverse hazard rate Contributions are collected by a risk-neutral common agent A who produces at cost C(q) the public good and whose utility function is U(q is twice differentiable and convex with C(0) = C (0) = 0 and C (∞) = ∞, where Inada conditions avoid corner solutions.
Benchmark. Let q FB (θ 1 θ 2 ) be the first-best level of public good. It is increasing in both arguments and satisfies the Lindahl-Samuelson conditions: Strategy space. Each principal P i may offer any nonnegative and continuous contribution schedule t i (·) defined on a compact interval Q = [0 Q ], whereQ is large enough (say larger than q FB (θ θ )).
Timing. The sequence of events is as follows.
• Stage 0: Principals privately learn their types θ i .
• Stage 2: The agent accepts or refuses any of those contracts. If he refuses all contracts, the game ends with zero payoff for all players.
• Stage 3: The agent produces the level of public good q. Payments are made according to the agent's acceptance decisions and the chosen level of public good.
Together with the principals' preferences, the information structure, and strategy spaces, this timing defines our common agency game under incomplete information . We consider pure-strategy perfect Bayesian equilibria (PBE) of (in short equilibrium). Let t i (· θ i ) denote an equilibrium strategy followed by principal P i when his type is θ i . Definition 1. A pair of strategy profiles {t 1 (· θ 1 ) t 2 (· θ 2 )} (θ 1 θ 2 )∈ 2 is an equilibrium of if and only if 16 Example 2 below provides an equilibrium characterization in the case of an exponential distribution.
Theorem 6 applies to beta density that may be zero atθ.
• principal P i (i = 1 2) with type θ i finds it optimal to offer the contribution schedule t i (· θ i ) given that he expects that principal P −i follows the strategy profile • the agent's updated beliefs on the principals' types follow Bayes' rule on the equilibrium path and are arbitrary elsewhere • the agent accepts contributions and chooses optimally the level of public good given those contributions and his beliefs on the principals' types.
Because of symmetry between players, we focus on symmetric equilibrium contributions, and we may sometimes omit subscripts when they are obvious.
Remark 1. Acceptance of all contributions is a weakly optimal strategy for the agent given that those contributions are nonnegative. Note that the restriction to nonnegative schedules is innocuous in this context. The agent would never choose an equilibrium output on the range of transfers offered by a given principal that are negative. He would prefer to refuse such schedule to increase his payoff. 17 Remark 2. The strategy space that we consider allows principals to offer only contribution schedules. We postpone to Section 7.3 the analysis of the case where principals may offer more complex communication mechanisms in line with the informed principal literature (say menus of such contribution schedules from which they may pick one). 18 19 Remark 3. Existence of an optimal output at Stage 3 follows from compactness of Q and continuity of the schedules. In the sequel, we impose further regularity assumptions on contributions to get sharper predictions.
Remark 4. In any separating equilibrium, the agent infers from each principal's contribution his type. In such equilibrium, the agent gets endogenous private information on both principals' types before making his own choice on the level of public good.
Remark 5. At Stages 2 and 3 of the game, the agent's decisions to accept and produce depend only on the contribution schedules he receives. In our private values context where the principals' types do not enter directly into the agent's utility function, these decisions do not depend on the agent's posterior beliefs following any offer made by one of the principals either on or off the equilibrium path. Hence, out-of-equilibrium beliefs that sustain the equilibrium are arbitrary. 20 17 In Section 7.3 we allow for negative transfers when principals offer inscrutable menus of mechanisms (i.e., menus of contributions schedules that do not reveal the principal's type) that are accepted or refused by the agent before he produces and the principals reveal their types. Ex post participation constraints are then replaced by interim ones and, in that case, it becomes quite natural to allow for transfers being possibly negative in some states of nature. 18 Tirole (1990, 1992) and Myerson (1983) among others. 19 There is no cheap-talk stage between players that could help them to replicate the existence of a mediator (Bárány 1992, Forges 1990, Gerardi 2002. 20 See the Appendix for details.

Overview
We proceed as follows to compute P i 's best response to a pure-strategy profile {t −i (· θ −i )} θ −i ∈ followed by P −i . First, we conjecture that P −i 's strategy is separating, i.e., P −i offers different contributions as his type changes. Our focus on separating equilibria is in the spirit of looking at equilibrium allocations that are informative as in Spence (1973) and Riley (1979). 21 Before choosing the level of public good, the agent gets endogenous private information on θ −i by simply observing the mere offer t −i (· θ −i ) he receives. Principal P i must thus design his own contribution with an eye on the information rent that the agent gets from this endogenous information. 22 Second, we first act as if the agent was perfectly informed on P i 's type when the latter chooses his best response to the strategy profile {t −i (q θ −i )} θ −i ∈ followed by P −i in a pure-strategy equilibrium. Third, we benefit from the private values environment (i.e., the principals' types do not enter directly into the agent's utility function) to show that the corresponding profile of contribution schedules is also a best response in the asymmetric information game . Deviating toward another contribution schedule is suboptimal for any out-ofequilibrium beliefs that the agent may hold following such an unexpected offer. Finally, we notice that P i 's best response is itself separating and conveys information on P i 's type to the agent. Therefore, the agent also gets endogenous private information on P i 's type by simply observing his mere offer. This verifies that the same techniques can also be used to compute P −i 's best response, so this approach holds the symmetric equilibrium we seek.

