An underinclusive classification excludes some similarly situated persons from the intended legislative benefit or detriment. Legislation that is designed to eliminate fraud in government but actually excludes executive branch employees from its regulatory grasp is underinclusive. Some classifications can be both underinclusive and overinclusive. The Supreme Court has developed a three-tiered approach to examine all such legislative classifications. Under the first tier of scrutiny, known as strict scrutiny, the Court will strike down any legislative classification that is not necessary to fulfill a compelling or overriding government objective.
Strict scrutiny is applied to legislation involving suspect classifications and fundamental rights.
A fundamental right is a right that is expressly or implicitly enumerated in the U.S. Constitution, such as freedom of speech or assembly. Most legislation reviewed by the Supreme Court under the strict scrutiny standard has been invalidated, because very few classifications are necessary to support a compelling government objective. The second tier of scrutiny used by the Court to review legislative classifications is known as heightened, or intermediate, scrutiny. Legislation will not survive heightened scrutiny unless the government can demonstrate that the classification is substantially related to an important societal interest. Gender classifications are examined under this middle level of review, as are classifications that burden extramarital children.