Foreign law is routinely used in American courts, but in everyday cases applying existing American legal rules related to family law, contract law, tort law, evidence law, and the like. Those American legal rules (such as "choice of law" rules) often expressly call for the consideration of foreign law. Let me offer a few such examples, starting in this post with family law.
Consider, for instance, Ghassemi v. Ghassemi, a 2008 Louisiana case. The Ghassemis were first cousins born in Iran. They married in Iran in 1976 and had a son born in Iran in 1977. The husband then came to the United States to study, while the wife and son stayed behind. The husband remained in America, and in 1995 arranged for the son to join him in America. The wife also eventually came to America, and in 2006 she petitioned for divorce from the husband.
To rule on the divorce petition, Louisiana courts first had to determine whether the Ghassemis were validly married. It turns out that Louisiana, like about half the other states, bars marriage between first cousins. If the Ghassemis had tried to get married in Louisiana, their marriage would not have been valid.
But the Ghassemis had married in Iran, not in Louisiana. And in such a situation, Louisiana law expressly provides that Louisiana must look to the law of the place where the marriage was entered into: "A marriage that is valid in the state [defined to include foreign countries] where contracted ... shall be treated as a valid marriage unless to do so would violate a strong public policy of [Louisiana] ...." This is a common legal rule in American states, stemming from longstanding principles favoring the validity of marriages:
Based on the universally espoused policy of favoring the validity of marriages if there is any reasonable basis for doing so (favor matrimonii), this Article authorizes the validation of marriages that are valid either in the state where contracted or in the state where the spouses were first domiciled as husband and wife.... This ancient policy of favor matrimonii and favor validatis is well entrenched in the substantive law of every state of the United States.
The Louisiana court therefore looked at Iranian law to determine if the Ghassemis were validly married. The court concluded that first cousin marriages were valid under Iranian law, and therefore the Ghassemis were validly married for purposes of Louisiana law. The court did not refuse to follow Louisiana law by applying Iranian law. Rather, it applied Iranian law because Louisiana law so required.
Moreover, the Louisiana court also looked to European law, Mexican law, and Canadian law:
Furthermore, we note that marriages between first cousins are widely permitted within the western world. "Such marriages were not forbidden at common law." Additionally, no European country prohibits marriages between first cousins. Marriages between first cousins are also legal in Mexico and Canada, in addition to many other countries.
Actually, the U.S. is unique among western countries in restricting first cousin marriages. Even so, such marriages may be legally contracted in [nineteen states] .... An additional six states ... also allow first cousin marriages subject to certain restrictions.
Yet this too is just the application of Louisiana law, rather than the replacement of Louisiana law with foreign law. Recall that the Louisiana statute provides that foreign marriages are recognized "unless to do so would violate a strong public policy of the state." A "strong public policy" must be something more than just a policy against entering into a certain kind of marriage in Louisiana - the whole point of the statute, after all, is to validate some marriages that could not have been validly entered into in Louisiana.
Rather, a "strong public policy" must be something stronger. In the words of the Ghassemi court, the question is whether, "although Louisiana law expressly prohibits the marriages of first cousins, such marriages are ... so 'odious' as to violate a strong public policy of this state." Looking at how such marriages are treated by sister states within the United States and by other countries within our Western culture, can help tell us how "odious" our culture generally sees such marriages as being. And the uniform recognition of such marriages in Europe and among our North American neighbors, coupled with the recognition of such marriages in half the states, suggests that the marriages are indeed not contrary to a "strong public policy" in Louisiana.
But what about polygamous marriages? What about same-sex marriages? Well, that's where the "strong public policy" exception might come in. American courts that have considered the question have indeed concluded that polygamous marriages are contrary to American public policy. (Note that one basis for so deciding is that polygamous marriages are not recognized in the United States or in most European countries.) And Louisiana law expressly provides that same-sex marriages are against Louisiana public policy and thus invalid, even if entered into elsewhere. So if a foreign marriage is seen as sufficiently improper by state courts or the state legislature, it need not be recognized; but many other foreign marriages are recognized by states even if they could not be lawfully entered into in those states.
(Of course, if the Supreme Court eventually concludes that the Constitution mandates equal recognition of same-sex marriages, then this provision of the Louisiana Civil Code would be invalidated, and Louisiana would have to recognize foreign same-sex marriages. But Louisiana would then also have to allow same-sex marriages within Louisiana itself; it wouldn't be foreign law that requires recognition of out-of-state same-sex marriages, but federal constitutional law that requires recognition of all same-sex marriages.)
And the Louisiana statute makes sense. I think it makes sense as to the definitions of which marriages are recognized: Whatever one might think of the merits of first cousin marriages, if those cousins have married - whether in Iran or Tennessee - it seems wrong to render them unmarried when they come to Louisiana, whether to live or to visit.
But beyond this, surely the determination of what is needed to be married must turn on the law of the place of marriage and not on the law of Louisiana. Louisiana law, for instance, requires a Louisiana marriage license, plus a 72-hour waiting period. Unsurprisingly, people who marry in Iran or Canada or England won't comply with Louisiana formalities. Why would they even have thought to comply with them?
Then, when they come to Louisiana - perhaps many years later - and there is a question of whether they are married for Louisiana law purposes, Louisiana courts shouldn't just say, "You didn't get a Louisiana license and wait 72 hours when you thought you got married, so therefore you aren't married." Louisiana courts instead need to inquire into whether the couple was properly married under the laws of the place where they married, just as the Louisiana statute provides.