Computing best responses
Following the procedure explained above, we assume that the agent is perfectly informed on P i 's type when the latter chooses his best response to the strategy profile {t −i (q θ −i )} θ −i ∈ followed by P −i . The Revelation Principle can be used to characterize any allocation that principal P i may achieve by deviating toward any possible contribution schedule t i (q θ i ). 23 We thus focus on revelation mechanisms {t D i (θ −i |θ i ) q D (θ −i |θ i )}θ −i ∈ that induce the agent to reveal to P i what he has learned by observing P −i 's offer. 21 Such equilibria aggregate information efficiently, which seems to be an interesting normative property, especially in view of assessing the ex post efficiency of equilibrium allocations. Section 7.2 analyzes instead the case of uninformative pooling equilibria. 22 This points at the role that contributions play in a common agency environment: learning over what Epstein and Peters (1999) call market information, i.e., over other principals' preferences that are reflected in their own offers to the agent. 23 Martimort and Stole (2002).
Letθ −i be the agent's report on θ −i (that he has learned from observing P −i 's offer) to P i in the truthful and direct revelation mechanism above. The agent's utility becomes Incentive compatibility yields the expression of the agent's information rent: For a fixed strategy profile Making P i 's endogenous screening problem about learning P −i 's type from the agent tractable requires further conditions on P −i 's contributions. The conditions below will thus be satisfied by the informative equilibrium under scrutiny. To simplify the analysis, we now also consider contribution schedules that are piecewise three times differentiable so that the equilibrium output is differentiable. 24 Definition 2. A nonnegative contribution t −i (q θ −i ) is increasing in type (IT) when, at any differentiability point (q θ −i ), Under IT, principal P −i contributes more if he has a greater valuation. Another natural requirement is that an upward shift in P −i 's valuation increases also the equilibrium quantity, i.e., the same Spence-Mirrlees property as for the principals' preferences holds also for the contribution schedules.
Definition 3. A nonnegative contribution t −i (q θ −i ) with margin p −i (q θ −i ) = ∂t −i (q θ −i )/∂q satisfies the Spence-Mirrlees Property (SMP) when, at any differentiability point (q θ −i ), Using standard techniques from the screening literature in monopolistic screening environments, the next lemma characterizes incentive compatible allocations that P i may induce by choosing his own contribution schedule. Lemma 1. Assume that P −i offers a nonnegative contribution t −i (q θ −i ). Any truthful and direct revelation mechanism {t D i (θ −i |θ i ) q D (θ −i |θ i )}θ −i ∈ that P i may offer to induce the allocation {U D (θ −i |θ i ) q D (θ −i |θ i )} θ −i ∈ satisfies the following properties.
Running Example continued. Suppose that principal P 2 with type θ 2 offers the revealing nonnegative contribution Notice that, on its positive range, t 2 (q θ 2 ) satisfies IT when q ≥ −2 dt 0 2 (θ 2 )/dθ 2 = 3 2 (θ 2 +θ) −θ, a property that holds for the equilibrium output given in (6) below. It also satisfies SMP. ♦ Turning now to participation constraints, the agent accepts P i 's offer when is the agent's rent when not taking P i 's contribution. Since t −i (q θ −i ) is nonnegative, the agent necessarily makes a nonnegative profit at any profile (θ i θ −i ).
Running Example continued. By taking only the contribution schedule defined in (4), the agent gets a reservation payoff where the second term in the right-hand side above is achieved by choosing the nonnegative outputq 2 (θ 2 ) = 1 4 (3θ 2 −θ) on the positive range of t 2 (q θ 2 ). ♦ If the agent were informed on P i 's type θ i , principal P i would solve the following mechanism design problem at a best response to any nonnegative SMP profile t −i (q θ −i ): subject to (2), (3), and (5).
A solution to P i (θ i ) is an allocation {U D (θ −i |θ i ) q D (θ −i |θ i )} (or, equivalently, a direct revelation mechanism {t D i (θ −i |θ i ) q D (θ −i |θ i )} that induces this allocation) 25 from which we can easily reconstruct the nonlinear contribution t i (q θ i ) offered by P i with the simple formula t i (q θ i ) = t D i (θ −i |θ i ) at q = q D (θ −i |θ i ). 26 The standard techniques for solving problems like P i (θ i ) in monopolistic screening environments consist in, first, neglecting the second-order condition (3), second, assuming that the participation constraint (5) binds only at θ i =θ to obtain an expression of the agent's rent U D (θ −i |θ i ), and, third, integrating by parts the expected rent left to the agent to get an expression of the principal's virtual surplus function.
As shown in the Appendix, these first three steps of the analysis lead to the reducedform problem A first difficulty is that the concavity of P i 's virtual surplus function in the maximand above depends on the other principal's offer t −i (q θ −i ), which is an equilibrium construction. A second difficulty comes from checking that the second-order condition (3) holds. It turns out that both difficulties can be handled together when t −i (q θ −i ) satisfies a couple of properties that are made explicit in condition (11) below.
Running Example continued. Let us find principal P 1 's best response to t 2 (q θ 2 ) and assume that his type is revealed through the contract offer to the agent. As we saw above, such best response can be computed by means of a direct revelation mechanism {t D 1 (θ 2 |θ 1 ) q D (θ 2 |θ 1 )}θ 2 ∈ . Using standard techniques, let us thus write the agent's payoff when taking both schedules as The equilibrium output being chosen on the positive range of t 2 (q θ 2 ), we immediately get from the Envelope Theorem that A key point for finding P 1 's best response consists in determining where the participation constraint necessary to induce the agent's acceptance of principal P 1 's contribution binds. Note thatÛ 2 (θ) = 0 and that the slope ofÛ 2 (θ 2 ) is lower than the slope of U D (θ 2 |θ 1 ) provided principal P 1 's marginal contribution is positive and induces more production than when the agent contracts only with principal P 2 . This yields immediately: When using the direct revelation mechanism {t D 1 (θ 2 |θ 1 ) q D (θ 2 |θ 1 )}θ 2 ∈ , principal P 1 's expected payoff becomes where the equality follows from using Pointwise optimization of P 1 's virtual surplus yields the following expression of the output induced at a best response to t 2 (q θ 2 ): Moreover, P 1 's marginal contribution at a best response is such that and where the second equality follows from using the expression of t 2 (q θ 2 ) given in (4) on its positive range. Integrating yields the following expression of P 1 's contribution for any output in its positive range: is chosen so that U D (θ|θ 1 ) = 0 for all θ 1 . 27 The contribution can finally be extended beyond the set of equilibrium outputs as in (4). A pair of such schedules forms thus a symmetric informative equilibrium in this quadratic-uniform example. ♦ The next subsection offers a general analysis of such equilibria.
−θ as will be the case for the output found in (6). This expression is symmetric to that obtained for t 0 2 (θ 2 ).

Symmetric informative equilibria
Consider symmetric equilibria that solve problems P i (θ i ) (or P i (θ i )) for both principals. An important step of our analysis below will consist in showing that the contribution schedule solution to P i (θ i ), which was derived assuming that the agent has complete information on P i 's type, is also a best response in , i.e., when P i is privately informed. Indeed, note that the incentive and participation constraints (2), (3), and (5) do not depend on the agent's beliefs on P i 's type, but only on the schedule that this principal offers. Hence, the agent's decisions to accept that contribution and to produce accordingly are also independent of his beliefs on P i 's type. Any deviation away from the contribution that P i would optimally offer had the agent been informed on his type is thus dominated for any out-of-equilibrium beliefs. At a symmetric informative equilibrium with contribution t(q θ) (resp. marginal contribution p(q θ)) satisfying SMP, we denote, respectively, the agent's output and rent This is the standard condition in screening models that says that the marginal surplus of the bilateral coalition between P i and the agent (left-hand side of (7)) is equal to the marginal cost of the latter's information rent (right-hand side of (7)). The difficulty comes from the fact that the marginal contribution p(q θ) is an equilibrium construction.
To complete the characterization of equilibrium marginal contributions, it is useful to rewrite the optimality condition for the agent's output given that he has accepted both contributions. This output must, on top of (7), also satisfy the following first-order condition of the agent's problem expressed in terms of nonlinear contributions: with the second-order condition Consider now an equilibrium output q(θ 1 θ 2 ) increasing in each argument. For any given level of the public good q = q(θ 1 θ 2 ), we can uniquely define the conjugate of type θ i as the type ψ(q θ i ) for principal P −i such that q(θ i ψ(q θ i )) = q. Using conditions (7) and (8), ψ(q θ) must be defined as From now on, we assume that These assumptions bear on an endogenous object, namely the equilibrium marginal contribution, but they allow a clear characterization of equilibrium properties.
Turning now to the output distortions and distributions of information rent in any such symmetric informative equilibrium, we obtain the following theorem.
Theorem 2. Any symmetric informative equilibrium with a marginal contribution p(q θ) satisfying (11) implements an output schedule q(·) and a rent profile U(·) such that

• efficiency arises when both principals have the highest valuation
• the agent's information rent is such that At a best response, a principal induces less output from the agent than what is ex post efficient for their bilateral coalition. This downward distortion reduces the information rent that the agent gets from his endogenous private knowledge on the other principal's type. This distortion is captured by the right-hand side of (7), which is positive thanks to SMP. In this common agency game, inefficiency comes from the screening problem that each principal faces in contracting with an agent who is endogenously privately informed on the other principal's type.
This downward distortion should be contrasted to the usual free-riding problem for public good provision found in centralized Bayesian mechanisms (Laffont and Maskin 1979, Rob 1989, Mailath and Postlewaite 1990. There free-riding comes from the contributors' incentives to underestimate their valuations when reporting to a single mechanism designer. Under common agency instead, principals do not hide their own valuations from the common agent, but each of these principals wants to screen the agent about the other principal's preferences. These are no longer contributors themselves who hide information, but the agent who might pretend having received less contributions from each principal than he really had.
Finally, the agent's rent is everywhere nonnegative and zero when at least one of the principals has the lowest possible valuation. The equilibrium allocation is generally not budget balanced, and may generate some surplus that accrues to the agent.

