In Mount Healthy City School District Board of Education v. Doyle, 429 U.S. 274 (1977), the school board had made a decision not to rehire an untenured teacher as a result of a series of incidents that included name calling and obscene gesturing to students, arguments with other school employees, and a telephone call to a local radio station to express discontent with a teacher dress code policy. Id. at 281-82. The district court had found for the employee on the grounds that the telephone call, an exercise of protected speech, was a "substantial part" of the decision not to rehire. Id. at 283. The Supreme Court reversed, specifically rejecting the argument that "the fact that the protected conduct played a 'substantial part' in the actual decision not to renew would necessarily amount to a constitutional violation justifying remedial action." Id. at 285. Analogizing to cases involving the voluntariness of a confession in a criminal case, the Court announced a burden-shifting approach that would "protect against the invasion of constitutional rights without commanding undesirable consequences not necessary to the assurance of those rights." Id. at 287. Thus, under Mount Healthy, when an employee asserts that he or she was fired by an employer in retaliation for his or her exercise of free speech, the employee must show "that his or her conduct was constitutionally protected, and that this conduct was a 'substantial factor'--or 'motivating factor' in the employer's decision not to rehire him or her." Id. Once the employee has made that prima facie case, the burden shifts to the employer to show "by a preponderance of the evidence that it would have reached the same decision as to employee's reemployment even in the absence of the protected conduct." Id.