anti-spousal testimony rule

The reason for the rule at common law disqualifying the wife is to protect the sanctity and tranquility of the marital relationship. When the good faith of the marital relation is pertinent and it is made to appear to the trial court, as it was here, that the relationship was entered into with no intention of the parties to live together as husband and wife but only for the purpose of using the marriage ceremony in a scheme to defraud, the ostensible spouses are competent to testify against each other.

Defined by the Supreme Court in - Lutwak v. United States, 344 U.S. 604 (1953).