Lindahl-Samuelson conditions and tractable examples
From condition (10) we have for all q = q(θ i θ −i ) and (θ i θ −i ) ∈ 2 . This condition can be rewritten as This differential equation in θ i can be integrated to get p(q θ i ). Since p(q θ i ) must remain bounded around θ i =θ for all q, we obtain Taking into account this expression of the marginal contributions and using (8) yields the modified Lindahl-Samuelson conditions Conditions (14) and (15) might sometimes suffice to characterize the marginal contribution and output at an equilibrium.
Example 1. Let us extend our Running Example for a general cost function. Assume that the principals' types are still independently and uniformly distributed on = [θ θ ].
The following marginal contribution, which is linear in type and satisfies SMP, is part of a symmetric equilibrium: p(q θ) = 1 2 θ − 1 6θ + 1 3 C (q) One can check that condition (7) holds, so it is a best response for each principal to offer such a marginal contribution given that the other principal also does so.

Existence and uniqueness
To get further insights on the structure of equilibria, it is useful to describe an equilibrium in terms of its isoquant lines θ 2 = ψ(q θ 1 ). Rewriting conditions (7) and (8) along such isoquants yields for all (q θ), where q is in the range of the equilibrium schedule of outputs q(·). 29 For a distribution with finite support, positive density, and |f (θ)| bounded, equations (17) and (18) are already quite informative on the shape of the marginal contribution at its boundaries on any isoquant. For any q such that 2θ ≥ C (q) ≥θ+θ−1/(2f (θ)), the highest type on the q-isoquant isθ, whereas the lowest typeθ(q) ≥θ is increasing in q. We show in the Appendix (Lemma 6) that marginal contributions at those boundaries satisfy Solving for (17) and (18) at a fixed q means looking for a function x(θ) = p(q θ) that is increasing in θ (and thus invertible) on a domain [θ(q) θ ] that satisfies the nonstandard functional equation with the boundary conditions 30 x(θ(q)) =θ(q) and Equation (19) is not a standard differential equation since it depends not only on the function and its (nonnegative) derivative, but also on its inverse. Standard results do not apply to guarantee existence and uniqueness of such a solution. Moreover, the boundary conditions (20) are such that (19) has a singularity atθ. Hence, the analysis needed to prove existence has to rely on a global approach. In this respect, a more tractable way to prove existence is to work with the equilibrium distribution of marginal prices. 31 Doing so turns out also to provide new intuition on how each principal computes his best response. Fix q and denote by G(p q) the cumulative distribution of marginal price p(q θ) on that isoquant, i.e., G(p q) = Pr[p(q θ) ≤ p]. Since we are interested in deriving equilibria with strictly increasing marginal contribution, G(p q) has no atom. Denote then by g(p q) = ∂G(p q)/∂p the corresponding density. A priori, only agents with type θ ≥θ(q) may lie on that isoquant q, and the boundary conditions (20) tell us that the range of prices p(q θ) must be [θ(q) C (q) −θ(q)]. By the monotonicity of p(θ q), we have G(p(q θ) q) = F(θ) and g(p(q θ)) ∂p(q θ)/∂θ = f (θ), and we may extend G(p q) for p ∈ [θ θ (q)] with the convention that types θ ≤θ(q) contribute their valuation at the margin, i.e., p(q θ) = θ.
The equilibrium condition (19) can be rewritten using the definition of G(· q) as From this, we obtain the functional equation The boundary conditions (20) yield The next theorem provides our existence result. For this, we need the following technical assumption on the hazard function: 32 30 We focus on the case 2θ ≥ C (q) ≥θ +θ − 1/(2f (θ)), i.e., outputs close enough to the first-best when both principals have the highest valuation, since it appears to be the most interesting. Lower output levels correspond to less stringent boundary conditions that are thus less constraining for the equilibrium characterization. 31 A similar trick is used by Leininger et al. (1989) for double auctions and Wilson (1993) for nonlinear pricing. 32 Since R(θ) = 0 and R (θ) = −1, it is straightforward that the convexity of the hazard function guarantees (23). Another sufficient condition is the log concavity of the density. Notice that Theorem 3. Assume that (23) holds. A solution G(p q) to the system (21)-(22) (alternatively, a solution p(q θ) to (19)- (20)) exists. This solution G(p q) (resp. p(q θ)) is increasing in p (resp. θ).
It is instructive to sketch the proof of Theorem 3. The first step is to consider the sequence of distributions of marginal contributions that each principal plays in turn at a best response to what the other offers, starting from the simple case where one principal, say P 1 , myopically offers a marginal contribution always equal to his own valuation. Under the weak condition (23), principal P 2 reacts by offering a distribution that, at each iteration, dominates in the sense of first-order stochastic dominance that offered at the round before. For P 1 , this is the reverse; each iterate is dominated by the previous one. Intuitively, a principal finds it worthwhile to offer higher marginal contributions if the other offers lower contributions and vice versa; the best-response mapping is monotonically decreasing. This iterative process converges toward a set of distributions that is stable in the following sense: if any principal offers a distribution of marginal prices from this set, the other principal's best response lies also in it. Schauder's Second Theorem 33 then guarantees the existence of a distribution in that stable set that is a fixed point.
Theorem 3 gives us the existence of a solution p(q θ) to (19) for a given isoquant q. We must also check that, as q increases, the corresponding ψ(q θ) derived from the knowledge of p(q θ) increases in q to ensure concavity of the principals' problems as requested by Theorem 1. Using that ψ(q θ (q)) =θ in any equilibrium and differentiating with respect to q yields ∂ψ(q θ)/∂q > 0 in the neighborhood ofθ(q). Hence, concavity holds when θ is small enough. The monotonicity of output follows from Theorem 2.
We have been silent so far about uniqueness, with respect to which we make the following statement. This result is of some importance for what follows. In the case of a uniform distribution, the unique separating equilibrium is given by (16) and, anticipating Theorem 6 below, it is interim efficient.
With an unbounded support, however,θ(q) is not properly defined and there is no boundary condition that must be satisfied by the price schedule atθ = +∞. This indeterminacy opens the door to a multiplicity of equilibria as shown by the example below.

Ex post inefficiency
The following necessary implementability condition makes it easy to check whether a given output schedule can be implemented as a common agency equilibrium.
Lemma 2. Any equilibrium output q(·) must satisfy (24) Condition (24) says that the expected virtual surplus (where marginal valuations θ i are replaced by their virtual values θ i − R(θ i )) is worth at least the whole surplus generated in the worst scenario where both principals have the lowest type. This is similar to the standard feasibility condition that arises in asymmetric information models with independent types once Bayesian incentive compatibility, ex post budget balancing, and individual rationality constraints are aggregated together. 35 In the contexts used so far in this literature, there is no restriction in the centralized mechanisms that an uninformed mediator can use to implement an allocation, and this condition turns out to be also sufficient: Given any output schedule satisfying the implementability condition, one can find transfers that are ex post budget-balanced, Bayesian incentive compatible, and individually rational for the informed players. Here, the added requirement is that the allocation should arise as the equilibrium of a common agency game and budget balancing is replaced by the weaker requirement that the agent's information rent is nonnegative. Condition (24) is here no longer sufficient for implementation as a common agency equilibrium. Indeed, such an allocation must also solve the functional equation (15).
Nevertheless, the necessary condition (24) is enough to get sharp results. Indeed, Examples 1 and 2 above already showed existence of ex post inefficient equilibria. Equipped with condition (24), it is straightforward to check that ex post inefficiency always arises.
Theorem 5. The first-best output q FB (θ 1 θ 2 ) never satisfies condition (24) and thus cannot be achieved at any common agency equilibrium under asymmetric information.
This result echoes the discussion after Theorem 2, but it sharpens it. Equipped with Theorems 2 and 5, we can conclude that there is always some downward distortion 34 See the proof in the Appendix. 35 Myerson and Satterthwaite (1983) develop such conditions for the case of bargaining, whereas Laffont and Maskin (1979), Güth and Hellwig (1987), Mailath and Postlewaite (1990), Ledyard and Palfrey (1999), and Hellwig (2003) did so for the case of public goods. of the equilibrium output below the first-best for at least a set of types with nonzero measure. This contrasts sharply with the case of complete information where common agency games have efficient equilibria sustained with "truthful" schedules.

