The standards for determining the proper pleading of an affirmative defense and for granting a motion to strike an affirmative defense have had a curious evolution. The standards appear never to have been considered by the Supreme Court. In two original jurisdiction cases, the Court struck an affirmative defense, see Washington v. Oregon, 52 S. Ct. 456, 76 L. Ed. 1301 (1932), and Missouri v. Chicago, Burlington, & Quincy R.R. Co., 241 U.S. 533, 36 S. Ct. 715, 60 L. Ed. 1148 (1916), but did not discuss the standard for granting a motion to strike an affirmative defense.
Uncertainty has sometimes resulted from the fact that district courts, which have made most of the rulings on sufficiency of affirmative defenses and motions to strike them, have not always distinguished between affirmative defenses in a timely filed answer and those later filed, either with or without court permission to amend an answer, especially those filed in late stages of litigation. Rule 12(a)(1)(A)(i) requires an answer to be filed within 21 days after service of a summons and complaint. Rule 15(a)(1)(A) provides that a party is entitled to amend its answer 21 days after serving it, Rule ...