A bright-line rule (or bright-line test) is a clearly defined rule or standard, generally used in law, composed of objective factors which leaves little or no room for varying interpretation. The purpose of a bright-line rule is to produce predictable and consistent results in its application.
Bright-line rules are usually standards established by courts in legal precedent or by legislatures in statutory provisions. Bright-line rules are often contrasted with its opposite balancing tests ( or fine line testing), where a result is dependent on weighing several factors, which could lead to inconsistent application of law or reduce objectivity.
 Debate in the US
In the United States, there is much scholarly legal debate between those favoring bright-line rules and those favoring balancing tests. While some legal scholars, such as Supreme Court Justice Antonin Scalia, have expressed a strong preference for bright-line rules, critics often argue that bright-line rules are overly-simplistic and can lead to harsh and unjust results. Supreme Court Justice Stephen Breyer noted that there are circumstances in which the application of bright-line rules would be inappropriate, stating that "no single set of legal rules can ever capture the ever changing complexity of human life." Over the course of the last three decades, many bright-line rules previously established in U.S. jurisprudence have been replaced with balancing tests.
 Example of a bright-line rule
- American statutory rape laws, including Romeo and Juliet laws – In most states, the age of the victim and the age of the accused are the only relevant factors determinative of guilt or innocence. Because it is a bright-line rule, there is no balancing test to examine factors such as mistake of the accused, the misrepresentation of age by the minor, or the minor's consent to sexual intercourse.
- In Michigan v. Summers, the Supreme Court announced the bright-line rule that law enforcement officers have the authority to detain the occupants of a residence while conducting a search for contraband. The rule was intended to provide clear rules to law enforcement personnel and avoid case-by-case analysis. Scholars have challenged the clarity and efficacy of this bright-line rule in practice.
 Notable cases containing bright-line rules
- Miranda v. Arizona
- Goldberg v. Kelly
- Michigan v. Summers
- Storer Doctrine
- SEC v. Chenery Corp., 332 U.S. 194 (1947)
- National Petroleum Refiners Assn. v. FTC, 482 F.2d 672 (D.C. Cir. 1973), cert. denied, 415 U.S. 951 (1974)
- Heckler v. Campbell, 461 U.S. 458 (1983)
- Bowen v. Georgetown University Hospital, 488 U.S. 204 (1988)
- Sameena, Inc. v. U.S. Air Force, 147 F.3d 1148 9th Cir. (1998)
- Evans v. the United Kingdom
- Georgia v. Randolph, 547 U.S. 103, 125, 126 S. Ct. 1515, 1529, 164 L. Ed. 2d 208, 229 (2006) (Breyer, J., concurring).
- Statutory Rape Laws by State
- Amir H. Ali, Following the Bright-Line of Michigan v. Summers, 45 Harvard Civil Rights-Civil Liberties Law Review 483 (2010)
- Language Log Discussion of the phrase, with examples and history
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