Both federal and state courts have their own evidenciary laws. While they are for the most part similar, there are some important differences. There are three primary sources for the laws of evidence. Like most laws, evidenciary laws can originate via common law, Constitutional rights, or state or federal statutes. Many of the rules derived from common law date back to ancient English law.
Common law refers to laws and legal traditions that have existed and been followed for so long that their legitimacy is eventually assumed and given the stamp of legal legitimacy. Many evidence laws are derived from common law. For example, the privilege that a wife may not be forced to testify against her husband dates back to ancient times and comes into American law via English common law. Even before this rule was written down in a statute, it had been followed by the courts for so long that it was seen as binding.
Several rules of evidence derive directly from the United States Constitution. The Fifth Amendment, for example, protects a defendant from being compelled to testify against himself in a criminal trial. Additionally, the Fourth Amendment protects against illegal searches and seizures. Because of the Fourth Amendment, evidence obtained without a warrant or probable cause cannot be used in court. Additionally, the Due Process Clause of the Fifth and Fourteenth Amendments has been used by the Supreme Court to justify additional evidenciary laws. For example, when a suspect is arrested, they must be read their Miranda rights. If these rights are not read to them, anything they tell police after being arrested cannot be used as evidence against them.
The legislatures of the states and the United States Congress have passed numerous laws regarding the use of evidence in trials. Some of these laws have simply codified pre-existing common law principles and standardized them. Others have created entirely new rules. Federally, the Federal Rules of Evidence govern most trials in the federal courts of the United States. This code of laws contains extensive rules as well as advisory notes to help guide their application. Evidence law in federal court is primarily guided by the Federal Rules of Evidence and supplemented by important additions from common law and the Constitution.