Why should foreign law ever be applied in American courts?, some people ask. Why should foreigners get the benefit of this law when they come to America? There are many answers to this, but I thought I’d offer a helpful illustration.

The broad point is that, when American courts apply foreign law, they apply it because American law calls for the application of foreign law. It’s not that American courts are somehow using foreign law to trump American law; rather, they are trying to follow American law. But why would American law consider foreign law? Here’s one example.

Say that Mahmoud (age 30) and Wafa (age 16) come on a visit to California, and have sex here. Having sex with an under-18-year-old is generally a crime in California:

Unlawful sexual intercourse is an act of sexual intercourse accomplished with a person who is not the spouse of the perpetrator, if the person is [under the age of 18 years].

If Mahmoud raises the defense, “I’m from France and it’s OK to have sex with under-18-year-olds there,” his defense will go nowhere. Likewise if his defense is “I’m from Washington State and it’s OK to have sex with under-18-year-olds there.” Foreign law, and out-of-state law, is in that context irrelevant.

But say Mahmoud raises the defense, “I’m from Pakistan, and Wafa and I were married there.” Then under California law Mahmoud wouldn’t be guilty (since the law refers to sex “with a person who is not the spouse of the perpetrator”), assuming his marriage is valid. Convicting him would be a denial of his rights under California law.

Now as it happens, in California, under-18-year-olds may not marry without a court order (and, generally, parental consent). But California law only says that these items are required to marry, and by the normal rules of statutory construction this applies only in California. (It might or might not be seen as applying also to Californians who leave California briefly to marry elsewhere, but we set aside here; cf. McDonald v. McDonald (Cal. 1936).) California doesn’t purport to regulate marriages in Pakistan, or to retroactively require California court approval for people married in Pakistan to have the rights of married couples in California. One can imagine a California law so providing (e.g., barring sex in California even among married couples when one spouse is under 18, unless a California court approves the marriage), but there is no such law, and I’ve never heard of any such law in an American state.

So indeed foreigners (or even people from outside the state) coming to California might have, in a sense, rights that Californians in California don’t possess: The visitors may have sex with their under-18-year-old spouses without any court approval of the marriage. But that’s just what California law calls for.

How, though, can California courts tell whether Mahmoud and Wafa are actually married? California courts can’t ask for a California marriage certificate, since Mahmoud and Wafa were married outside the state (and indeed outside the country). Instead, following the well-established American rule for determining whether a marriage is valid — which is embodied in a California statute — they ask whether the marriage was “valid by the laws of the jurisdiction in which the marriage was contracted” (here, Pakistan). If it complied with all the relevant formalities and substantive preconditions under Pakistani law, then it is valid for California law purposes, and Mahmoud isn’t guilty of statutory rape. But if the marriage was invalid under Pakistani law, then Mahmoud isn’t married, and he probably is guilty. (I say “probably” because it might be that, if Mahmoud sincerely believed he was married, or especially if he reasonably believed he was married, he wouldn’t be criminally punishable, since he wouldn’t have had the requisite “mens rea,” or culpable mental state, as to one element of the crime: the “person who is not the spouse” element. But if the marriage is valid under Pakistani law, then all that is irrelevant.)

Of course, some marriages are invalid under California law even if they are valid under Pakistani law; polygamous marriages would probably qualify, as would, say, father-daughter marriages, or perhaps marriages with 8-year-olds. They would be rejected under an exception to the general rule that marriages are valid in California if they are valid in the place where they were entered into: the exception for marriages that are “contrary to public policy.” But generally the fact that California formalities weren’t complied with in the foreign marriage (which is inevitable), or even that the marriage would be substantively invalid in California but not in a way that would shock California courts — e.g., when the age of marriageability is 16 rather than 18 — doesn’t render the foreign marriage invalid for California purposes. (Of course, this leaves some interesting and difficult line-drawing questions, but that’s what courts are supposed to do in applying the “contrary to public policy” standard. In our legal system, founded on the common law, courts are often asked to make these sorts of policy judgments when a statute doesn’t preclude the judgments.)

So to determine whether Mahmoud is guilty of statutory rape for having sex with Wafa, we need to look to Pakistani law to determine if their marriage is valid, and if Mahmoud is married to Wafa under Pakistani law, he is probably not guilty of the crime. As a result, he is free to have sex with this 16-year-old in California even though Californians generally aren’t, at least in the absence of a court order authorizing the marriage. But, again, that’s not because Pakistani law somehow trumps California law. It’s because, in this particular scenario, this is the result that California law demands — and, I think, quite reasonably so.