The topic of same-sex marriage in the US provides a way to explore the complexity of the US legal system, along with some legal research resources.
As a federal system, the US has national and state laws covering marriage. Although marriage has traditionally been regulated by state law, in 1996 Congress passed Public Law 104-199, the so-called Defense of Marriage Act (DOMA). This federal law is codified in two sections of the U.S. Code: 1 U.S.C. § 7 and 28 U.S.C. § 1738C. It defines the term “marriage” as “a legal union between one man and one woman as husband and wife” and the term “spouse” to mean “a person of the opposite sex who is a husband or a wife.” The main effect of DOMA is to prevent same-sex married couples from collecting various federal benefits that are available to opposite-sex married couples.
Ordinarily, if two people are married in one US state, their marriage is considered valid in all other US states. DOMA provides, however, that no US state is required to recognize a same-sex marriage from another state (or another country). Currently, nine US states allow same-sex couples to marry. Nine other states have provisions in their state constitutions that bar same-sex marriages.
In many areas of law (particularly criminal and family law), it is possible for US states to have widely-varying rules about rights and obligations. If the US Constitution mandates specific rights, however, states must provide them.
On March 26-27, the United States Supreme Court heard oral arguments in two cases about same-sex marriage. The Supreme Court can invalidate state and federal laws if they are inconsistent with the Constitution (the “supreme law of the land”). Because the Constitution provides for “equal protection” of law, advocates for same-sex marriage argue that banning such marriages, or refusing to recognize them, violates the Constitution. The Supreme Court might issue a broad ruling accepting or rejecting this argument. The nine Supreme Court justices, who are divided ideologically, may also choose to rule more narrowly, and limit their ruling to the state whose action is being challenged (California).
When a case is heard at the US Supreme Court, non-parties may submit amicus curiae (“friend of the court”) briefs, in addition to the parties’ own briefs. All of these briefs are available online.
The US Supreme Court is often hostile to foreign-law arguments, but two of the amicus briefs make arguments based on foreign law. One group of scholars, opposing same-sex marriage, says banning such marriages is consistent with other countries’ laws. Other experts claim that refusing to recognize same-sex marriage would tarnish the US’s human rights standing, in contrast to many other nations that recognize marriage equality.
The Supreme Court is expected rule on the two cases sometime in June 2013.
 “Public Laws” are available free online from 1995.
 Usually, researchers want a codified version of federal laws that includes all laws currently in force, called the United States Code.
 One reliable source of information on state laws is the National Conference of State Legislatures (NCSL). See the NCSL’s webpage on same-sex marriage.
 The Cornell Legal Information Institute has a directory of state legal websites here.
 The cases are United States v. Windsor and Hollingsworth v. Perry.