In American law, the clear statement rule is a guideline for statutory construction, instructing courts to not interpret a statute in a way that will have particular consequences unless the statute makes unmistakably clear its intent to achieve that result. Such rules "insist that a particular result can be achieved only if the text (and not legislative history) says so in no uncertain terms." Popkin, Statutes in Court 201 (1999).
Such rules are commonly applied in areas implicating the structural constitution, such as federalism - sovereign immunity or preemption, for example - where there is a strong interest against implicit abridgment of traditional understandings. For example, while Congress can abrogate the states' sovereign immunity in some situations, see Seminole Tribe v. Florida, 517 U.S. 44 (1996); Alden v. Maine, 527 U.S. 706 (1999), it cannot do so implicitly: it must "mak[e] its intention unmistakably clear in the language of the statute." Atascadero State Hospital v. Scanlon, 473 U.S. 234, 242 (1985). Preemption is another area susceptible to clear statement rules. Congress may preempt a field of regulation, "occup[ying] a field[ and] leaving no room for any claim under state law," Pollit v. Health Care Service Corp., 558 F.3d 615 (7th Cir. 2009), but it doesn't have to. When a law is construed to preempt, the result is a broad and indiscriminate extinguishment of substantive and remedial state law, and sensitive to this problem, "the Court has on occasion"--Wyeth v. Levine, for example—expressed support for a clear statement rule: it will find preemption only when Congress expresses preemptive intent clearly in the statute." Kendall, Redefining Federalism 49 (2005).
Some argue that Chief Justice John Marshall imposed such a rule: "where fundamental values were at stake, statutes would not be interpreted to impair such values, absent a clear statement in the legislation," Popkin, at 73, and the court more recently applied a similar approach in Hamdan v. Rumsfeld, 548 U.S. 557 (2006). Similarly, a clear statement rule governs exceptions to the rule that legislation must be considered as addressed to the future, not to the past." Greene v. United States, 376 U.S. 149, 160 (1964). Because "statutory retroactivity has long been disfavored," and is in many instances forbidden by the ex post facto clause of the Constitution, see Landgraf v. USI Film Products, 511 U.S. 244 (1994), "[a]bsent a clear statement from Congress that an amendment should apply retroactively, we presume that it applies only prospectively to future conduct, at least to the extent that it affects substantive rights, liabilities, or duties." United States v. Seale, 542 F.3d 1033 (5th Cir. 2008) (internal quotation marks omitted). As the Supreme Court explained in Landgraf, "a requirement that Congress first make its intention clear helps ensure that Congress itself has determined that the benefits of retroactivity outweigh the potential for disruption or unfairness." Such rules do, therefore, have some life in the area of substantive rights as well as enforcement of constitutional structure.
The court does not apply clear statement rules in all areas, however. In many cases, the court has found "implied" prohibitions and causes of action in statutes, a result that would be precluded (or at least hampered) by clear statement rules. For example, Title IX prohibits gender discrimination by recipients of federal education funding. Does that bare prohibition also provide an implied cause of action to an individual so discriminated against? Yes, the court held in Cannon v. University of Chicago, 441 U.S. 677 (1979). Does the prohibition on discrimination also imply a prohibition on, and cause of action for, retaliation against someone who complains of such discrimination? Yes, the court held in Jackson v. Birmingham Board of Education, 544 U.S. 167 (2005). Similarly, ADEA prohibits age discrimination. Does that also imply a prohibition on retaliation against someone who complains of such discrimination? Yes, the court held in Gomez-Perez v. Potter, 128 S. Ct. 29 (2008).