According to Hart and Holmström (1987), moral hazard models can be subdivided in models with hidden action and models with hidden information. In the former case, after the contract has been signed the agent chooses an action (such as an effort level) that cannot be observed by the principal. In the latter case, after the contract has been signed there is a random draw by nature that determines the agent's type (such as his valuation for a good or his costs of effort). In the literature, two reasons have been discussed why moral hazard may imply that the first-best solution (the solution that would be attained under complete information) is not achieved.