The words of the Constitution do not change whether they are being applied to immigration or same-sex marriage, or whether the statute is from California, Massachusetts or Arizona. The 10th Amendment is often cited to support the constitutionality of Arizona's immigration law as a matter of "states' rights." That same 10th Amendment is cited to support the unconstitutionality of the Defense of Marriage Act, which prohibits federal recognition of Massachusetts' same-sex marriages. To agree with one outcome and not the other can be misconstrued as partisan. If the 10th Amendment is good for the goose, it must be good for the gander, although whether conservatives or liberals are ganders is a bit unclear.
But though it may seem that the neutral principles expressed in the 10th Amendment demand uniform results, this isn't quite true. The amendment states: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people." And therein lies the rub. What powers are delegated to the federal government by the Constitution? And which are not? And how do we decide?
The text of the Constitution is the obvious place to start. The Constitution provides that the federal government has powers of "naturalization" and regulating commerce with foreign nations (Article I, Section 8). It also prohibits states from entering treaties (Article I, Section 10). While immigration (as opposed to citizenship after immigration) is not specifically mentioned in the Constitution, there is a provision that comes close. Article I, Section 9 specifically limits congressional power: "The migration or importation of such persons as any of the states now existing shall think proper to admit, shall not be prohibited by the Congress prior to the year one thousand eight hundred and eight, but a tax or duty may be imposed on such importation, not exceeding ten dollars for each person."
The provision is, of course, rooted in the slave trade. But by limiting congressional power until a certain date — 1808 — the implication is clear that migration of persons was intended to be an ordinary federal, rather than state, power.
In addition to the text, the history of constitutional interpretation is another guidepost to who has what powers. The DOMA litigation is not the first time Massachusetts has relied on the 10th Amendment. Massachusetts passed a statute barring state vendors from doing business with Myanmar (previously Burma). The U.S. Supreme Court unanimously struck down the statute under the "supremacy clause," not even mentioning the 10th Amendment. It was sufficient that there were presidential and congressional powers to develop a comprehensive national and international strategy.
Marriage, of course, does not appear in the Constitution, so judicial interpretation is where we find guidance on this issue. The federal courts shy away from family law. The U.S. Supreme Court has declared only a handful of state marriage laws unconstitutional, the most famous example being the Virginia statute criminalizing interracial marriage. But when striking down congressional statutes, the high court has repeatedly touted marriage and family law as the unquestionable domain of state, rather than federal, power. No matter how contentious they may seem, divorces and child custody disputes rarely become federal cases.
Finally, there are also our common practices and understandings about the difference between immigration and marriage. If you have a passport, its navy blue cover bears the seal "United States of America," not, for example, Colorado or California. You need not present your passport when you cross the George Washington Bridge or the Hoover Dam. On the other hand, if you have a marriage certificate, it is embossed with the name of the state in which you obtained the license, rather than "the United States." If you have a divorce decree, it likewise bears the name of the state court in which you obtained the judgment.
By constitutional text, established interpretation and everyday practice, immigration is a federal matter and marriage is a state concern. When it comes to applying a neutral principle such as the 10th Amendment — powers not given to the federal government are reserved for the states or people — the very impartiality of the principle may yield inconsistent outcomes.
Ruthann Robson is a professor at City University of New York School of Law.