Interim inefficiency
Under asymmetric information, one can still be interested in the normative properties of common agency equilibria provided that interim efficiency is used as the welfare criterion. We now investigate circumstances under which an equilibrium might be interim efficient.
Interim efficient allocations are obtained as solutions of a centralized mechanism design problem. 36 An uninformed mediator offers a centralized mechanism to both principals, who then report their types to this mediator. This mediator maximizes a weighted sum of the principals' and the agent's utilities with the weights given to different types of principals being possibly different. Because we want to replicate a symmetric common agency equilibrium with such a centralized mechanism, we consider symmetric weights that do not depend on the principal's identity.
Lemma 3. An interim efficient profile q(θ 1 θ 2 ) nondecreasing in each argument (resp. increasing) is such that there exist positive social weights α(θ) > 0 37 such that Equation (25) is again a Lindhal-Samuelson condition under asymmetric information where valuations are replaced by virtual valuations reflecting the weights that different types have in the social welfare function that is maximized by the uninformed mediator in charge of finding such interim efficient allocation.
Examples 1 and 2 continued. For a uniform distribution having support = [θ θ ], the solution found in (16) remains interim efficient with the uniform weight α(θ) = 1 2 36 Holmström and Myerson (1983). 37 We focus on the case where all types receive a positive social weight in the social welfare criterion.
Without this assumption, we would get the unpalatable conclusion that giving only a Dirac mass to typesθ trivially achieves efficiency since the equilibrium output has no distortion at the top. Also, given that we focused above on separating equilibria with strictly monotonically increasing allocations as described in Theorem 1, we restrict to social weights that induce monotonically increasing allocations as well. 38 This inequality captures the possibility that the common agent receives a positive weight in the social welfare function maximized by the uninformed mediator. Remember that Theorem 2 shows that, in any common agency symmetric equilibrium, the agent gets a nonnegative ex post rent U(θ 1 θ 2 ) that should be accounted for when evaluating welfare. This distinguishes our notion of interim efficiency from that used when it is assumed that budget is always balanced ex post (as in Ledyard and Palfrey 1999).
for all θ. Even though the type distribution has unbounded support, positive results can also be found for Example 2 with the uniform weight α(θ) = 0, i.e., principals have no weight in the social welfare objective maximized by the uninformed mediator. ♦ Altogether (15) and (25) show that any increasing candidate function b(·) must solve the functional equation This condition is rather stringent. As a result, it is not surprising that there are few candidates for such b(·) and F(·) functions that altogether ensure interim efficiency.
Theorem 6. A symmetric equilibrium of a common agency game is interim efficient if and only if the inverse hazard rate R(θ) is linear.
Theorem 7. Public intervention through an uninformed mediator improves on the equilibrium outcome unless the inverse hazard rate R(θ) is linear.
Although the common agency institution implements an interim efficient allocation for linear inverse hazard rates, beyond that case, players strictly gain from appealing to an uninformed mediator to collect contributions and move the outcome toward the interim efficiency frontier with a centralized mechanism. This is an important insight. Contribution games under asymmetric information are unlikely to be efficient even in the weaker sense of interim efficiency. Beyond the linear inverse hazard rate case, those games entail too much screening with each principal 39 Note that f (θ) = 0 for that density, so Lemma 6 in the Appendix does not apply. In particular, trying to learn the other's type through the agent compared to a more centralized design with an uninformed mediator collecting direct messages from the privately informed principals.

Discussion
This section investigates a few extensions of our basic framework and discusses some modeling assumptions.

Delegation
The output distortion in our common agency game comes from the fact that each principal tries to screen the agent's endogenous information. This indirect communication seems overly costly. An alternative to the game of voluntary contributions could be for one principal, say P −i , to provide the public good himself and to have direct communication between principals. 40 Assuming that the agent has no particular advantage in producing the public good himself, this would amount to considering that the principals' objective functions are now, respectively, Consider now the case where a mechanism is designed by an uninformed mediator who gives all bargaining power to principal P i . It is straightforward to check that the optimal output obtained this way solves 41 42 The valuation θ −i of the principal with no bargaining power is replaced by the lower virtual valuation θ −i − R(θ −i ).
Furthermore, assuming now that types are uniformly distributed on [θ θ ] and that each principal might have all bargaining power with probability 1 2 , we find C (q(θ 1 θ 2 )) = 1 2 C (q 1 (θ 1 θ 2 )) + C (q 2 (θ 2 θ 1 )) where q(θ 1 θ 2 ) is the (unique from Theorem 4) equilibrium output obtained in the common agency game that is defined by (16). Indeed, under common agency, both principals have the same bargaining power and their valuations θ i are replaced by virtual valuations θ i − 1 2 R(θ i ) with only a weight 1 2 on the inverse hazard rate distortion term. From this, we obtain immediately the following proposition.
Proposition 1. Assume that types are uniformly distributed on [θ θ ]. Then the average output implemented with asymmetric bargaining situations 1 2 (q 1 (θ 1 θ 2 ) + q 2 (θ 2 θ 1 )) is greater (resp. lower, equal) than the equilibrium output under common agency if C (·) is concave (resp. convex, linear). 40 Matthews and Postlewaite (1989) analyze the gains of allowing unmediated communication between bidders in a double auction. Contrary to us, they do not give any productive role to one of those bidders. 41 Assuming that θ is small enough to get positive output and marginal contributions. 42 This result arises also when principal P i offers himself the mechanism (Mylovanov 2005).
When C(q) = q 2 /2 as in our Running Example, the equilibrium output under common agency is the exact mean between those implemented by delegating the contracting power to either principal with probability 1 2 . Because virtual valuations under common agency only entail half of the inverse hazard rate distortions, common agency reduces output fluctuations around that mean. Strict concavity of the surplus function implies that an ex ante efficiency criterion would select common agency rather than an institution that delegates all contracting power to either principal with probability 1 2 . This result justifies our focus on a game with voluntary contributions in the first place.

Pooling and bunching
Pooling Equilibria. Our focus on separating informative equilibria where principals reveal their types through their offers makes the agent's endogenous information vis-àvis each of them clearer. This also makes the analysis of information aggregation more relevant by stressing the most favorable case for it.
In contrast, it is possible to construct uninformative equilibria. In such equilibria, all types of a given principal pool and offer the same contribution that specifies a payment for a given output target q * , the agent learns nothing from observing that contribution, and the other principal has nothing to screen about and is forced to agree on this output target if any production takes place. Consider thus the forcing contribution t * (q) = 1 2 C(q * ) > 0 for q = q * > 0 0 for q = q * .
When both principals offer this contract, they share equally the cost of producing q * . Denote byŴ (θ) = max q θq − C(q) andq(θ) = arg max q θq − C(q), respectively, the aggregate payoff of a bilateral coalition between a principal with type θ and the agent, and the corresponding optimal (increasing) output.
Proposition 2. Assume that 2θq * − C(q * ) ≥ 2Ŵ (θ) and q * ≥q(θ). 43 There exists a pooling equilibrium in which both principals offer t * (q) whatever their types. This equilibrium is sustained with arbitrary out-of-equilibrium beliefs.
Running Example continued. The forcing contributions above give us an example of a nondifferentiable equilibrium. Assuming that the cost function is quadratic, we immediately observe that the best such symmetric forcing contracts 44 implement an output equal to q * =θ +θ, giving an ex ante welfare worth W P = 1 2 (θ +θ) 2 . Instead, tedious computations show that the linear equilibrium yields a lower ex ante welfare worth W S = W P − θ 2 /16. In other words, principals are somewhat able to weaken competition with those rather inflexible contracts. ♦ 43 It can be easily seen that the set of such q * is nonempty when q FB (θ θ ) ≥q(θ), i.e., 2θ ≥θ. 44 For this to be an equilibrium, we need to check the condition 2θq * − C(q * ) ≥ 2Ŵ (θ) from Proposition 2.
Bunching. The pooling equilibria above are such that all types of principals offer the same contribution and the agent chooses a fixed output. Starting from the separating equilibria stressed above, one can construct other equilibria that still induce the agent to choose a fixed output if his type belongs to some interval with a positive measure even though the principals' types are revealed through contract offers. Coming back to our Running Example, consider indeed the schedules where q * ∈ (3θ −θ 2θ) and t 0 i (θ i ) = (q * ) 2 /12 − (θ i /2 −θ/6)q * . Those schedules are such that all types (θ 1 θ ) or (θ θ 2 ) get zero rent. They are discontinuous at some q * that lies in the range of equilibrium outputs defined in (6) for the informative equilibrium. It can be checked that for all pairs (θ 1 θ 2 ) such that 3 2 (θ 1 + θ 2 ) −θ ≤ q * , the agent chooses q * when offered those contributions. Bunching arises due to the discontinuity at q * . 45

