In reviewing the constitutionality of a statute, law, or ordinance, U.S. courts employ one of three levels of review: rational basis, intermediate scrutiny, or strict scrutiny.
The nature of the right at issue determines which level of scrutiny will be used. Where a fundamental right or a suspect classification is at issue, a higher level of scrutiny is used.
The idea of applying a different level of scrutiny based on the potential right infringed has its origins in the United States v. Carolene Products Company (1938). The court in Carolene employed the rational basis test; however, footnote 4 of Carolene leaves open the possibility of the Court using a higher level of scrutiny in cases dealing with fundamental rights.
The ‘rational basis’ test, the most lenient of the three tests, reviews whether the law at issue is ‘rationally related’ to a ‘legitimate’ government interest.
United States v. Carolene Products Company, 304 U.S. 144 (1938).