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, the supreme law of the land is the United States Constitution and, under the Constitution's Supremacy Clause, laws enacted by Congress and treaties to which the U.S. is a party. These form the basis for federal laws under the federal constitution in the United States, circumscribing the boundaries of the jurisdiction of federal law and the laws in the fifty U.S. states and in the territories.
Notably, a statute does not disappear automatically merely because it has been found unconstitutional; it must be deleted by a subsequent statute. 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 any court that does so will be reversed by the Supreme Court. 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.
Also, certain practices traditionally allowed under English common law were outlawed by the Constitution, such as bills of attainder and general search warrants.
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 British Commonwealth 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. 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.
Some adherents of originalism and strict constructionism such as Justice Antonin Scalia of the United States Supreme Court argue that American courts should never look for guidance to post-Revolution cases from legal systems outside of the United States, regardless of whether the reasoning is persuasive, with the sole exception of cases interpreting international treaties to which the United States is a signatory. This position follows inevitably from the philosophy of originalism, which posits not only that the Constitution is the ultimate source of judicial authority in the U.S., but that the only proper analysis of the document consists of discerning the document's original meaning at the time of its adoption. Therefore, discussion of British law that postdates the Constitution is irrelevant as it sheds no light on the original meaning of the Constitution. Others, such as Justices Anthony Kennedy and Stephen Breyer, disagree, and cite foreign law from time to time, where they believe it is informative, persuasive, useful or helpful. However, 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.
In the beginning, federal law traditionally focused on areas where there was a express grant of power to the federal government in the federal Constitution, like the military, money, foreign affairs (especially international treaties), tariffs, intellectual property (specifically patents and copyrights), and mail. Since the start of the 20th century, aggressive interpretations of the Commerce and Spending Clauses of the Constitution have enabled federal law to expand into areas like aviation, telecommunications, railroads, pharmaceuticals, antitrust, and trademarks. In some areas, like aviation and railroads, the federal government has developed a comprehensive scheme that preempts virtually all state law, while in others, like family law, a relatively small number of federal statutes (generally covering interstate and international situations) interacts with a much larger body of state law. In areas like antitrust and trademark, there are powerful laws at both the federal and state levels that coexist with each other.
The fifty American states are separate sovereigns with their own state constitutions and state governments. They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the federal Senate.
The law of most of the states is based on the common law of England; the notable exception is Louisiana, whose law is based upon the Napoleonic Code. The passage of time has led to state courts and legislatures expanding, overruling, or modifying the common law; as a result, the laws of any given state invariably differ from the laws of its sister states.
Many American states have codified some or all of their statutory law into legal codes. Codification was an idea borrowed from the civil law through the efforts of American lawyer David Dudley Field. New York's codes are known as "Laws." California and Texas simply call them "Codes." Other states use terms such as "Revised Statutes" or "Compiled Statutes" for their compilations. California, New York, and Texas have separate subject-specific codes, while all other states and the federal government use a single code divided into numbered titles.
In some states, codification is often treated as a mere restatement of the common law. Judges are free to liberally interpret the codes unless and until their interpretations are specifically overridden by the legislature. In other states, there is a tradition of strict adherence to the plain text of the codes.
The advantage of codification is that once the state legislature becomes accustomed to writing new laws as amendments to an existing code, the code will usually reflect democratic sentiment as to what the current law is (though the entire state of the law must always be ascertained by reviewing case law to determine how judges have interpreted a particular codified statute).
In contrast, in jurisdictions with uncodified statutes, like the United Kingdom, determining what the law is can be a more difficult process. One has to trace back to the earliest relevant Act of Parliament, and then identify all later Acts which amended the earlier Act, or which directly overrode it. For example, when the UK decided to create a Supreme Court of the United Kingdom, lawmakers had to identify every single Act referring to the House of Lords that was still good law, and then amend all of those laws to refer to the Supreme Court.
All states have somewhat similar laws in regard to "higher crimes" (or felonies), such as murder and rape, although penalties for these crimes may vary from state to state. Capital punishment is permitted in some states but not others. Three strikes laws in certain states impose harsh penalties on repeat offenders.
Some states distinguish between two levels: felonies and misdemeanors (minor crimes). Generally, most felony convictions result in lengthy prison sentences as well as subsequent probation, large fines, and orders to pay restitution directly to victims; while misdemeanors may lead to a year or less in jail and a substantial fine. To simplify the prosecution of traffic violations and other relatively minor crimes, some states have added a third level, infractions. These may result in fines and sometimes the loss of one's driver's license, but no jail time.
For public welfare offenses where the state is punishing merely risky (as opposed to injurious) behavior, there is significant diversity across the various states. For example, punishments for drunk driving varied greatly prior to 1990. State laws dealing with drug crimes still vary widely, with some states treating possession of small amounts of drugs as a misdemeanor offense or as a medical issue and others categorizing the same offense as a serious felony.
