A characteristic of a statute or administrative rule dealing with First Amendment rights and other fundamental liberty interests, whereby the statute prohibits some conduct but fails to prohibit other, similar conduct.
An underinclusive law is not necessarily unconstitutional or invalid. The U.S. Supreme Court has recognized that all laws are underinclusive and selective to some extent. If a law is substantially underinclusive, however, it may be unconstitutional.
The case of Church of Lukumi Babalu Aye, Inc. v. City of Hialeah, 508 U.S. 520, 113 S. Ct. 2217, 124 L. Ed. 2d 472 (1993), illustrates unconstitutional underinclusiveness. The Church of Lukumi Babalu Aye is a religious sect that practices Santeria, which involves the ritual killing of animals. Shortly after officials of the city of Hialeah, Florida, learned that the church had purchased property in that city, the city passed certain ordinances for the stated purpose of promoting public health and preventing cruelty to animals. Because the ordinances prohibited the ritual killing of animals, the church's practice of animal sacrifice was made illegal.
According to the Supreme Court, the ordinances infringed on the freedom of the church to practice its religion. Furthermore, the ordinances were so underinclusive in their attempt to promote public health and prevent animal cruelty that they violated the First Amendment to the U.S. Constitution. The ordinances failed to punish other, nonreligious conduct that endangered the city's interest in animal Welfare, such as fishing or hunting for sport. The ordinances also failed to cover other, nonreligious animal killing that threatened the city's interest in public health. The ordinances did not, for example, prevent hunters from bringing animal carcasses to their homes. Ultimately, the Court concluded, the ordinances had "every appearance of a prohibition that society is prepared to impose upon Santeria worshippers but not upon itself."
If a law infringes on constitutionally protected free speech, press, or associational rights, it may be unconstitutionally underinclusive if it is based on the content of the speech or somehow regulates ideas. In R.A.V. v. City of St. Paul, 505 U.S. 377, 112 S. Ct. 2538, 120 L. Ed. 2d 305 (1992), the Supreme Court struck down a hate speech ordinance that prohibited "the display of a symbol which one knows or has reason to know 'arouses anger, alarm or resentment in others on the basis of race, color, creed, religion or gender.'" A youth in St. Paul, Minnesota, had been prosecuted under the ordinance for burning a cross in the yard of an African American family. The Court held that the law was unconstitutionally underinclusive under the First Amendment because it punished only certain speech addressing particular topics; the law addressed the content, rather than the manner, of the speech.
A law is not necessarily invalid just because it is underinclusive. For example, a statute that prohibited the use of loudspeaker systems near a hospital might be underinclusive for failing to prohibit shouting or the use of car horns in the same area. This type of underinclusiveness concerns only the manner of delivering speech, however, and is therefore more likely to pass constitutional scrutiny than a statute that prohibits speech on particular subjects.