The U.S. Supreme Court somewhat controversially cited foreign (non-British) case law in Texas v. Lawrence (2003, a gay rights case) and Roper v. Simmons (2005). This led to a backlash and the Court has studiously avoided citations to foreign and international courts in more recent decisions. This hasn’t stopped amici from filing briefs both pro and con gay marriage rights based on foreign and international law (the pro brief cites a paper of mine with Larry Helfer).
There are two ways in which international and comparative examples may matter. First, even if the Court is unlikely to cite foreign or international case law some judges, most importantly Justice Kennedy, are known to value the comparative perspective. There was an easy opportunity for the con side here. Justice Scalia famously noted in his dissent in Roper that:
To invoke alien law when it agrees with ones own thinking, and ignore it otherwise, is not reasoned decision making, but sophistry.
In other words, he accuses liberals of resorting to foreign law only when it confirms their predispositions. There are judicial opinions, most notably in the European Court of Human Rights, which find that international human rights law does not require legalization of gay marriage. The pro brief does not deny this but highlights that nowhere gay marriage has been been taken away after it had been legalized. Moreover, there is a distinct trend towards recognition of equality and gay marriage. The brief also appeals to the notion that the Justices may be concerned about the waning international influence of the Court. Opinio Juris has more on the two briefs.
The second, and perhaps more important, way in which international examples matter is in the evidence they provide for the beneficial or deleterious effects of gay marriage. In the oral arguments, Justice Kennedy mentioned the “sociological evidence” several times (here is a brief by the American Sociological Association). Here is what Justice Alito had to say:
I think it was first adopted in The Netherlands in 2000. So there isn’t a lot of data about its effect. And it may turn out to be a — a good thing; it may turn out not to be a good thing, as the supporters of Proposition 8 apparently believe. But you want us to step in and render a decision based on an assessment of the effects of this institution which is newer than cell phones or the Internet?
Clearly, Alito is not yet persuaded by the social science evidence but suggests that he might be at some future point. Moreover, this suggests that the comparative social science evidence may be important. Would that be sufficient for a political scientist to claim that such research is “in the national interest?”