The United States Supreme Court frequently bases federal constitutional doctrine on state law, (1) often doing so by counting states' laws in a variety of doctrinal contexts to determine the legislative consensus among the States. For instance, state counting is used to determine the "evolving standards of decency" that define the meaning of "cruel and unusual punishment" under the Eighth and Fourteenth Amendments, (2) to determine whether some method of conducting jury trials is consistent with the Sixth and Fourteenth Amendments, (3) and to decide whether a state practice is consistent with traditions of ordered liberty implied by substantive due process. (4) But across this doctrinal variety, state counting involves two common elements: judicial use of state law to inform the content of federal constitutional doctrine, and judicial evaluation of states' laws collectively rather than singly to determine a state "consensus." When counting states, the Court treats the States as one large decision-making body whose members reach a single consensus.
The oddity of treating the States as a single collective decision maker has not been lost on scholars. At least one has argued that such use of state law actually undermines the purposes of federalism, which she identifies as permitting states to express the diverse preferences of their respective residents. (5) This criticism of state counting is mistaken. To understand why and how state counting might be valuable, it is important first to draw a distinction between two different ways in which the Court could be counting states. First, the Court could be using the state legislatures' consensus as a source of national law. Alternatively, the Court could be using the state legislatures' consensus as a limit on national law. In the first case, the Court would count the States' laws to determine the States" consensus position on an issue and then enforce that position against outlier states. In the second case, the Court would determine the States' consensus to place an outside limit on the judiciary's enforcement of its own view of the constitutional norm. In effect, the second version of state counting uses the States' consensus as a sort of collective veto over judicial review, not as an independent source of federal constitutional norms.
This Essay argues that the Supreme Court counts states largely for the second purpose of limiting judicial power. Seen as a mechanism of judicial self-limitation, the Court's practice of counting states is not inconsistent with federalism but is rather a natural extension of the federal principles already in the Constitution. Moreover, understanding state counting as a mechanism of judicial self-limitation helps explain why the Court tends to be casual about the details of how it counts states. One might justly complain that state counting does little to protect the novel policymaking experiments of outlier states from judicial review, but such protection is probably impossible absent restrictions on judicial review so severe that they would permit "experiments" such as Jim Crow that few would want to accept.
I. WHY STATE COUNTING DOES NOT PROVIDE A SOURCE OF FEDERAL LAW
The assertion that the Court's state-counting decisions might best be viewed as using state "consensus" to provide limits on, rather than sources of, federal law needs some defense, because it is in tension with the Court's own account of how it uses state law in at least some doctrinal contexts. The Court's Eighth Amendment opinions, for instance, assert that state consensus actually supplies the content of the rule that the Court enforces against the States. (6) According to the Court, "The clearest and most reliable objective evidence of contemporary values is the legislation enacted by the country's legislatures." (7) Laws that depart from the "consensus" established by these legislatures are deemed to be so "unusual" that they violate the Eighth Amendment's ban on cruel and unusual punishment. …