It has also been criticized that it is based on erroneous assumptions about the intrinsic distribution of power and information in the labour/employer relationship.  On the other hand, conservative scientists in the field of law and economics, such as Professors Richard A. Epstein and Richard Posner, state that employment after authorization is an important factor that underlyes the strength of the U.S. economy. There are no precise rules on what may appear in an employment contract, so it is up to the employer and the potential worker to negotiate the exact terms of the agreement. Wording that expressly prohibits dismissal without justification in the contract is the best way for a worker to protect himself after having ensed up labour law. If an employee is hired, they will probably get some kind of contract that they can read and sign. There are different types of documents, and they can even vary within the same company. However, such a contract is not a legal requirement and is only implied. Thirty-six U.S. states (and the District of Columbia) also recognize an implied contract as a waiver of authorization.  According to the implicit exception in the contract, an employer cannot dismiss an employee „when a tacit contract is concluded between an employer and an employee, even if there is no explicit written act on the employment relationship.“  Proving the terms of a tacit contract is often difficult and the burden of proof lies with the dismissed employee.