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Mechanism design

Mechanism design is a branch of economics, social choice theory, and game theory that deals with designing games (or mechanisms) to implement a given social choice function. Because it starts at the end of the game (the optimal result) and then works backwards to find a game that implements it, it is sometimes called reverse game theory.[citation needed]

The Stanley Reiter diagram above illustrates a game of mechanism design. The upper-left space depicts the type space and the upper-right space X the space of outcomes. The social choice function maps a type profile to an outcome. In games of mechanism design, agents send messages in a game environment . The equilibrium in the game can be designed to implement some social choice function .

Mechanism design has broad applications, including traditional domains of economics such as market design, but also political science (through voting theory) and even networked systems (such as in inter-domain routing).[1]

Mechanism design studies solution concepts for a class of private-information games. Leonid Hurwicz explains that "in a design problem, the goal function is the main given, while the mechanism is the unknown. Therefore, the design problem is the inverse of traditional economic theory, which is typically devoted to the analysis of the performance of a given mechanism."[2]

The 2007 Nobel Memorial Prize in Economic Sciences was awarded to Leonid Hurwicz, Eric Maskin, and Roger Myerson "for having laid the foundations of mechanism design theory."[3] The related works of William Vickrey that established the field earned him the 1996 Nobel prize.

Intuition edit

In an interesting class of Bayesian games, one player, called the "principal", would like to condition his behavior on information privately known to other players. For example, the principal would like to know the true quality of a used car a salesman is pitching. He cannot learn anything simply by asking the salesman, because it is in the salesman's interest to distort the truth. However, in mechanism design, the principal does have one advantage: He may design a game whose rules influence others to act the way he would like.

Without mechanism design theory, the principal's problem would be difficult to solve. He would have to consider all the possible games and choose the one that best influences other players' tactics. In addition, the principal would have to draw conclusions from agents who may lie to him. Thanks to the revelation principle, the principal only needs to consider games in which agents truthfully report their private information.

Foundations edit

Mechanism edit

A game of mechanism design is a game of private information in which one of the agents, called the principal, chooses the payoff structure. Following Harsanyi (1967), the agents receive secret "messages" from nature containing information relevant to payoffs. For example, a message may contain information about their preferences or the quality of a good for sale. We call this information the agent's "type" (usually noted   and accordingly the space of types  ). Agents then report a type to the principal (usually noted with a hat  ) that can be a strategic lie. After the report, the principal and the agents are paid according to the payoff structure the principal chose.

The timing of the game is:

  1. The principal commits to a mechanism   that grants an outcome   as a function of reported type
  2. The agents report, possibly dishonestly, a type profile  
  3. The mechanism is executed (agents receive outcome  )

In order to understand who gets what, it is common to divide the outcome   into a goods allocation and a money transfer,   where   stands for an allocation of goods rendered or received as a function of type, and   stands for a monetary transfer as a function of type.

As a benchmark the designer often defines what would happen under full information. Define a social choice function   mapping the (true) type profile directly to the allocation of goods received or rendered,

 

In contrast a mechanism maps the reported type profile to an outcome (again, both a goods allocation   and a money transfer  )

 

Revelation principle edit

A proposed mechanism constitutes a Bayesian game (a game of private information), and if it is well-behaved the game has a Bayesian Nash equilibrium. At equilibrium agents choose their reports strategically as a function of type

 

It is difficult to solve for Bayesian equilibria in such a setting because it involves solving for agents' best-response strategies and for the best inference from a possible strategic lie. Thanks to a sweeping result called the revelation principle, no matter the mechanism a designer can[4] confine attention to equilibria in which agents truthfully report type. The revelation principle states: "To every Bayesian Nash equilibrium there corresponds a Bayesian game with the same equilibrium outcome but in which players truthfully report type."

This is extremely useful. The principle allows one to solve for a Bayesian equilibrium by assuming all players truthfully report type (subject to an incentive compatibility constraint). In one blow it eliminates the need to consider either strategic behavior or lying.

Its proof is quite direct. Assume a Bayesian game in which the agent's strategy and payoff are functions of its type and what others do,  . By definition agent i's equilibrium strategy   is Nash in expected utility:

 

Simply define a mechanism that would induce agents to choose the same equilibrium. The easiest one to define is for the mechanism to commit to playing the agents' equilibrium strategies for them.

 

Under such a mechanism the agents of course find it optimal to reveal type since the mechanism plays the strategies they found optimal anyway. Formally, choose   such that

 

Implementability edit

The designer of a mechanism generally hopes either

  • to design a mechanism   that "implements" a social choice function
  • to find the mechanism   that maximizes some value criterion (e.g. profit)

To implement a social choice function   is to find some transfer function   that motivates agents to pick  . Formally, if the equilibrium strategy profile under the mechanism maps to the same goods allocation as a social choice function,

 

we say the mechanism implements the social choice function.

Thanks to the revelation principle, the designer can usually find a transfer function   to implement a social choice by solving an associated truthtelling game. If agents find it optimal to truthfully report type,

 

we say such a mechanism is truthfully implementable (or just "implementable"). The task is then to solve for a truthfully implementable   and impute this transfer function to the original game. An allocation   is truthfully implementable if there exists a transfer function   such that

 

which is also called the incentive compatibility (IC) constraint.

In applications, the IC condition is the key to describing the shape of   in any useful way. Under certain conditions it can even isolate the transfer function analytically. Additionally, a participation (individual rationality) constraint is sometimes added if agents have the option of not playing.

