The second highest hurdle for laws to clear in equal protection cases is for laws based on gender (sex). State governments have passed many laws that treat men and women differently, but unlike race-based laws, the courts have adopted an “intermediate strategy” to determine if these laws are constitutional. The logic underpinning this determination is that there are more legitimate reasons to treat people differently based on their gender than on their race, but that the long history of gender-based discrimination means that we must be initially skeptical about those reasons. The states thus have a higher hurdle to clear with gender-based laws than with other laws, but not so high a hurdle to clear as with race-based laws.
Under the intermediate scrutiny standard, the Supreme Court has upheld a state law that punished men but not women for sexual intercourse if the woman was younger than 18. It upheld a federal law requiring males to register for the draft but not women, while striking down another law that awarded widows a survivor’s benefit but not widowers. However, it also required Mississippi University for Women, a state-supported all-female institution, to admit men to its nursing program.
Finally, the lowest and easiest hurdle for laws to clear exists for laws treating people differently for reasons other than their race or gender. According to judicial doctrines, these need only have a “rational basis” for their existence and be linked to a “legitimate state purpose.” For example, the progressive income tax used by the federal government and 34 states treats people unequally: Wealthier taxpayers are assessed a higher tax rate than poorer taxpayers. Advocates of progressive taxes justify this nominally unequal treatment by pointing out there is a rational basis for such unequal treatment in that wealthier individuals have greater discretionary income than poor individuals, and can thus afford to pay more.
Raising revenue to fund its operations and activities is also clearly a legitimate state function. Progressive income tax rates are thus acceptable under the equal protection clause. However, were a state to impose a higher income tax rate on men than on women, or a tax on whites but not on blacks, it would be a clear violation of the clause.
In short, if a state passes a law that treats people differently because of their race or national origin, it must have a compelling reason for doing so; if it passes a law based on gender, it must have a very good reason for doing so. Finally, if it treats people differently for reasons other than race, national origin, or gender, it must merely have a reasonable (or good) reason for doing so.