Most classifications are either overinclusive or underinclusive. An overinclusive classification contains all persons who are similarly situated and also persons who should not be included. Legislation that is intended to protect poor and fragile elderly people but actually extends to all senior citizens is overinclusive. 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.