Judicial invalidation of government action on the ground that it was arbitrary—and thereby supposedly violated the Equal Protection Clause—is one powerful tool of the judicial activist. What reason is there to construe the Equal Protection Clause to invite such judicial micromanagement, and what principles meaningfully constrain a judgment that an action was arbitrary?

In today’s decision in Engquist v. Oregon Department of Agriculture, Chief Justice Roberts’s majority opinion for six justices (himself, Scalia, Kennedy, Thomas, Breyer—most notably—and Alito) makes some progress in confining the scope of judicial invalidation of supposedly arbitrary government actions. The specific question before the Court was “whether a public employee can state a claim under the Equal Protection Clause by alleging that she was arbitrarily [or vindictively or maliciously] treated differently from other similarly situated employees, with no assertion that the different treatment was based on the employee’s membership in any particular class” (e.g., race, sex, national origin). (For some reason, such a claim has come to be said to be based on a “class-of-one” theory of equal protection.) The Chief Justice’s answer: no.

The Chief Justice’s opinion relies partly on the traditional distinction between the government’s acting as regulator and its acting as proprietor, including employer. But the real ground on which it distinguishes precedent is that cases permitting claims of arbitrariness involved “a clear standard against which departures, even for a single plaintiff, could be readily assessed.” By contrast, “some forms of state action”—including, but certainly not limited to, employment decisions—“by their nature involve discretionary decisionmaking based on a vast array of subjective, individualized assessments.… In such situations, allowing a challenge based on the arbitrary singling out of a particular person would undermine the very discretion that state officials are entrusted to exercise.” (Slip op. at 11.) Thus, although the Court’s holding is limited to the public-employment context, its rationale promises to apply more broadly—and to confine judicial invalidation of a government action on the basis of supposed arbitrariness to those situations in which the government has arbitrarily departed from a clear standard.