Communication
Our previous analysis focused on a particular strategy space for competing principals: the space of nonlinear contributions. Although it is quite natural, it might restrict communication since all information revelation takes place through the choice of a particular schedule and thus happens prior to the agent's choices on acceptance and production. One may wonder whether there would be any gain for principals to send messages to their common agent after the offer stage or, equivalently, to offer a menu of such contributions from which they will later pick one after the agent's acceptance.
Suppose that principal P i can offer any more general mechanism consisting of a collection of contribution schedulest i (q ·) = {t i (q θ i )}θ i ∈ . The output q is the agent's choice andθ i is a message sent by that principal at a communication stage that takes place following the agent's acceptance. From the Revelation Principle, there is no loss of generality from focusing on such direct communication when computing best responses in any pure strategy equilibrium. For technical reasons, we assume thatt i (q θ i ) is continuous in q andθ i . We consider also the following sequence of events where the agent chooses an output after principals have picked schedules within the menu of contributions they respectively proposed. Finally, communication opens new possibilities for contracting and, in particular, principals may find it attractive to offer "inscrutable" menus of contribution schedules that do not reveal their types, letting the agent only break even between accepting or declining such an offer in expectations. Accordingly, the strategy space of contributions is enlarged by allowing also for negative transfers if needed. Payments follow according to the principals' and the agent's choices. Denote by * the game thereby modified by appending these communication possibilities. 45 To check that those schedules are best responses to each other, one has only to check that the forcing contract region (i.e., q < q * ) does not induce deviation for each principal. The condition is similar to that in Proposition 2, We now show the robustness of any separating equilibrium in as defined through Theorem 1 to such an extension of the strategy space. An equilibrium in yields payoffs to principals that remain equilibrium payoffs in * . Given t(q θ) a separating (equilibrium) strategy in , we define a degenerate extension of this strategy in * as a collection of contribution schedules t * (q ·|θ) such that t * (q θ i |θ) = t(q θ) for allθ i ∈ . With such a degenerate extension, principal P i 's contribution does not depend on his messageθ i . Proposition 3. Take any symmetric equilibrium in corresponding to the contribution schedule t(q θ i ). There exists a perfect Bayesian equilibrium of * such that principal P i with type θ i offers a degenerate menu t * (q θ i |θ i ) = t(q θ i ) for all (θ i θ i ). 46 When communication is allowed, we may also ask whether there is the possibility of sustaining inscrutable equilibria in which all principals pool and offer the same menu of contributions, so that nothing is learned by the agent and his acceptance decision takes place in expectations. Such ex ante acceptance could relax participation constraints and reduce screening distortions.
Two remarks are in order. First, Section 7.2 shows that pooling equilibria with forcing contributions exist, which proves existence of such inscrutable equilibria. However, pooling is by and large induced by the nature of those nondifferentiable contributions that force all types of principals to agree on equal sharing of the cost of implementing a given output target. Second, moving back to more flexible differentiable contributions, the next proposition shows an impossibility result.
Proposition 4. There does not exist any perfect Bayesian equilibrium of * such that both principals P i pool whatever their types θ i and offer the same inscrutable menu of differentiable contribution schedules {t(q θ i )}θ i ∈ .
The intuition behind this proposition can be grasped in two steps. First, observe that, with an inscrutable offer by principal P 2 , principal P 1 and the agent have symmetric but incomplete information on θ 2 at the time of contracting. Under such ex ante contracting, it is well known that the differentiable "sell-out" contribution schedule t S 1 (q θ 1 ) = θ 1 q − V S (θ 1 ) (where V S (θ 1 ) is principal P 1 's payoff ) maximizes the bilateral 46 The literature on informed principal problems (Myerson 1983, Maskin and Tirole 1990 in monopolistic screening environments stresses the value of pooling offers where different types of principals offer the same mechanism (a menu of contribution schedules), delaying communication to a later stage. Such delayed communication is attractive when the agent is risk-averse (because it allows pooling of incentive constraints) or under common values (because it avoids signaling distortions). With private values and risk neutrality, no such benefit arises as shown by Mylovanov (2005) in a model like ours with a continuum of types. Allowing communication does not break equilibrium. Proposition 3 confirms that result. (The proof in the Appendix constructs the out-of-equilibrium beliefs explicitly and uses the compactness of the menu to prove that each principal finds it optimal to offer the informative mechanism t * (q ·|θ i ) at a best response.) More precisely, each principal has also in his best-response correspondence in * a degenerate menu of contributions that are all equal to his equilibrium strategy in and all information revelation takes place at the offer stage. In a related vein, Peters (2003) finds conditions under which principals do not gain from offering more than a take-it-or-leave-it offer in common agency environments with complete information on their preferences. payoff of the coalition between that principal and the agent. Provided P 2 's offer is itself differentiable in q, this is the unique way to maximize this bilateral payoff. Offering the menu {t S (q θ 1 )}θ 1 ∈ is thus part of an inscrutable best response for principal P 1 . By the same token, principal P 2 also offers a menu of sell-out contracts. Given those offers, the agent chooses an efficient output. But such ex post efficient allocation cannot be implemented from Theorem 5, which yields a contradiction.