Necessity edit

Consider a setting in which all agents have a type-contingent utility function  . Consider also a goods allocation   that is vector-valued and size   (which permits   number of goods) and assume it is piecewise continuous with respect to its arguments.

The function   is implementable only if

 

whenever   and   and x is continuous at  . This is a necessary condition and is derived from the first- and second-order conditions of the agent's optimization problem assuming truth-telling.

Its meaning can be understood in two pieces. The first piece says the agent's marginal rate of substitution (MRS) increases as a function of the type,

 

In short, agents will not tell the truth if the mechanism does not offer higher agent types a better deal. Otherwise, higher types facing any mechanism that punishes high types for reporting will lie and declare they are lower types, violating the truthtelling IC constraint. The second piece is a monotonicity condition waiting to happen,

 

which, to be positive, means higher types must be given more of the good.

There is potential for the two pieces to interact. If for some type range the contract offered less quantity to higher types  , it is possible the mechanism could compensate by giving higher types a discount. But such a contract already exists for low-type agents, so this solution is pathological. Such a solution sometimes occurs in the process of solving for a mechanism. In these cases it must be "ironed." In a multiple-good environment it is also possible for the designer to reward the agent with more of one good to substitute for less of another (e.g. butter for margarine). Multiple-good mechanisms are an ongoing problem in mechanism design theory.

Sufficiency edit

Mechanism design papers usually make two assumptions to ensure implementability:

  1.  

This is known by several names: the single-crossing condition, the sorting condition and the Spence–Mirrlees condition. It means the utility function is of such a shape that the agent's MRS is increasing in type.

  1.  

This is a technical condition bounding the rate of growth of the MRS.

These assumptions are sufficient to provide that any monotonic   is implementable (a   exists that can implement it). In addition, in the single-good setting the single-crossing condition is sufficient to provide that only a monotonic   is implementable, so the designer can confine his search to a monotonic  .

Highlighted results edit

Revenue equivalence theorem edit

Vickrey (1961) gives a celebrated result that any member of a large class of auctions assures the seller of the same expected revenue and that the expected revenue is the best the seller can do. This is the case if

  1. The buyers have identical valuation functions (which may be a function of type)
  2. The buyers' types are independently distributed
  3. The buyers types are drawn from a continuous distribution
  4. The type distribution bears the monotone hazard rate property
  5. The mechanism sells the good to the buyer with the highest valuation

The last condition is crucial to the theorem. An implication is that for the seller to achieve higher revenue he must take a chance on giving the item to an agent with a lower valuation. Usually this means he must risk not selling the item at all.

Vickrey–Clarke–Groves mechanisms edit

The Vickrey (1961) auction model was later expanded by Clarke (1971) and Groves to treat a public choice problem in which a public project's cost is borne by all agents, e.g. whether to build a municipal bridge. The resulting "Vickrey–Clarke–Groves" mechanism can motivate agents to choose the socially efficient allocation of the public good even if agents have privately known valuations. In other words, it can solve the "tragedy of the commons"—under certain conditions, in particular quasilinear utility or if budget balance is not required.

Consider a setting in which   number of agents have quasilinear utility with private valuations   where the currency   is valued linearly. The VCG designer designs an incentive compatible (hence truthfully implementable) mechanism to obtain the true type profile, from which the designer implements the socially optimal allocation

 

The cleverness of the VCG mechanism is the way it motivates truthful revelation. It eliminates incentives to misreport by penalizing any agent by the cost of the distortion he causes. Among the reports the agent may make, the VCG mechanism permits a "null" report saying he is indifferent to the public good and cares only about the money transfer. This effectively removes the agent from the game. If an agent does choose to report a type, the VCG mechanism charges the agent a fee if his report is pivotal, that is if his report changes the optimal allocation x so as to harm other agents. The payment is calculated

 

which sums the distortion in the utilities of the other agents (and not his own) caused by one agent reporting.

Gibbard–Satterthwaite theorem edit

Gibbard (1973) and Satterthwaite (1975) give an impossibility result similar in spirit to Arrow's impossibility theorem. For a very general class of games, only "dictatorial" social choice functions can be implemented.

A social choice function f() is dictatorial if one agent always receives his most-favored goods allocation,

 

The theorem states that under general conditions any truthfully implementable social choice function must be dictatorial if,

  1. X is finite and contains at least three elements
  2. Preferences are rational
  3.  

Myerson–Satterthwaite theorem edit

Myerson and Satterthwaite (1983) show there is no efficient way for two parties to trade a good when they each have secret and probabilistically varying valuations for it, without the risk of forcing one party to trade at a loss. It is among the most remarkable negative results in economics—a kind of negative mirror to the fundamental theorems of welfare economics.

Shapley value edit

Phillips and Marden (2018) proved that for cost-sharing games with concave cost functions, the optimal cost-sharing rule that firstly optimizes the worst-case inefficiencies in a game (the price of anarchy), and then secondly optimizes the best-case outcomes (the price of stability), is precisely the Shapley value cost-sharing rule.[5] A symmetrical statement is similarly valid for utility-sharing games with convex utility functions.

Examples edit

Price discrimination edit

Mirrlees (1971) introduces a setting in which the transfer function t() is easy to solve for. Due to its relevance and tractability it is a common setting in the literature. Consider a single-good, single-agent setting in which the agent has quasilinear utility with an unknown type parameter  

 

and in which the principal has a prior CDF over the agent's type  . The principal can produce goods at a convex marginal cost c(x) and wants to maximize the expected profit from the transaction

 

subject to IC and IR conditions

 
 

The principal here is a monopolist trying to set a profit-maximizing price scheme in which it cannot identify the type of the customer. A common example is an airline setting fares for business, leisure and student travelers. Due to the IR condition it has to give every type a good enough deal to induce participation. Due to the IC condition it has to give every type a good enough deal that the type prefers its deal to that of any other.

