An evidentiary standard that applies in certain employment discrimination cases. (Whether or not the but for evidentiary standard applies depends upon (1) the particular jurisdiction in which a case is filed and (2) the exact type of discrimination alleged.) Under this standard, employee-plaintiffs must prove that the employer's discriminatory act for which a claim is being brought (e.g., termination, reassignment, demotion) was the primary factor that motivated the employer-defendant's particular action. For example, assume an employee is terminated for low productivity that had been documented during the previous 6 months. At the exit interview, the employee is referred to as "one of our older workers" by the company's human resources manager. Under the "but for" evidentiary standard, the worker must prove that he was terminated primarily because of his age and not as a result of low productivity. Although the remark about being an "older worker" may have been discriminatory, by itself, it would still not be sufficient to prove a discrimination case if the worker's lack of productivity was the employer's primary motivation for terminating the employee.

Employers favor the "but for" standard because it sets up a relatively high level of proof to support a discrimination case.