Conclusion
Let us summarize the main findings of our analysis.
First, modeling private information on the principals' preferences in a common agency game justifies the use of nonlinear contributions for screening purposes, whereas such strategy space is given a priori in previous complete information models. Doing so introduces incentive compatibility conditions that replace the truthfulness requirement used earlier on. Under asymmetric information, principals reveal their types to the common agent through their mere offer of contributions and try to learn about the types of others that have been endogenously learned by the common agent from observing these offers.
Second, ex post inefficiency always arises at equilibrium, contrary to complete information models. The reason is not the standard free-riding phenomenon stressed by the centralized mechanism design approach, but it comes now from the desire of each principal to screen the agent about the endogenous information he has learned from observing others' offers. The common agent at the nexus of all information sets may indeed pretend that each principal contributes less than what he really does.
Third, the weaker criterion of interim efficiency may be satisfied by some separating equilibria only when the inverse hazard rate of the types distribution is linear. This suggests that principals might generally find it worth agreeing on more centralized mechanisms to improve on the equilibrium outcome achieved with voluntary contributions.
Fourth, and from a more technical viewpoint, we developed techniques to prove the existence of at least one differentiable equilibrium that solves a complex functional equation linking the marginal contribution, its type derivative, and its inverse. The difficulty in solving that equation comes from having boundary conditions at both ends of the types interval. Existence has to follow from a global approach. The techniques we developed are likely to be valuable beyond the specific examples analyzed here to tackle existence in other settings where principals offer contribution schedules in an effort to control a common agent's choice. Uniqueness is proved for the uniform distribution.
Finally, although we restricted principals to make single take-it-or-leave-it offers, we show that the separating equilibria we focus on are robust when principals may entertain more complex communication with their agent. Other extensions that were investigated dealt with the existence of pooling and uninformative equilibria, and the possibility of direct communication between principals. The latter provided a justification for the common agency institution as a means of maximizing ex ante welfare compared with more random and asymmetric allocations of the bargaining power between principals.
A few other extensions of our framework would be worthwhile to pursue. Indeed, we have so far focused on the case where principals have no means to communicate with each other. The motivation for doing so is twofold. First, and from a practical viewpoint, this may be viewed as describing equilibrium behavior when principals do not know each other before contributing or when opening communication channels between principals is prohibitively costly or even forbidden. Returning to our earlier motivating examples, the first situation may capture what happens when different sovereign countries contribute to a transnational public good, whereas the second case is more likely when different governmental bodies, separated by "Chinese walls," contribute to the financing of a public good. Second, and from a theoretical viewpoint, this focus on noncommunication between principals gives us a reference point to analyze, in future research, the benefits of adding either direct or mediated communication. Following Agastya et al. (2007), who study equilibria in a game of voluntary contributions for a 0-1 project appended with a cheap-talk stage, we conjecture that more equilibria might arise when such communication is possible.
A particular way by which communication takes place is when principals contribute sequentially. 47 Distortions might then depend on whether offers are publicly observable by subsequent principals. In the latter case, we would be back to an analysis of the Stackelberg timing, whereas our previous analysis focused on simultaneous offers. 48 In the former case, we would have also to take into account how the first contributors may manipulate beliefs of subsequent contributors to reduce their own contribution. 49

Appendix
Proof of Lemma 1. The proof is standard (see, for instance, Laffont and Martimort 2001, Chapter 3) and is thus omitted.
Proof of Theorem 1. First, we assume that the agent is informed on P i 's type and we transform problem P i (θ i ) to get P i (θ i ). Then we compute P i 's best response {t i (q θ i )} θ i ∈ to a strategy profile {t −i (q θ −i )} θ −i ∈ satisfying IT and SMP. From this, we derive the optimality conditions (7). To do so, we also assume quasi-concavity of the agent's problem and the fact that the participation constraint (5) binds only at θ −i =θ. Second, we show that these conditions are indeed satisfied.
• Pointwise optimization: Consider P i 's best response to a strategy profile {t −i (q θ −i )} θ −i ∈ used by P −i and satisfying IT. Thus U D (θ −i |θ i ) is weakly increasing in θ −i and (5) is binding only at θ −i =θ provided that the marginal contribution p i (q θ i ) is positive (we show this last claim below). Integrating by parts, we then obtain 47 See the related work of Marx and Matthews (2000). 48 With exogenous private information, Martimort (1996b) shows that distortions are exacerbated in a Stackelberg equilibrium compared with Nash. We conjecture that the same result would be true here also. 49 Pavan and Calzolari (2009) analyze sequential common agency games with exogenous information.
Inserting this latter expression into P i 's objective function and neglecting the secondorder condition (3) (that will be checked below), we obtain the reduced-form problem where S i (q θ i θ −i ) denotes principal P i 's virtual surplus defined as Surplus S i (q θ i θ −i ) is concave (resp. strictly concave) in q when which is true when Equation (29) yields the first condition in (11) for a symmetric equilibrium (where the index −i has been suppressed). Under strict concavity, optimizing pointwise the virtual surplus in (27) gives thus a unique output q D (θ −i |θ i ) (which is interior sinceQ is large enough) implemented at a best response that satisfies Hence condition (7) holds at a symmetric equilibrium satisfying IT and SMP. Differentiating (30) with respect to θ i , we obtain which yields the monotonicity property, under strict concavity ∂q D ∂θ i > 0 Therefore, principal P i offers different output schedules as his type changes so that the family t i (q θ i ) is separating in θ i . Differentiating (30) with respect to θ −i , we obtain Differentiating (28) with respect to θ −i allows us to simplify (31) to get Hence, the other monotonicity property and SMP ensures that the second-order condition for the agent's problem (3) holds when Again (32) yields the second condition in (11) at a symmetric equilibrium with an output schedule increasing in both arguments.
• Implementation of the best response through a nonlinear contribution t i (q θ i ): At a best response to P −i 's offer t −i (q θ −i ) (with margin p −i (q θ −i )), P i cannot do better than offering himself a direct revelation mechanism that implements the increasing output q D (·|θ i ) satisfying (which gives (7) at a symmetric equilibrium). Denote the inverse function for q D (θ −i |θ i ) by θ D −i (·|θ i ). We can reconstruct the nonlinear schedule t i (q θ i ) that P i could as well offer as t i (q θ i ) = t D i (θ D −i (q|θ i )|θ i ) for q in the range of q D (·|θ i ). For q ≥ q D (θ|θ i ), we extend that schedule in a smooth-pasting way with a constant slope p i (q θ i ) = θ i , i.e., t i (q θ i ) = t i (q D (θ|θ i ) θ i ) + θ i (q − q D (θ|θ i )), where t i (q D (θ|θ i ) θ i ) = t i (q D (θ|θ i ) θ i ) + q D (θ|θ i ) q D (θ|θ i ) p i (q θ i ) dq and t i (q D (θ|θ i ) θ i ) is determined through the binding participation constraint U D (θ|θ i ) =Û −i (θ). Note that this upward extension satisfies IT and SMP. For q ≤ q D (θ|θ i ), t i (q θ i ) is also extended in a smooth-pasting way below q D (θ|θ i ) as a nonnegative schedule by the formula t i (q θ i ) = max{0 t i (q D (θ|θ i ) θ i ) + q q D (θ|θ i ) p(x θ i ) dx}, where we take the extension p(x θ i ) = θ ī θ ∂p(q D (θ|y) y)/∂θ i dy for all x ≤ q D (θ|θ i ). This downward extension satisfies IT and SMP by construction.
Written in terms of contribution schedules, the first-and second-order conditions for the agent's problem can be expressed as and (8) and (9) at a symmetric equilibrium.
• The agent's participation constraint (5) binds atθ: We proceed with several lemmata. Define first the output levelq −i (θ −i ) when the agent does not take P i 's contribution as an arbitrary selection in the correspondence arg max q∈Q t Lemma 4. Assume that p i (q θ i ) ≥ 0 for all (q θ i ) and that t i (q θ i ) satisfies SMP. Then, for any (θ i θ −i ), we have Proof. By the definitions ofq(θ −i ) and q D (θ −i |θ), respectively, we have Adding up these inequalities, we get Since marginal transfers are positive, the last inequality is true only if q D (θ −i |θ) ≥q(θ −i ). Moreover, if p i (q θ i ) satisfies SMP, p i (q D (θ −i |θ i ) θ i ) ≥ p i (q D (θ −i |θ i ) θ ) and, from (33), we obtain Quasi-concavity of the agent's problem at (θ θ −i ) yields finally q D (θ −i |θ i ) ≥ q D (θ −i |θ).