A trick given by Mirrlees (1971) is to use the envelope theorem to eliminate the transfer function from the expectation to be maximized,

 
 

Integrating,

 

where   is some index type. Replacing the incentive-compatible   in the maximand,

 

after an integration by parts. This function can be maximized pointwise.

Because   is incentive-compatible already the designer can drop the IC constraint. If the utility function satisfies the Spence–Mirrlees condition then a monotonic   function exists. The IR constraint can be checked at equilibrium and the fee schedule raised or lowered accordingly. Additionally, note the presence of a hazard rate in the expression. If the type distribution bears the monotone hazard ratio property, the FOC is sufficient to solve for t(). If not, then it is necessary to check whether the monotonicity constraint (see sufficiency, above) is satisfied everywhere along the allocation and fee schedules. If not, then the designer must use Myerson ironing.

Myerson ironing edit

 
It is possible to solve for a goods or price schedule that satisfies the first-order conditions yet is not monotonic. If so it is necessary to "iron" the schedule by choosing some value at which to flatten the function.

In some applications the designer may solve the first-order conditions for the price and allocation schedules yet find they are not monotonic. For example, in the quasilinear setting this often happens when the hazard ratio is itself not monotone. By the Spence–Mirrlees condition the optimal price and allocation schedules must be monotonic, so the designer must eliminate any interval over which the schedule changes direction by flattening it.

Intuitively, what is going on is the designer finds it optimal to bunch certain types together and give them the same contract. Normally the designer motivates higher types to distinguish themselves by giving them a better deal. If there are insufficiently few higher types on the margin the designer does not find it worthwhile to grant lower types a concession (called their information rent) in order to charge higher types a type-specific contract.

Consider a monopolist principal selling to agents with quasilinear utility, the example above. Suppose the allocation schedule   satisfying the first-order conditions has a single interior peak at   and a single interior trough at  , illustrated at right.

  • Following Myerson (1981) flatten it by choosing   satisfying
     
    where   is the inverse function of x mapping to   and  is the inverse function of x mapping to  . That is,   returns a   before the interior peak and   returns a   after the interior trough.
  • If the nonmonotonic region of   borders the edge of the type space, simply set the appropriate   function (or both) to the boundary type. If there are multiple regions, see a textbook for an iterative procedure; it may be that more than one troughs should be ironed together.

Proof edit

The proof uses the theory of optimal control. It considers the set of intervals   in the nonmonotonic region of   over which it might flatten the schedule. It then writes a Hamiltonian to obtain necessary conditions for a   within the intervals

  1. that does satisfy monotonicity
  2. for which the monotonicity constraint is not binding on the boundaries of the interval

Condition two ensures that the   satisfying the optimal control problem reconnects to the schedule in the original problem at the interval boundaries (no jumps). Any   satisfying the necessary conditions must be flat because it must be monotonic and yet reconnect at the boundaries.

As before maximize the principal's expected payoff, but this time subject to the monotonicity constraint

 

and use a Hamiltonian to do it, with shadow price  

 

where   is a state variable and   the control. As usual in optimal control the costate evolution equation must satisfy

 

Taking advantage of condition 2, note the monotonicity constraint is not binding at the boundaries of the   interval,

 

meaning the costate variable condition can be integrated and also equals 0

 

The average distortion of the principal's surplus must be 0. To flatten the schedule, find an   such that its inverse image maps to a   interval satisfying the condition above.

See also edit

Notes edit

  1. ^ Penna, Paolo; Ventre, Carmine (July 2014). "Optimal collusion-resistant mechanisms with verification". Games and Economic Behavior. 86: 491–509. doi:10.1016/j.geb.2012.09.002. ISSN 0899-8256.
  2. ^ L. Hurwicz & S. Reiter (2006) Designing Economic Mechanisms, p. 30
  3. ^ "The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel 2007" (Press release). Nobel Foundation. October 15, 2007. Retrieved 2008-08-15.
  4. ^ In unusual circumstances some truth-telling games have more equilibria than the Bayesian game they mapped from. See Fudenburg-Tirole Ch. 7.2 for some references.
  5. ^ Phillips, Matthew; Marden, Jason R. (July 2018). "Design Tradeoffs in Concave Cost-Sharing Games". IEEE Transactions on Automatic Control. 63 (7): 2242–2247. doi:10.1109/tac.2017.2765299. ISSN 0018-9286. S2CID 45923961.

