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.
Putting that all to the side, the arguments in the brief in support of a constitutional right to same sex marriage seem problematic for another reason. While that brief discusses the decisions of constitutional courts, it also counts statutes that authorize same sex marriage. It seems then that the briefly treats the statute and constitutional decisions of other countries as largely equivalent.
But there are significant differences between the two, especially as to the ease of adopting and eliminating a provision. That another country adopts a statute, but not a constitutional provision, allowing same sex marriage might be an argument for the US to adopt that statute; it is less obvious why it is an argument for the US Constitution to be interpreted to require same sex marriage. In fact, the arguments suggests the opposite conclusion.
Of course, many people who favor looking at foreign law don’t view constitutional and statutory law the way I do. They believe that constitutional law can be updated by judges and therefore the differences between statutory and constitutional law are not as stark as the original Constitution establishes. But even under that view, there are differences and those differences seem relevant.