Thus, we necessarily haveq
Proof. Using the Envelope Theorem, we get where the last inequality follows from (34) and the fact that t −i (q θ −i ) satisfies SMP. Therefore, Lemma 5 shows that the agent's participation constraint (5) binds necessarily atθ, and U D (θ|θ i ) =Û −i (θ) at any P i 's best response to a strategy profile {t −i (q θ −i )} θ −i ∈ satisfying IT and SMP used by P −i when this best response also satisfies SMP and has a positive marginal contribution.
• Out-of-equilibrium beliefs and best responses in : The analysis above has assumed that the agent was informed on P i 's type when the latter computes his best response to the strategy profile {t −i (q θ −i )} θ −i ∈ satisfying IT and SMP used by P −i . We show first that the strategy profile {t i (q θ i )} θ i ∈ is also a best response to {t −i (q θ −i )} θ −i ∈ in the game where the agent is a priori uninformed on P i 's type. Second, we show that any off-equilibrium beliefs sustain the strategy profile {t i (q θ i )} θ i ∈ as a best response in the game where principals are privately informed.
Consider the collection of strategies {t i (q θ i )}θ i ∈ . These strategies are all distinct so that, if played in a separating equilibrium, P i reveals his type θ i to the agent when he chooses t i (q θ i ). We want to prove that this menu of contributions is incentive compatible for P i . Denote byṼ i (θ i θ i ) principal P i 's payoff when his type is θ i and he picks the strategy t i (q θ i ) for someθ i ∈ . Denote also V i (θ i ) =Ṽ i (θ i θ i ) as the equilibrium payoff.
Facing the contributions t i (q θ i ) and t −i (q θ −i ), the agent chooses the quantity q D (θ −i |θ i ). PayoffṼ i (θ i θ i ) can be written as We can now compute Since S i (· θ i θ −i ) is a strictly concave function with critical point at q = q D (θ −i |θ i ) and ∂q D (θ −i |θ i )/∂θ i ≥ 0, we have ∂Ṽ i ∂θ i (θ i θ i ) ≥ 0 (resp. =) if and only if θ i ≥θ i (resp. =) Condition (35) shows then that the collection of strategies {t i (q θ i )}θ i ∈ is incentive compatible from principal P i 's viewpoint.
Consider now a deviation by principal P i with type θ i to a contribution schedule z i (q) such that z i (q) / ∈ {t i (q θ i )}θ i ∈ . Facing the contributions z i (q) and t −i (q θ −i ), the agent chooses a quantityq(θ −i |θ i ) in the correspondence arg max q z i (q) + t −i (q θ −i ) − C(q) regardless of his beliefs on P i 's type. Now observe that an upper bound on the payoff for such deviation is obtained when we replace the agent's participation constraint (5) by the weaker requirement In this relaxed problem, the agent's participation constraint binds necessarily atθ only. The payoff in any such deviation is thus no greater than where the right-hand side is principal P i 's payoff when he offers t i (q θ i ). This proves that t i (q θ i ) is a best response in when principal P i 's type is θ i .
Proof of Theorem 2.
Moreover, we have which, taken in tandem with (37), defines the transfer t(q(θ i θ j ) θ i ) on the range of equilibrium outputs q(θ i θ j ). Note also that and thus ∂t ∂θ i (q(θ i θ j ) θ i ) ≥ 0 for θ j ≥θ when SMP holds. Using (2), we deduce that the agent's rent is everywhere nonnegative and zero only when θ i =θ for at least one i.

Boundaries conditions for the system (18)
Lemma 6. The following properties hold.
Proof of Theorem 3. Fix q such that 2θ ≥ C (q) ≥θ +θ − 1/(2f (θ)). The boundary condition (39) can be used to integrate (21) and get G(· q) as a solution to Consider now the mapping (·) such that An equilibrium distribution G(· q) (defined on [θ(q) C (q) −θ(q)] and extended on [θ C (q) −θ(q)] as explained in the text) is thus a fixed point of the mapping (·). Several facts immediately follow from the definition (45).

Consider the function
This is not a distribution admitting a density function as required by our formalism. However, we may still apply twice to it the mapping (·) above to generate such distribution. For p ∈ [θ(q) C (q) −θ(q)), we have One can check that and 2 (F) = (I) Moreover, to avoid infinite terms in the denominator on the right-hand side of (45), we want to find a condition ensuring that the mapping (·) will be onto and that the distribution of price at any iteration starting from (I)(·) never crosses F(·). A sufficient condition is that (I)(p) ≥ F(p) for all p ∈ [θ(q) C (q) −θ(q)]. This amounts to Note that χ(θ(q)) = 0 and that χ(·), which is quasi-concave under the assumptioṅ R(p) ≤ 0, achieves its maximum at p * < C (q) −θ(q) such thatθ + p * − C (q) = (1 − F(p * ))/f (p * ). Hence, (46) holds when χ(C (q) −θ(q)) > 0. This last condition holds when (1 − F(x))/θ − x increases with x; a sufficient condition is min θ∈ θ + R(θ) =θ.
Finally, (·) is continuous on N. To show that, consider two distributions G and H in N. We have Now fix arbitrarily small. There exists η such that for p ≥ C (q) −θ(q) − η, both right-hand sides above are less than and thus | (G)(p) − (H)(p)| ≤ 2 . For p ∈ [θ(q) C (q) −θ(q) − η], the right-hand side of (47) can be made arbitrarily small, say less than 2 , by taking H close enough to G with respect to · ∞ . By, gathering everything, (G) − (H) ∞ = sup p | (G)(p) − (H)(p)| ≤ 2 , which ensures continuity. Therefore (·) is a compact mapping from N onto N * ⊆ N. The existence of G(· q) then follows Schauder's Second Theorem (Burton 2005, p. 184), which states that a compact mapping on a convex nonempty subset of a Banach space N has a fixed point.
Proof of Example 2 continued. Equations (17) and (18) first can be transformed into a system of first-order differential equations to get both the marginal contribution of a given type and the identity of his conjugate. Using (17), we get From equation (44), we obtain ∂p ∂θ (q θ) = − ∂p ∂θ (q ψ(q θ)) ∂ψ ∂θ (q θ) Differentiating (51) with respect to θ, using the last expression, and replacing θ by ψ(q θ) in (51) and (17) yield The solutions to this differential equation do not depend on q and we denote u(θ) = θ − 1/r − p(q θ). We look for such nonnegative solutions u(·) with 0 <u(θ) ≤ 1, where the last inequality is needed to satisfy SMP. Equation (52) can also be written as where λ is some constant. Since the function φ + ln(1 − φ) is monotonically decreasing on [0 1), it is invertible. Denote by G(·) its inverse defined over R − . We obtaiṅ Take now any initial value u(θ) ∈ (0 θ − 1/r) and consider the solution u(·) to (53) with this initial condition when λ > u(θ) − (1/r) ln(1 + ru(θ)). The function u(·) is nonnegative, strictly increasing, and has a slope less than 1, so that it never reaches the boundary v(θ) = θ − 1/r. Using the Theorem of Uniqueness for the solution to such a differential equation (Hirsch and Smale 1974, p. 164), it can also be shown that such a solution converges without extending it toward a limit u ∞ defined as λ = u ∞ − (1/r) ln(1 + ru ∞ ).
Proof of Lemma 2. Denote P i 's ex post payoff for a given pair (θ i θ −i ) as Simple algebra gives From the fact that U(θ θ ) = 0 in any symmetric equilibrium, we must have Indeed, we haveθ − p(q(θ θ ) θ ) > 0 from SMP and (7). Using (8), we get where the last inequality follows from the strict convexity of C(·), C(0) = 0, and the fact that q(θ θ ) > 0 when p(q θ ) > 0 and C (0) = 0. We also obtain the following expressions of the partial derivatives of V (·): and Integrating (56) yields for some function φ(·). Because U(θ θ −i ) = 0 for all θ −i , one gets Inserting the expressions obtained from (57) and (58) into (54) yields Simple integrations by parts show that Because in any equilibrium U(θ 1 θ 2 ) ≥ 0, we must have, from (59), Integrating by parts the left-hand side above yields the inequality To get (24), note that φ (θ −i ) ≥ 0 from (58) and that φ(θ) > 0 is given by (55). In passing, using (57), integrating by parts, and taking into account that U(θ i θ ) = 0 show also that Hence, the principals' interim participation constraints are satisfied.
Proof of Theorem 5. Define first Integrating by parts and using d( Using the definition of q FB (·) to simplify the last integral yields Therefore, taking expectations with respect to θ 2 yields The first term can again be integrated by parts to get where the last integral is zero by the definition of q FB (·). Gathering everything, we get Hence, (24) does not hold for the first-best.
Proof of Lemma 3. The uninformed mediator offers a centralized mechanism Denote P i 's expected payoff when his type is θ i as Denote also the agent's payoff as and Voluntary participation by the principals and the agent requires, respectively, The uninformed mediator maximizes now the objective function 51 (60), (62), and (63) for some weights α (·) to be made precise below. The characterization of those interim efficient allocations then follows closely Ledyard and Palfrey (1999). First, (60) implies where we use symmetry to set V 1 (θ) = V 2 (θ) = V (θ) ≥ 0. Then observe that (61), which is checked ex post.
Reciprocally, the fact that a common agency equilibrium satisfies (25) implies that one can find transfers that implement the corresponding output. Take T i (θ i θ −i ) = t(q(θ i θ −i ) θ i ), where t(·) is the symmetric contribution schedule.
We now prove a lemma that significantly restricts the kind of equilibrium schedules that may be sought.
Condition (66) is of course very demanding, since, taken with the equilibrium conditions, it fully characterizes the equilibrium Q(·) along the diagonal.