References edit

  • Clarke, Edward H. (1971). "Multipart Pricing of Public Goods" (PDF). Public Choice. 11 (1): 17–33. doi:10.1007/BF01726210. JSTOR 30022651. S2CID 154860771.
  • Gibbard, Allan (1973). "Manipulation of voting schemes: A general result" (PDF). Econometrica. 41 (4): 587–601. doi:10.2307/1914083. JSTOR 1914083.
  • Groves, Theodore (1973). "Incentives in Teams" (PDF). Econometrica. 41 (4): 617–631. doi:10.2307/1914085. JSTOR 1914085.
  • Harsanyi, John C. (1967). "Games with incomplete information played by "Bayesian" players, I-III. part I. The Basic Model". Management Science. 14 (3): 159–182. doi:10.1287/mnsc.14.3.159. JSTOR 2628393.
  • Mirrlees, J. A. (1971). (PDF). Review of Economic Studies. 38 (2): 175–208. doi:10.2307/2296779. JSTOR 2296779. Archived from the original (PDF) on 2017-05-10. Retrieved 2016-08-12.
  • Myerson, Roger B.; Satterthwaite, Mark A. (1983). "Efficient Mechanisms for Bilateral Trading" (PDF). Journal of Economic Theory. 29 (2): 265–281. doi:10.1016/0022-0531(83)90048-0. hdl:10419/220829.
  • Satterthwaite, Mark Allen (1975). "Strategy-proofness and Arrow's conditions: Existence and correspondence theorems for voting procedures and social welfare functions". Journal of Economic Theory. 10 (2): 187–217. CiteSeerX 10.1.1.471.9842. doi:10.1016/0022-0531(75)90050-2.
  • Vickrey, William (1961). "Counterspeculation, Auctions, and Competitive Sealed Tenders" (PDF). The Journal of Finance. 16 (1): 8–37. doi:10.1111/j.1540-6261.1961.tb02789.x.

