Background
In the United States, the law is derived from various sources. These sources are constitutional law, statutory law, treaties, administrative regulations, and the common law. At both the federal and state levels, the law of the United States was originally largely derived from the common law system of English law, which was in force at the time of the Revolutionary War. However, U.S. law has diverged greatly from its English ancestor both in terms of substance and procedure, and has incorporated a number of civil law innovations. Thus, most U.S. law consists primarily of state law, which can and does vary greatly from one state to the next.
Constitutionality
Where Congress enacts a statute that conflicts with the Constitution , the Supreme Court may find that law unconstitutional and declare it invalid. A statute does not disappear automatically merely because it has been found unconstitutional; a subsequent statute must delete it. Many federal and state statutes have remained on the books for decades after they were ruled to be unconstitutional. However, under the principle of stare decisis, no sensible lower court will enforce an unconstitutional statute, and the Supreme Court will reverse any court that does so. Conversely, any court that refuses to enforce a constitutional statute (where such constitutionality has been expressly established in prior cases) will risk reversal by the Supreme Court.
American Common Law
As common law courts, U.S. courts have inherited the principle of stare decisis. American judges, like common law judges elsewhere, not only apply the law, they also make the law, to the extent that their decisions in the cases before them become precedent for decisions in future cases.
The actual substance of English law was formally "received" into the United States in several ways. First, all U.S. states except Louisiana have enacted "reception statutes" which generally state that the common law of England (particularly judge-made law) is the law of the state to the extent that it is not repugnant to domestic law or indigenous conditions. Some reception statutes impose a specific cutoff date for reception, such as the date of a colony's founding, while others are deliberately vague. Thus, contemporary U.S. courts often cite pre-Revolution cases when discussing the evolution of an ancient judge-made common law principle into its modern form, such as the heightened duty of care traditionally imposed upon common carriers.
Second, a small number of important British statutes in effect at the time of the Revolution have been independently reenacted by U.S. states. Two examples that many lawyers will recognize are the Statute of Frauds (still widely known in the U.S. by that name) and the Statute of 13 Elizabeth (the ancestor of the Uniform Fraudulent Transfers Act). Such English statutes are still regularly cited in contemporary American cases interpreting their modern American descendants.
However, it is important to understand that despite the presence of reception statutes, much of contemporary American common law has diverged significantly from English common law. The reason is that although the courts of the various Commonwealth nations are often influenced by each other's rulings, American courts rarely follow post-Revolution Commonwealth rulings unless there is no American ruling on point, the facts and law at issue are nearly identical, and the reasoning is strongly persuasive.
Early on, American courts, even after the Revolution, often did cite contemporary English cases. This was because appellate decisions from many American courts were not regularly reported until the mid-19th century; lawyers and judges, as creatures of habit, used English legal materials to fill the gap. But citations to English decisions gradually disappeared during the 19th century as American courts developed their own principles to resolve the legal problems of the American people. The number of published volumes of American reports soared from eighteen in 1810 to over 8,000 by 1910. By 1879, one of the delegates to the California constitutional convention was already complaining: "Now, when we require them to state the reasons for a decision, we do not mean they shall write a hundred pages of detail. We [do] not mean that they shall include the small cases, and impose on the country all this fine judicial literature, for the Lord knows we have got enough of that already. "
Today, in the words of Stanford law professor Lawrence Friedman: American cases rarely cite foreign materials. Courts occasionally cite a British classic or two, a famous old case, or a nod to Blackstone; but current British law almost never gets any mention. Foreign law has never been cited as binding precedent, but merely as a reflection of the shared values of Anglo-American civilization or even Western civilization in general.