Q(θ)
Moreover, using 2p(Q(θ) θ) = C (Q(θ)) and differentiating with respect to θ yield 2 ∂p ∂q (Q(θ) θ) − C (Q(θ)) Q (θ) + ∂p ∂θ (Q(θ) θ) = 0 Finally, we have 2θ − C (Q(θ)) = R(θ) ∂p ∂θ (Q(θ) θ) = 4 − C (Q(θ))Q(θ) R(θ) Proof of Proposition 3. Suppose that P −i with type θ −i offers t * (q ·|θ −i ) such that t * (q θ i |θ −i ) = t(q θ −i ) for allθ −i ∈ on the equilibrium path when playing in * . This choice reveals, of course, all information on P −i 's type to the agent who gets endogenous private information on P −i 's type against P i from that exactly as when playing . Consider P i 's best response. First, notice that P i can achieve the same payoff as in by offering also the degenerate menu t * (q ·|θ i ) such that t * (q θ i |θ i ) = t(q θ i ) for all θ i ∈ . Indeed, the agent's decision to accept that degenerate menu and to produce accordingly are the same as in .
Suppose now that P i makes any other offer, say a menut i (q ·) = t * (q ·|θ i ). We want to find out-of-equilibrium beliefs for the agent that make offering this menu a suboptimal strategy for the deviating principal. Consider first the lower envelope of the offered menu defined as z i (q) = minθ i ∈ t i (q θ i ) for all q ∈ Q. By continuity oft i (q ·) inθ i and compactness of , the Theorem of the Maximum ensures that such a lower envelope z i (q) is well defined and continuous in q. Define also accordingly any arbitrary selection within the nonempty compact values and upper semicontinuous correspondence arg minθ i ∈ t i (q θ i ) asθ 0 i (q). For any θ −i , define also q(θ −i ) to be a measurable selector from the nonempty compact values correspondence arg max q∈Q z i (q) + t(q θ −i ) − C(q). Such a selector exists from the Measurable Maximum Theorem (Aliprantis and Border 1999, p. 570) since the above maximand is a Carathéodory function. Such a measurable selector allows us to compute the deviating principal's expected payoff in a meaningful way. Choose now out-of-equilibrium beliefs that put mass 1 onθ 0 i (q(θ −i )) following any deviation by principal P i . These beliefs minimize the agent's rent from his endogenous private information. Using the definition of z i (q), observe that, following the deviating menu offert i (q ·), P i gets thus at most the expected payoff E θ −i [θ i q(θ −i ) − z i (q(θ −i ))].
Note then that the contribution z i (q) could also have been offered when playing and accepted by any type of the agent if max q∈Q z i (q) + t(q θ −i ) − C(q) ≥ 0 for any type θ −i . Such contribution implements the output schedule q(θ −i ). Then, by definition of the equilibrium strategy t(q θ i ) in , we necessarily have where q(θ i θ −i ) is the equilibrium output in . This ends the proof that the deviating offert i (q ·) is dominated.
Proof of Proposition 4. Take any menu of differentiable contribution schedules {t * 2 (q θ 2 )}θ 2 ∈ that is incentive compatible for principal P 2 and inscrutable, i.e., all types of that principal offer this menu and the agent's prior beliefs on principal P 2 's types are unchanged following such an offer. By either accepting or refusing this menu, the agent getsÛ 2 = max 0 E θ 2 max q t * 2 (q θ 2 ) − C(q) Take a menu of contribution schedules {t 1 (q θ 1 )}θ 1 ∈ that is incentive compatible for principal P 1 and also inscrutable. For the agent to accept both menus of contributions, the following participation constraint must hold: E (θ 1 θ 2 ) max q {t 1 (q θ 1 ) + t * 2 (q θ 2 ) − C(q)} ≥Û 2 For differentiable schedules, incentive compatibility for the agent implies the following first-order condition at any equilibrium output q(θ 1 θ 2 ): ∂t 1 ∂q (q(θ 1 θ 2 ) θ 1 ) + ∂t * 2 ∂q (q(θ 1 θ 2 ) θ 2 ) = C (q(θ 1 θ 2 )) Lemma 8. In any best response to the inscrutable menu {t * 2 (q θ 2 )}θ 2 ∈ , principal P 1 with type θ 1 gets Proof. Consider principal P 1 with type θ 1 . He can always deviate by offering a degenerate menu {t 1 (q θ 1 )}θ 1 ∈ such that t 1 (q θ 1 ) = t S 1 (q θ 1 ) for allθ 1 , where t S 1 (q θ 1 ) is the sell-out contract t S 1 (q θ 1 ) = θ 1 q − V S 1 (θ 1 ) with V S 1 (θ 1 ) satisfying (76) and being the principal's deviation payoff. Such a sell-out contract aligns the objective of principal P 1 with that of the agent. It induces an output q(θ 1 θ 2 ) that is efficient for their bilateral coalition (given the contributions received from principal P 2 ) and it maximizes their expected bilateral payoff when expectations are taken over principal P 2 's type, which is unknown at the time of acceptance in any inscrutable equilibrium. This output is thus such that θ 1 + ∂t * 2 ∂q (q(θ 1 θ 2 ) θ 2 ) = C (q(θ 1 θ 2 )) Finally, V S 1 (θ 1 ) is adjusted to leave the agent indifferent between taking this degenerate menu, in which case his beliefs on the principal's deviating types are irrelevant, or not.
Last, at any best response in the game * , principal P 1 gets precisely V S 1 (θ 1 ) whatever his type. Indeed, such best response would give a set of incentive compatible payoffs (V 1 (θ 1 )) θ 1 ∈ for principal P 1 that, by definition, must weakly Pareto dominate the payoff vector (V S 1 (θ 1 )) θ 1 ∈ . However, the payoff vector (V S 1 (θ 1 )) θ 1 ∈ is undominated within the set of payoffs achievable with incentive compatible allocations and thus there cannot be other equilibrium payoffs.
To see that the payoff vector (V S 1 (θ 1 )) θ 1 ∈ is undominated, observe first that this payoff vector also maximizes the ex ante payoff of principal P 1 , namely E (θ 1 θ 2 ) [θ 1 q − t 1 (q θ 1 )], over the set of all incentive feasible allocations that induce the agent to accept principal P 1 's contract. Indeed, because of risk neutrality and ex ante contracting, the best ex ante incentive compatible mechanism obviously implements the bilateral efficient output that solves (77). It does so with menus of contributions {t 1 (q θ 1 )} θ 1 ∈ of the form t 1 (q θ 1 ) = θ 1 q − α(θ 1 ), which leave the agent a residual claimant for his output