Justice Ginsburg recently gave a speech in which she offered a qualified defense of the practice of consulting foreign law. The speech has already received exceedingly intense criticism. In fact Supreme Court references to foreign law have produced some of the most passionate criticisms of the Court in recent years. This is itself a bit of a mystery. (Maybe some people think, wrongly, that the cautious practice that Justice Ginsburg defends will mean that the United States will "lose its sovereignty"?)
In any case Justice Ginsburg's remarks may well be on track. As she emphasizes, the decisions of foreign tribunals may well provide relevant information. As Eric Posner and I have discussed (in a paper available on the website of the AEI-Brookings Joint Center on Regulatory Policy, and forthcoming in the Stanford Law Review), it is possible to formalize this simple intuition by reference to the Condorcet Jury Theorem. In brief, the Jury Theorem says that if most members of a group are more than 50% likely to be right, the chance that a majority of the group will be right approaches 100% as the size of the group expands. (The Jury Theorem provides a hint about why and when large groups can be "wise.") If thirty state courts have chosen to do X, and if no state court has decided not to do X, it might well make sense for the thirty-first state court to do X too. So too at the international level: If fifty national courts have chosen to do X, there may well be reason for the United States Supreme Court to do X as well.
The beauty of the Jury Theorem is that it helps explain why it might also be a mistake to consult foreign law. If most nations are likely to err, then there's no reason to pay attention to them. If other nations are relevantly different, they're best ignored. (Some theories of constitutional interpretation, such as originalism, suggest that they're relevantly different most of the time.) And if other nations are just following one another, they're not making independent judgments, so the Jury Theorem doesn't apply.
The relevant considerations can be turned, without much difficulty, into a framework for analyzing when state courts should pay attention to the courts of other states, and so too for national supreme courts. (Eric Posner and I try to do that.) It wouldn't be senseless to conclude that state courts should generally consult other state courts (even on the meaning of state constitutions), and also that the United States Supreme Court should generally refrain from examining other nations' judgments (at least if the question is the meaning of our national constitution). But this is not an obvious conclusion, and it is at least illuminating to learn (as we try to show) that some national courts seem to follow an implicit Condorcetian logic in their own practices. The very fact that it is pretty standard to consult the practices of other nations suggests a Condorcetian reason for doing exactly that.
Of course there are many complexities here. But the underlying question is not a simple one, and if it is properly analyzed, it is hard to justify the intensity of the opposition to Justice Ginsburg's cautious position.