oath, vocal affirmation of the truth of one's statements, generally made by appealing to a deity. From the earliest days of human history, calling upon the gods of a community to witness the truth of a statement or the solemnity of a promise has been commonly practiced. The force of the oath depends on the belief that supernatural powers will punish falsehood spoken under oath or the violation of a promise. The oath thus performs wide legal and quasi-legal functions. It was the basis of the medieval process of compurgation. It is still used in legal proceedings today: Thus, a jury is sworn in, and a witness takes an oath before testifying. In modern times, the force of the oath is strengthened by punishment for perjury. Difficulties have arisen in cases of atheists or of persons with religious scruples against oath-taking (e.g., Quakers), but statutes have now generally been modified so that a witness may affirm his intention to tell the truth without appealing to a deity. The main classes of oaths are the assertory oath, which concerns past or present facts, and the promissory oath, which refers to future conduct (such as that taken by an alien upon naturalization or by a high government official on assuming office). In the 1950s, fear of Communist subversion led many governmental and educational institutions to institute loyalty oaths, which required employees to swear to their non-membership in the Communist party or other presumptively subversive organizations. In the 1960s these loyalty oath requirements were invalidated by the Supreme Court.