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We study full surplus extraction and implementation in dynamic environments. We exploit intertemporal correlations of agents' types to construct within-period ex post incentive compatible mechanisms. First, we formulate one-shot environments, in which a single agent has a hidden type and the planner observes a public signal about the agent's type after a type-contingent allocation is chosen. We propose necessary and sufficient conditions for full surplus extraction (strong detectability) and for implementability of the targeted allocation rule (weak detectability) in this one-shot problem. We decompose the general dynamic problem into one-shot problems, and obtain sufficient conditions for surplus extraction and implementation.

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We study the design of self-enforcing mechanisms that rely on neither a trusted third party (e.g., court, trusted mechanism designer) nor a long-term relationship. Instead, we use a smart contract written on blockchains as a commitment device. We design the digital court, a smart contract that identifies and punishes agents who reneged on the agreement. The digital court substitutes the role of legal enforcement in the traditional mechanism design paradigm. We show that, any agreement that is implementable with legal enforcement can also be implemented with enforcement by the digital court. To pursue a desirable design of the digital court, we study a way to leverage truthful reports made by a small fraction of behavioral agents. Our digital court has a unique equilibrium as long as there is a positive fraction of behavioral agents, and it gives correct judgment in the equilibrium if honest agents are more likely to exist than dishonest agents. The platform for smart contracts is already ready in 2020; thus, self-enforcing mechanisms proposed in this paper can be used practically, even now. As our digital court can be used for implementing general agreements, it does not leak the detailed information about the agreement even if it is deployed on a public blockchain (e.g., Ethereum) as a smart contract. (2019), which demonstrated that smart contracts could substitute one of the role of courts but enhance illegal cartelization. This paper further articulates the convenience and social risk of blockchain enforcement by considering a general mechanism design framework and showing the possibility of (i) unique implementation through behavioral mechanism design, (ii) prevention of false charges, and (iii) privacy preservation. JEL Codes: D47, D82, L86 This paper supersedes Matsushima