Further reading edit

External links edit

mechanism, design, branch, economics, social, choice, theory, game, theory, that, deals, with, designing, games, mechanisms, implement, given, social, choice, function, because, starts, game, optimal, result, then, works, backwards, find, game, that, implement. Mechanism design is a branch of economics social choice theory and game theory that deals with designing games or mechanisms to implement a given social choice function Because it starts at the end of the game the optimal result and then works backwards to find a game that implements it it is sometimes called reverse game theory citation needed The Stanley Reiter diagram above illustrates a game of mechanism design The upper left space 8 displaystyle Theta depicts the type space and the upper right space X the space of outcomes The social choice function f 8 displaystyle f theta maps a type profile to an outcome In games of mechanism design agents send messages M displaystyle M in a game environment g displaystyle g The equilibrium in the game 3 M g 8 displaystyle xi M g theta can be designed to implement some social choice function f 8 displaystyle f theta Mechanism design has broad applications including traditional domains of economics such as market design but also political science through voting theory and even networked systems such as in inter domain routing 1 Mechanism design studies solution concepts for a class of private information games Leonid Hurwicz explains that in a design problem the goal function is the main given while the mechanism is the unknown Therefore the design problem is the inverse of traditional economic theory which is typically devoted to the analysis of the performance of a given mechanism 2 The 2007 Nobel Memorial Prize in Economic Sciences was awarded to Leonid Hurwicz Eric Maskin and Roger Myerson for having laid the foundations of mechanism design theory 3 The related works of William Vickrey that established the field earned him the 1996 Nobel prize Contents 1 Intuition 2 Foundations 2 1 Mechanism 2 2 Revelation principle 2 3 Implementability 2 3 1 Necessity 2 3 2 Sufficiency 3 Highlighted results 3 1 Revenue equivalence theorem 3 2 Vickrey Clarke Groves mechanisms 3 3 Gibbard Satterthwaite theorem 3 4 Myerson Satterthwaite theorem 3 5 Shapley value 4 Examples 4 1 Price discrimination 4 2 Myerson ironing 4 2 1 Proof 5 See also 6 Notes 7 References 8 Further reading 9 External linksIntuition editIn an interesting class of Bayesian games one player called the principal would like to condition his behavior on information privately known to other players For example the principal would like to know the true quality of a used car a salesman is pitching He cannot learn anything simply by asking the salesman because it is in the salesman s interest to distort the truth However in mechanism design the principal does have one advantage He may design a game whose rules influence others to act the way he would like Without mechanism design theory the principal s problem would be difficult to solve He would have to consider all the possible games and choose the one that best influences other players tactics In addition the principal would have to draw conclusions from agents who may lie to him Thanks to the revelation principle the principal only needs to consider games in which agents truthfully report their private information Foundations editMechanism edit A game of mechanism design is a game of private information in which one of the agents called the principal chooses the payoff structure Following Harsanyi 1967 the agents receive secret messages from nature containing information relevant to payoffs For example a message may contain information about their preferences or the quality of a good for sale We call this information the agent s type usually noted 8 displaystyle theta nbsp and accordingly the space of types 8 displaystyle Theta nbsp Agents then report a type to the principal usually noted with a hat 8 displaystyle hat theta nbsp that can be a strategic lie After the report the principal and the agents are paid according to the payoff structure the principal chose The timing of the game is The principal commits to a mechanism y displaystyle y nbsp that grants an outcome y displaystyle y nbsp as a function of reported type The agents report possibly dishonestly a type profile 8 displaystyle hat theta nbsp The mechanism is executed agents receive outcome y 8 displaystyle y hat theta nbsp In order to understand who gets what it is common to divide the outcome y displaystyle y nbsp into a goods allocation and a money transfer y 8 x 8 t 8 x X t T displaystyle y theta x theta t theta x in X t in T nbsp where x displaystyle x nbsp stands for an allocation of goods rendered or received as a function of type and t displaystyle t nbsp stands for a monetary transfer as a function of type As a benchmark the designer often defines what would happen under full information Define a social choice function f 8 displaystyle f theta nbsp mapping the true type profile directly to the allocation of goods received or rendered f 8 8 X displaystyle f theta Theta rightarrow X nbsp In contrast a mechanism maps the reported type profile to an outcome again both a goods allocation x displaystyle x nbsp and a money transfer t displaystyle t nbsp y 8 8 Y displaystyle y hat theta Theta rightarrow Y nbsp Revelation principle edit Main article Revelation principle A proposed mechanism constitutes a Bayesian game a game of private information and if it is well behaved the game has a Bayesian Nash equilibrium At equilibrium agents choose their reports strategically as a function of type 8 8 displaystyle hat theta theta nbsp It is difficult to solve for Bayesian equilibria in such a setting because it involves solving for agents best response strategies and for the best inference from a possible strategic lie Thanks to a sweeping result called the revelation principle no matter the mechanism a designer can 4 confine attention to equilibria in which agents truthfully report type The revelation principle states To every Bayesian Nash equilibrium there corresponds a Bayesian game with the same equilibrium outcome but in which players truthfully report type This is extremely useful The principle allows one to solve for a Bayesian equilibrium by assuming all players truthfully report type subject to an incentive compatibility constraint In one blow it eliminates the need to consider either strategic behavior or lying Its proof is quite direct Assume a Bayesian game in which the agent s strategy and payoff are functions of its type and what others do u i s i 8 i s i 8 i 8 i displaystyle u i left s i theta i s i theta i theta i right nbsp By definition agent i s equilibrium strategy s 8 i displaystyle s theta i nbsp is Nash in expected utility s i 8 i arg max s i S i 8 i p 8 i 8 i u i s i s i 8 i 8 i displaystyle s i theta i in arg max s i in S i sum theta i p theta i mid theta i u i left s i s i theta i theta i right nbsp Simply define a mechanism that would induce agents to choose the same equilibrium The easiest one to define is for the mechanism to commit to playing the agents equilibrium strategies for them y 8 8 S 8 Y displaystyle y hat theta Theta rightarrow S Theta rightarrow Y nbsp Under such a mechanism the agents of course find it optimal to reveal type since the mechanism plays the