The Fourteenth Amendment to the Constitution, ratified in the wake of the Civil War, provides in part that "No state shall . . . deny to any person within its jurisdiction the equal protection of the laws." This simple phrase, known as the Equal Protection Clause, has spawned a complex body of judicial doctrine. But its original purpose was to ensure that the recently defeated Southern states did not infringe on the rights of the newly emancipated slaves.

Initially, it seemed as though the clause might be limited only to claims of racial discrimination. In its first interpretation of the Equal Protection Clause in Slaughter-House Cases (1873), the Supreme Court held that a group of Louisiana butchers could not rely on the clause to challenge a state monopoly. "We doubt very much whether any action of a State not directed by way of discrimination against [African Americans] as a class, or on account of their race, will ever be held to come within the purview of this provision," the Court said.

But in fact the Court soon began to expand the reach of the clause beyond its core requirement of equal treatment of the races, holding that it was essentially a directive that all persons "similarly situated" should be treated alike. The issue then became how to determine which groups were in fact similarly situated. A general "rationality" requirement was read into the provision: in order for legislation to pass muster, any distinction it drew between groups of people-any "classification," in legal parlance-had to be rationally related to the legislation's purpose. In other words, for the law to treat one group of people differently than another its defender simply had to show that there was a reason for the discrepancy that went beyond mere hostility to the targeted group.

A similar requirement was read into another provision of the Fourteenth Amendment, the Due Process Clause: "Nor shall any state deprive any person of life, liberty, or property, without due process of law." From the turn of the century through the 1930s, a conservative Supreme Court frequently used the Due Process Clause-and to a lesser extent the Equal Protection Clause-to strike down economic legislation and social reforms on the ground that they were "unreasonable." In the most groundbreaking of these cases, Lochner v. New York (1905), the Court ruled that a state law setting a daily ten-hour limit on the working hours of bakers was an unreasonable interference with the freedom of workers and employers to enter into contracts.

After 1937, a reaction against the so-called Lochner era set in. The Supreme Court began to apply the rationality requirements of both the Equal Protection and Due Process Clauses so leniently as to render them virtually meaningless. "State legislatures are presumed to have acted within their constitutional power despite the fact that, in practice, their laws result in some inequality," the Court said in 1961, in a fairly typical formulation. "A statutory discrimination will not be set aside if any state of facts reasonably may be conceived to justify it."

But while this permissive attitude prevailed in the context of social and economic legislation, the Equal Protection Clause had not been rendered completely powerless. In 1938, a footnote in an otherwise unremarkable case called United States v. Carolene Products Co. laid the groundwork for much of the Supreme Court's later elaboration of the Equal Protection Clause. Justice Harlan Fiske Stone, writing for the Court, observed that a more searching equal protection review might be appropriate when "legislation appears on its face to be within a specific prohibition of the Constitution, such as those of the first ten amendments." Similarly, the Court might want to take a harder look at "statutes directed at particular religious . . . or national . . . or racial minorities." The theory behind this approach was that the Court might need to step in when the ordinary political process was not adequate to ensure justice-either because the legislation interfered with rights that were central to that process, or because it discriminated against "discrete and insular minorities" who were likely to be victims of prejudice and lacked sufficient power to protect their rights in the political arena.

Under the liberal Warren Court of the 1960s, the split approach outlined in Carolene Products solidified into a rigid "two-tier" system of evaluating equal protection claims. The Court reviewed ordinary, run-of-the mill challenges to social and economic legislation under its low-level "rationality" test. Those claims only had to pass the easy, reasonableness standard. But two kinds of claims warranted "strict scrutiny": claims that legislation infringed on a "fundamental right," such as the right to vote, the right to interstate travel, or the right to appeal in a criminal case; and claims that legislation had created a "suspect classification." A classification was "suspect" if it was based on a group's race, ethnicity, or religion-essentially the "discrete and insular minorities" of the Carolene Products footnote. Discrimination premised on these characteristics, the Court said, was so unlikely to be related to a legitimate state objective that it was in effect presumed to be the product of prejudice and hostility. In order to pass the strict scrutiny test, a legislative classification had to be "narrowly tailored" to achieve a "compelling state interest." This standard proved so difficult to meet that the strict scrutiny test was sometimes referred to as "strict in theory and fatal in fact."

After Warren E. Burger succeeded Earl Warren as chief justice in 1969, discontent with the two-tier standard surfaced both on and off the Court. Both Justices John Paul Stevens and Thurgood Marshall criticized the Court's equal protection jurisprudence, with Stevens declaring that there was "only one Equal Protection Clause," and Marshall advocating a "sliding scale" approach. "A principled reading of what this Court has done reveals that it has applied a spectrum of standards in reviewing discrimination allegedly violative of the Equal Protection Clause," Marshall wrote in a 1973 dissent. "This spectrum clearly comprehends variations in the degree of care with which the Court will scrutinize particular classifications, depending, I believe, on the constitutional and societal importance of the interest adversely affected and the recognized invidiousness of the basis upon which the particular classification is drawn."

While the Court failed to embrace either Stevens's or Marshall's views, it did begin to tinker with its two-tier approach so as to render it more flexible. In some cases, the Court applied the rationality test in a relatively rigorous fashion, occasionally using it to strike down legislation as unconstitutional. The justices also expanded the range of closely reviewed classifications beyond race and ethnicity to include illegitimacy and gender. For these last two categories, the Court devised an intermediate equal protection test, falling between rationality and strict scrutiny: legislation that discriminated against women or those of illegitimate birth had to be "substantially related" to achieving "an important governmental objective."

This intermediate standard has been developed primarily in cases of discrimination against women. But women do not fit neatly into the Carolene Products mold of "discrete and insular minorities": they are not discrete or insular, nor are they a minority. In order to justify giving them the benefit of heightened scrutiny, the Court has had to rely on another strand of the equal protection doctrine: the idea that people should not be subjected to discrimination on the basis of characteristics that are "immutable"-distinctions, like gender or race, over which they have no control-and that bear no relation to ability. Women, like racial minorities, have historically been subject to severe restrictions on such activities as voting, attending college, and working as lawyers-restrictions that were based on stereotype rather than on the actual capabilities of individual members of the group.

The four cases that follow are landmarks in the development of the Court's gender discrimination doctrine. As the Court grappled with the appropriate method of evaluating this category of equal protection claim, the justices first tried applying the rationality test in an unusually rigorous way (Reed v. Reed); then came to the brink of adopting gender as a full-fledged suspect classification (Frontiero v. Richardson); and finally settled on an intermediate standard that appeared to represent a workable compromise (Craig v. Boren). In the eyes of some Court watchers, the final case, United States v. Virginia (1996), appeared to raise the standard to the highest level. In any event, the decision has left some questions about the Court's future course in this area.