The New York Times runs an interesting story about dueling amicus briefs submitted in the gay marriage case before the Supreme Court about the extent of gay marriage in other countries, especially constitutional and liberal democracies. The briefs disagree about the extent of gay marriage, in part because they use different measures for how to count the countries.
The question whether the Supreme Court ought to consider the laws of other countries in deciding whether to interpret the Constitution is one that has been actively debated in the law review literature. I have largely neglected this literature, since it seems so obvious what the correct originalist answers are. (But see this post by co-blogger John McGinnis, who does write in the area.) First, considering the present law of other countries is obviously not directly relevant to the original meaning of a Constitution written in late 18th century America. Second, it is theoretically possible that the meaning of the clauses might make what other countries do relevant (for example, a clause that asks whether a rule is practicable), but I have not really seen a persuasive argument that attributes any such meaning to one of our constitutional clauses.