strategies they found optimal anyway Formally choose y 8 displaystyle y theta nbsp such that 8 i arg max 8 i 8 8 i p 8 i 8 i u i y 8 i 8 i 8 i 8 i p 8 i 8 i u i s i 8 s i 8 i 8 i displaystyle begin aligned theta i in amp arg max theta i in Theta sum theta i p theta i mid theta i u i left y theta i theta i theta i right 5pt amp sum theta i p theta i mid theta i u i left s i theta s i theta i theta i right end aligned nbsp Implementability edit The designer of a mechanism generally hopes either to design a mechanism y displaystyle y nbsp that implements a social choice function to find the mechanism y displaystyle y nbsp that maximizes some value criterion e g profit To implement a social choice function f 8 displaystyle f theta nbsp is to find some transfer function t 8 displaystyle t theta nbsp that motivates agents to pick f 8 displaystyle f theta nbsp Formally if the equilibrium strategy profile under the mechanism maps to the same goods allocation as a social choice function f 8 x 8 8 displaystyle f theta x left hat theta theta right nbsp we say the mechanism implements the social choice function Thanks to the revelation principle the designer can usually find a transfer function t 8 displaystyle t theta nbsp to implement a social choice by solving an associated truthtelling game If agents find it optimal to truthfully report type 8 8 8 displaystyle hat theta theta theta nbsp we say such a mechanism is truthfully implementable or just implementable The task is then to solve for a truthfully implementable t 8 displaystyle t theta nbsp and impute this transfer function to the original game An allocation x 8 displaystyle x theta nbsp is truthfully implementable if there exists a transfer function t 8 displaystyle t theta nbsp such that u x 8 t 8 8 u x 8 t 8 8 8 8 8 displaystyle u x theta t theta theta geq u x hat theta t hat theta theta forall theta hat theta in Theta nbsp which is also called the incentive compatibility IC constraint In applications the IC condition is the key to describing the shape of t 8 displaystyle t theta nbsp in any useful way Under certain conditions it can even isolate the transfer function analytically Additionally a participation individual rationality constraint is sometimes added if agents have the option of not playing Necessity edit Consider a setting in which all agents have a type contingent utility function u x t 8 displaystyle u x t theta nbsp Consider also a goods allocation x 8 displaystyle x theta nbsp that is vector valued and size k displaystyle k nbsp which permits k displaystyle k nbsp number of goods and assume it is piecewise continuous with respect to its arguments The function x 8 displaystyle x theta nbsp is implementable only if k 1 n 8 u x k u t x 8 0 displaystyle sum k 1 n frac partial partial theta left frac partial u partial x k left partial u partial t right right frac partial x partial theta geq 0 nbsp whenever x x 8 displaystyle x x theta nbsp and t t 8 displaystyle t t theta nbsp and x is continuous at 8 displaystyle theta nbsp This is a necessary condition and is derived from the first and second order conditions of the agent s optimization problem assuming truth telling Its meaning can be understood in two pieces The first piece says the agent s marginal rate of substitution MRS increases as a function of the type 8 u x k u t 8 M R S x t displaystyle frac partial partial theta left frac partial u partial x k left partial u partial t right right frac partial partial theta mathrm MRS x t nbsp In short agents will not tell the truth if the mechanism does not offer higher agent types a better deal Otherwise higher types facing any mechanism that punishes high types for reporting will lie and declare they are lower types violating the truthtelling IC constraint The second piece is a monotonicity condition waiting to happen x 8 displaystyle frac partial x partial theta nbsp which to be positive means higher types must be given more of the good There is potential for the two pieces to interact If for some type range the contract offered less quantity to higher types x 8 lt 0 displaystyle partial x partial theta lt 0 nbsp it is possible the mechanism could compensate by giving higher types a discount But such a contract already exists for low type agents so this solution is pathological Such a solution sometimes occurs in the process of solving for a mechanism In these cases it must be ironed In a multiple good environment it is also possible for the designer to reward the agent with more of one good to substitute for less of another e g butter for margarine Multiple good mechanisms are an ongoing problem in mechanism design theory Sufficiency edit Mechanism design papers usually make two assumptions to ensure implementability 8 u x k u t gt 0 k displaystyle frac partial partial theta frac partial u partial x k left partial u partial t right gt 0 forall k nbsp This is known by several names the single crossing condition the sorting condition and the Spence Mirrlees condition It means the utility function is of such a shape that the agent s MRS is increasing in type K 0 K 1 such that u x k u t K 0 K 1 t displaystyle exists K 0 K 1 text such that left frac partial u partial x k partial u partial t right leq K 0 K 1 t nbsp This is a technical condition bounding the rate of growth of the MRS These assumptions are sufficient to provide that any monotonic x 8 displaystyle x theta nbsp is implementable a t 8 displaystyle t theta nbsp exists that can implement it In addition in the single good setting the single crossing condition is sufficient to provide that only a monotonic x 8 displaystyle x theta nbsp is implementable so the designer can confine his search to a monotonic x 8 displaystyle x theta nbsp Highlighted results editRevenue equivalence theorem edit Main article Revenue equivalence Vickrey 1961 gives a celebrated result that any member of a large class of auctions assures the seller of the same expected revenue and that the expected revenue is the best the seller can do This is the case if The buyers have identical valuation functions which may be a function of type The buyers types are independently distributed The buyers types are drawn from a continuous distribution The type distribution bears the monotone hazard rate property The mechanism sells the good to the buyer with the highest valuation The last condition is crucial to the theorem An implication is that for the seller to achieve higher revenue he must take a chance on giving the item to an agent with a lower valuation Usually this means he must risk not selling the item at all Vickrey Clarke Groves mechanisms edit Main article Vickrey Clarke Groves mechanism The Vickrey 1961 auction model was later expanded by Clarke 1971 and Groves to treat a public choice problem in which a public project s cost is borne by all agents e g whether to build a municipal bridge The resulting Vickrey Clarke Groves mechanism can motivate agents to choose the socially efficient allocation of the public good even if agents have privately known valuations In other words it can solve the tragedy of the commons under certain conditions in particular quasilinear utility or if budget balance is not required Consider a setting in which I displaystyle I nbsp number of agents have quasilinear utility with private valuations v x t 8 displaystyle v x t theta nbsp where the currency t displaystyle t nbsp is