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A yellow-dog contract (a yellow-dog clause of a contract, or an ironclad oath) is an agreement between an employer and an employee in which the employee agrees, as a condition of employment, not to be a member of a labor union. In the United States, such contracts were, until the 1930s, widely used by employers to prevent the formation of unions, most often by permitting employers to take legal action against union organizers. In 1932, yellow-dog contracts were outlawed in the United States under the Norris-LaGuardia Act.
The term yellow-dog clause can also have a different meaning: non-compete clauses within or appended to a non-disclosure agreement to prevent an employee from working for other employers in the same industry.
In the 1870s, a written agreement containing a pledge not to join a union was commonly referred to as the "Infamous Document." This strengthens the belief that American employers in their resort to individual contracts were consciously following English precedents. This anti-union pledge was also called an "iron clad document," and from this time until the close of the 19th century "iron-clad" was the customary name for the non-union promise. Beginning with New York in 1887, sixteen states wrote on their statute books declarations making it a criminal act to force employees to agree not to join unions. The Congress of the United States incorporated in the Erdman Act of 1898 a provision relating to carriers engaged in interstate commerce.
During the last decade of the 19th century and the opening years of the 20th, the individual, anti-union promise declined in importance as an instrument in labor warfare. Its novelty had worn off; workers no longer felt themselves morally bound to live up to it and union organizers, of course, wholly disregarded it. In the early 20th century, the individual, anti-union promise was resorted to frequently in coal mining and in the metal trades. And it was not membership in a union that was usually prohibited, but participation in those essential activities without which membership is valueless.
In 1910, the International United Brotherhood of Leather Workers on Horse Goods, following an unsuccessful conference with the National Saddlery Manufacturers' Association, called a national strike in the saddlery industry for the 8-hour day. The strike proved a failure, and a large number of employers required oral or written promises to abandon and remain out of the organization as a condition of re-employment.
In the case Adair v. United States, the United States Supreme Court's majority held that the provision of the Erdman Act relating to discharge, because it would compel an employer to accept or retain the personal services of another person against the employer's will, was a violation of the Fifth Amendment to the Constitution, which declares that no person shall be deprived of liberty or property without due process of law. The court was careful, however, to restrict the decision to the provision relating to discharge, and to express no opinion as to the remainder of the law. The section of the Erdman Act making it criminal to force employees to sign anti-union agreements therefore remained unadjudicated.
The term yellow dog started appearing in the spring of 1921, in leading articles and editorials devoted to the subject which appeared in the labor press. Typical was the comment of the editor of the United Mine Workers' Journal:
Even though they were forbidden in the private sector by the Norris-LaGuardia Act in 1932, yellow dog contracts were allowed in public sector, including many government jobs, such as teachers, until the 1960s, beginning with precedent established in 1915 with Frederick v. Ownens.