The law of criminal procedure in the United States consists of an massive overlay of federal constitutional case law interwoven with the federal and state statutes that actually provide the foundation for the creation and operation of law enforcement agencies and prison systems as well as the proceedings in criminal trials. Due to the perennial inability of legislatures in the U.S. to enact statutes that would actually force law enforcement officers to respect the constitutional rights of criminal suspects and convicts, the federal judiciary gradually developed the exclusionary rule as a method to enforce such rights. In turn, the exclusionary rule spawned a family of judge-made remedies for the abuse of law enforcement powers, of which the most famous is the Miranda warning. The writ of habeas corpus is often used by suspects and convicts to challenge their detention, while the Civil Rights Act of 1871 is used by suspects to recover tort damages for police brutality.
The law of civil procedure governs process in all judicial proceedings involving lawsuits between private parties. Traditional common law pleading was replaced by code pleading in most states by the turn of the 20th century, and was subsequently replaced again in nearly all states by modern notice pleading. Most states have adopted rules of civil procedure closely modeled after the Federal Rules of Civil Procedure (including rule numbers). New York and California are the notable exceptions, although certain key portions of their civil procedure laws have been modified by their legislatures to bring them closer to federal civil procedure.
American civil procedure has several notable features, including extensive pretrial discovery, heavy reliance on live testimony obtained at deposition or elicited in front of a jury, and aggressive pretrial "law and motion" practice designed to result in a pretrial disposition (that is, summary judgment) or a settlement.
Contract law covers obligations established by agreement (express or implied) between private parties. Generally, contract law in transactions involving the sale of goods has become highly standardized nationwide as a result of the widespread adoption of the Uniform Commercial Code. However, there is still significant diversity in the interpretation of other kinds of contracts, depending upon the extent to which a given state has codified its common law of contracts or adopted portions of the Restatement (Second) of Contracts.
Parties are permitted to agree to arbitrate disputes arising from their contracts. Under the Federal Arbitration Act (which has been interpreted to cover all contracts arising under federal or state law), arbitration clauses are generally enforceable unless the party resisting arbitration can show unconscionability or fraud or something else which undermines the entire contract.
Tort law covers the entire imaginable spectrum of wrongs which humans can inflict upon each other, and of course, partially overlaps with wrongs also punishable by criminal law. Although the American Law Institute has attempted to standardize tort law through the development of several versions of the Restatement of Torts, many states have chosen to adopt only certain sections of the Restatements and to reject others. Thus, because of its immense size and diversity, American tort law cannot be easily summarized.
For example, a few jurisdictions allow actions for negligent infliction of emotional distress even in the absence of physical injury to the plaintiff, but most do not. For any particular tort, states differ on the causes of action, types and scope of remedies, statutes of limitations, and the amount of specificity with which one must plead the cause. With practically any aspect of tort law, there is a "majority rule" adhered to by most states, and one or more "minority rules."
Notably, the most broadly influential innovation of 20th century American tort law was the rule of strict liability for defective products, which originated with judicial glosses on the law of warranty. In 1963, Roger J. Traynor of the Supreme Court of California threw away legal fictions based on warranties and imposed strict liability for defective products as a matter of public policy in the landmark case of Greenman v. Yuba Power Products.. The American Law Institute subsequently adopted a slightly different version of the Greenman rule in Section 402A of the Restatement (Second) of Torts, which was published in 1965 and was very influential throughout the United States. Outside the U.S., the rule was adopted by the European Economic Community in the Product Liability Directive of July 1985, by Australia in July 1992, and by Japan in June 1994.
By the 1990s, the avalanche of American cases resulting from Greenman and Section 402A had become so complicated that another restatement was needed, which occurred with the 1997 publication of the Restatement (Third) of Torts: Product Liability.
Apart from model codes, the American Law Institute has also created Restatements of the Law which are widely used by lawyers and judges to simplify the task of summarizing the current status of the common law. Instead of listing long, tedious citations of old cases that may not fit very well together (in order to invoke the long-established principles supposedly contained in those cases), or citing a treatise which may reflect the view of only one or two authors, they can simply cite a Restatement section (which is supposed to reflect the consensus of the American legal community) to refer to a particular common law principle.
States have delegated lawmaking powers to thousands of agencies, townships, counties, cities, and special districts. And all the state constitutions, statutes and regulations are subject to judicial interpretation like their federal counterparts.
Thus, at any given time, the average American citizen is subject to the rules and regulations of several dozen different agencies at the federal, state, and local levels, depending upon one's current location and behavior.
Many states in the southwest that were originally Mexican territory have inherited several unique features from the civil law that governed when they were part of Mexico. These states include Arizona, California, Nevada, New Mexico, and Texas. For example, these states all have a community property system for the property of married persons (Idaho, Washington, and Wisconsin have also adopted community property systems, but they did not inherit them from a previous civil law system that governed the state). Another example of civil law influence in these states can be seen in the California Civil Code, where the law of contracts is treated as part of the law of obligations (though the rules actually codified are clearly derived from the common law).
Many of the western states, including California, Colorado, New Mexico, and Wyoming use a system of allocating water rights known as the prior appropriation doctrine, which is derived from Spanish civil law. It should be noted that each state has modified the doctrine to suit its own internal conditions and needs.