valued linearly The VCG designer designs an incentive compatible hence truthfully implementable mechanism to obtain the true type profile from which the designer implements the socially optimal allocation x I 8 argmax x X i I v x 8 i displaystyle x I theta in underset x in X operatorname argmax sum i in I v x theta i nbsp The cleverness of the VCG mechanism is the way it motivates truthful revelation It eliminates incentives to misreport by penalizing any agent by the cost of the distortion he causes Among the reports the agent may make the VCG mechanism permits a null report saying he is indifferent to the public good and cares only about the money transfer This effectively removes the agent from the game If an agent does choose to report a type the VCG mechanism charges the agent a fee if his report is pivotal that is if his report changes the optimal allocation x so as to harm other agents The payment is calculated t i 8 j I i v j x I i 8 I i 8 j j I i v j x I 8 i 8 I 8 j displaystyle t i hat theta sum j in I i v j x I i theta I i theta j sum j in I i v j x I hat theta i theta I theta j nbsp which sums the distortion in the utilities of the other agents and not his own caused by one agent reporting Gibbard Satterthwaite theorem edit Main article Gibbard Satterthwaite theorem Gibbard 1973 and Satterthwaite 1975 give an impossibility result similar in spirit to Arrow s impossibility theorem For a very general class of games only dictatorial social choice functions can be implemented A social choice function f is dictatorial if one agent always receives his most favored goods allocation for f 8 i I such that u i x 8 i u i x 8 i x X displaystyle text for f Theta text exists i in I text such that u i x theta i geq u i x theta i forall x in X nbsp The theorem states that under general conditions any truthfully implementable social choice function must be dictatorial if X is finite and contains at least three elements Preferences are rational f 8 X displaystyle f Theta X nbsp Myerson Satterthwaite theorem edit Main article Myerson Satterthwaite theorem Myerson and Satterthwaite 1983 show there is no efficient way for two parties to trade a good when they each have secret and probabilistically varying valuations for it without the risk of forcing one party to trade at a loss It is among the most remarkable negative results in economics a kind of negative mirror to the fundamental theorems of welfare economics Shapley value edit Main article Shapley value Phillips and Marden 2018 proved that for cost sharing games with concave cost functions the optimal cost sharing rule that firstly optimizes the worst case inefficiencies in a game the price of anarchy and then secondly optimizes the best case outcomes the price of stability is precisely the Shapley value cost sharing rule 5 A symmetrical statement is similarly valid for utility sharing games with convex utility functions Examples editPrice discrimination edit Mirrlees 1971 introduces a setting in which the transfer function t is easy to solve for Due to its relevance and tractability it is a common setting in the literature Consider a single good single agent setting in which the agent has quasilinear utility with an unknown type parameter 8 displaystyle theta nbsp u x t 8 V x 8 t displaystyle u x t theta V x theta t nbsp and in which the principal has a prior CDF over the agent s type P 8 displaystyle P theta nbsp The principal can produce goods at a convex marginal cost c x and wants to maximize the expected profit from the transaction max x 8 t 8 E 8 t 8 c x 8 displaystyle max x theta t theta mathbb E theta left t theta c left x theta right right nbsp subject to IC and IR conditions u x 8 t 8 8 u x 8 t 8 8 8 8 displaystyle u x theta t theta theta geq u x theta t theta theta forall theta theta nbsp u x 8 t 8 8 u 8 8 displaystyle u x theta t theta theta geq underline u theta forall theta nbsp The principal here is a monopolist trying to set a profit maximizing price scheme in which it cannot identify the type of the customer A common example is an airline setting fares for business leisure and student travelers Due to the IR condition it has to give every type a good enough deal to induce participation Due to the IC condition it has to give every type a good enough deal that the type prefers its deal to that of any other A trick given by Mirrlees 1971 is to use the envelope theorem to eliminate the transfer function from the expectation to be maximized let U 8 max 8 u x 8 t 8 8 displaystyle text let U theta max theta u left x theta t theta theta right nbsp d U d 8 u 8 V 8 displaystyle frac dU d theta frac partial u partial theta frac partial V partial theta nbsp Integrating U 8 u 8 0 8 0 8 V 8 d 8 displaystyle U theta underline u theta 0 int theta 0 theta frac partial V partial tilde theta d tilde theta nbsp where 8 0 displaystyle theta 0 nbsp is some index type Replacing the incentive compatible t 8 V x 8 8 U 8 displaystyle t theta V x theta theta U theta nbsp in the maximand E 8 V x 8 8 u 8 0 8 0 8 V 8 d 8 c x 8 E 8 V x 8 8 u 8 0 1 P 8 p 8 V 8 c x 8 displaystyle begin aligned amp mathbb E theta left V x theta theta underline u theta 0 int theta 0 theta frac partial V partial tilde theta d tilde theta c left x theta right right amp mathbb E theta left V x theta theta underline u theta 0 frac 1 P theta p theta frac partial V partial theta c left x theta right right end aligned nbsp after an integration by parts This function can be maximized pointwise Because U 8 displaystyle U theta nbsp is incentive compatible already the designer can drop the IC constraint If the utility function satisfies the Spence Mirrlees condition then a monotonic x 8 displaystyle x theta nbsp function exists The IR constraint can be checked at equilibrium and the fee schedule raised or lowered accordingly Additionally note the presence of a hazard rate in the expression If the type distribution bears the monotone hazard ratio property the FOC is sufficient to solve for t If not then it is necessary to check whether the monotonicity constraint see sufficiency above is satisfied everywhere along the allocation and fee schedules If not then the designer must use Myerson ironing Myerson ironing edit nbsp It is possible to solve for a goods or price schedule that satisfies the first order conditions yet is not monotonic If so it is necessary to iron the schedule by choosing some value at which to flatten the function In some applications the designer may solve the first order conditions for the price and allocation schedules yet find they are not monotonic For example in the quasilinear setting this often happens when the hazard ratio is itself not monotone By the Spence Mirrlees condition the optimal price and allocation schedules must be monotonic so the designer must eliminate any interval over which the schedule changes direction by flattening it Intuitively what is going on is the designer finds it optimal to bunch certain types together and give them the same contract Normally the designer motivates higher types to distinguish themselves by giving them a better deal If there are insufficiently few higher types on the margin the designer does not find it worthwhile to grant lower types a concession called their information rent in order to charge higher types a type specific contract Consider a monopolist principal selling to agents with quasilinear utility the example