As the fundamental law of the land, the United States Constitution imposes on public employers duties that limit their ability to make employment choices on the basis of certain characteristics — race, sex, national origin, religion, age, disability, citizenship, and others. The primary constitutional constraint is found in the Equal Protection Clause. The strength of the prohibition varies, however, depending on which so-called level of scrutiny the courts decide to use when reviewing a particular governmental decision. For example, the prohibition against race discrimination is perhaps the strongest, because the courts apply a strict scrutiny test to decisions that are racially based, while decisions based on disability are illegal only if they lack a rational basis.