Rational basis, intermediate, and strict scrutiny tiers
When a law classifies people based on protected characteristics or restricts fundamental rights, courts apply different levels of review under equal protection doctrine. The rational basis test is the most deferential: a law survives this review if it is rationally related to a legitimate government interest. Courts assume the legislature had a rational basis and uphold the law unless the challenger proves there is no conceivable rational reason for it. This standard rarely strikes down laws.
Intermediate scrutiny applies to classifications based on gender or legitimacy. The law must serve an important government interest and be substantially related to achieving that interest. This is more demanding than rational basis but less stringent than strict scrutiny. A law requiring men and women to have different work hours would fail intermediate scrutiny because any gender-based classification must be substantially related to an important interest, not merely rationally related to a legitimate one.
Strict scrutiny for suspect classifications
Strict scrutiny applies to laws classifying people by race, national origin, or alienage. The government must prove the law serves a compelling interest and is narrowly tailored to achieve that interest. Almost no law survives strict scrutiny. A law prohibiting people of a certain race from voting fails immediately. Even affirmative action programs designed to remedy discrimination face strict scrutiny, and many have been struck down because race-conscious classifications are disfavored even when intended to help historically marginalized groups.
The tiers reflect the principle that some classifications are inherently suspect because of a history of discrimination or because the trait is immutable and bears no relationship to ability to perform. Age classifications receive rational basis review because age is not immutable and lacks a strong history of systematic discrimination. Religion receives its own framework under the Free Exercise and Establishment Clauses. These tiers channel judicial skepticism; the more stringent the test, the harder a law must work to survive.