above Suppose the allocation schedule x 8 displaystyle x theta nbsp satisfying the first order conditions has a single interior peak at 8 1 displaystyle theta 1 nbsp and a single interior trough at 8 2 gt 8 1 displaystyle theta 2 gt theta 1 nbsp illustrated at right Following Myerson 1981 flatten it by choosing x displaystyle x nbsp satisfying ϕ 2 x ϕ 1 x V x x 8 1 P 8 p 8 2 V 8 x x 8 c x x d 8 0 displaystyle int phi 2 x phi 1 x left frac partial V partial x x theta frac 1 P theta p theta frac partial 2 V partial theta partial x x theta frac partial c partial x x right d theta 0 nbsp where ϕ 1 x displaystyle phi 1 x nbsp is the inverse function of x mapping to 8 8 1 displaystyle theta leq theta 1 nbsp and ϕ 2 x displaystyle phi 2 x nbsp is the inverse function of x mapping to 8 8 2 displaystyle theta geq theta 2 nbsp That is ϕ 1 displaystyle phi 1 nbsp returns a 8 displaystyle theta nbsp before the interior peak and ϕ 2 displaystyle phi 2 nbsp returns a 8 displaystyle theta nbsp after the interior trough If the nonmonotonic region of x 8 displaystyle x theta nbsp borders the edge of the type space simply set the appropriate ϕ x displaystyle phi x nbsp function or both to the boundary type If there are multiple regions see a textbook for an iterative procedure it may be that more than one troughs should be ironed together Proof edit The proof uses the theory of optimal control It considers the set of intervals 8 8 displaystyle left underline theta overline theta right nbsp in the nonmonotonic region of x 8 displaystyle x theta nbsp over which it might flatten the schedule It then writes a Hamiltonian to obtain necessary conditions for a x 8 displaystyle x theta nbsp within the intervals that does satisfy monotonicity for which the monotonicity constraint is not binding on the boundaries of the interval Condition two ensures that the x 8 displaystyle x theta nbsp satisfying the optimal control problem reconnects to the schedule in the original problem at the interval boundaries no jumps Any x 8 displaystyle x theta nbsp satisfying the necessary conditions must be flat because it must be monotonic and yet reconnect at the boundaries As before maximize the principal s expected payoff but this time subject to the monotonicity constraint x 8 0 displaystyle frac partial x partial theta geq 0 nbsp and use a Hamiltonian to do it with shadow price n 8 displaystyle nu theta nbsp H V x 8 u 8 0 1 P 8 p 8 V 8 x 8 c x p 8 n 8 x 8 displaystyle H left V x theta underline u theta 0 frac 1 P theta p theta frac partial V partial theta x theta c x right p theta nu theta frac partial x partial theta nbsp where x displaystyle x nbsp is a state variable and x 8 displaystyle partial x partial theta nbsp the control As usual in optimal control the costate evolution equation must satisfy n 8 H x V x x 8 1 P 8 p 8 2 V 8 x x 8 c x x p 8 displaystyle frac partial nu partial theta frac partial H partial x left frac partial V partial x x theta frac 1 P theta p theta frac partial 2 V partial theta partial x x theta frac partial c partial x x right p theta nbsp Taking advantage of condition 2 note the monotonicity constraint is not binding at the boundaries of the 8 displaystyle theta nbsp interval n 8 n 8 0 displaystyle nu underline theta nu overline theta 0 nbsp meaning the costate variable condition can be integrated and also equals 0 8 8 V x x 8 1 P 8 p 8 2 V 8 x x 8 c x x p 8 d 8 0 displaystyle int underline theta overline theta left frac partial V partial x x theta frac 1 P theta p theta frac partial 2 V partial theta partial x x theta frac partial c partial x x right p theta d theta 0 nbsp The average distortion of the principal s surplus must be 0 To flatten the schedule find an x displaystyle x nbsp such that its inverse image maps to a 8 displaystyle theta nbsp interval satisfying the condition above See also editAlgorithmic mechanism design Alvin E Roth Nobel Prize market design Assignment problem Contract theory Implementation theory Incentive compatibility Revelation principle Smart market MetagameNotes edit Penna Paolo Ventre Carmine July 2014 Optimal collusion resistant mechanisms with verification Games and Economic Behavior 86 491 509 doi 10 1016 j geb 2012 09 002 ISSN 0899 8256 L Hurwicz amp S Reiter 2006 Designing Economic Mechanisms p 30 The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel 2007 Press release Nobel Foundation October 15 2007 Retrieved 2008 08 15 In unusual circumstances some truth telling games have more equilibria than the Bayesian game they mapped from See Fudenburg Tirole Ch 7 2 for some references Phillips Matthew Marden Jason R July 2018 Design Tradeoffs in Concave Cost Sharing Games IEEE Transactions on Automatic Control 63 7 2242 2247 doi 10 1109 tac 2017 2765299 ISSN 0018 9286 S2CID 45923961 References editClarke Edward H 1971 Multipart Pricing of Public Goods PDF Public Choice 11 1 17 33 doi 10 1007 BF01726210 JSTOR 30022651 S2CID 154860771 Gibbard Allan 1973 Manipulation of voting schemes A general result PDF Econometrica 41 4 587 601 doi 10 2307 1914083 JSTOR 1914083 Groves Theodore 1973 Incentives in Teams PDF Econometrica 41 4 617 631 doi 10 2307 1914085 JSTOR 1914085 Harsanyi John C 1967 Games with incomplete information played by Bayesian players I III part I The Basic Model Management Science 14 3 159 182 doi 10 1287 mnsc 14 3 159 JSTOR 2628393 Mirrlees J A 1971 An Exploration in the Theory of Optimum Income Taxation PDF Review of Economic Studies 38 2 175 208 doi 10 2307 2296779 JSTOR 2296779 Archived from the original PDF on 2017 05 10 Retrieved 2016 08 12 Myerson Roger B Satterthwaite Mark A 1983 Efficient Mechanisms for Bilateral Trading PDF Journal of Economic Theory 29 2 265 281 doi 10 1016 0022 0531 83 90048 0 hdl 10419 220829 Satterthwaite Mark Allen 1975 Strategy proofness and Arrow s conditions Existence and correspondence theorems for voting procedures and social welfare functions Journal of Economic Theory 10 2 187 217 CiteSeerX 10 1 1 471 9842 doi 10 1016 0022 0531 75 90050 2 Vickrey William 1961 Counterspeculation Auctions and Competitive Sealed Tenders PDF The Journal of Finance 16 1 8 37 doi 10 1111 j 1540 6261 1961 tb02789 x Further reading editChapter 7 of Fudenberg Drew Tirole Jean 1991 Game Theory Boston MIT Press ISBN 978 0 262 06141 4 A standard text for graduate game theory Chapter 23 of Mas Colell Whinston Green 1995 Microeconomic Theory Oxford Oxford University Press ISBN 978 0 19 507340 9 A standard text for graduate microeconomics Milgrom Paul 2004 Putting Auction Theory to Work New York Cambridge University Press ISBN 978 0 521 55184 7 Applications of mechanism design principles in the context of auctions Noam Nisan A Google tech talk on mechanism design Legros Patrick Cantillon Estelle 2007 What is mechanism design and why does it matter for policy making Centre for Economic Policy Research Roger B Myerson 2008 Mechanism Design The New Palgrave Dictionary of Economics Online Abstract Diamantaras Dimitrios 2009 A Toolbox for Economic Design New York Palgrave Macmillan ISBN 978 0 230 61060 6 A graduate text specifically focused on mechanism design External links editEric Maskin Nobel Prize Lecture delivered on 8 December 2007 at Aula Magna Stockholm University Retrieved from https en wikipedia org w index php title Mechanism design amp oldid 1214580003, wikipedia, wiki, book, books, library,

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