Evidence

Evidence is not relevant unless its authenticity can be demonstrated. A letter in which the defendant admits her guilt in a tax-fraud trial is inadmissible unless the prosecution can first show that the defendant actually wrote it. Blood-stained clothing is irrelevant without some connection to the issues of the trial, such as evidence that the clothing belonged to the accused murderer. The process of linking a piece of evidence to a case—of authenticating or identifying the evidence—is frequently referred to as laying a foundation. Under the Federal Rules of Evidence, a foundation is sufficient if a reasonable juror would find it more probably true than not true that the evidence is what the party offering it claims it to be.

The most basic way to lay an evidentiary foundation is to demonstrate that a witness has personal knowledge. For example, the witness may testify that he wrote the letter, or that he saw the plaintiff sign the contract, or that he found the bullet in the kitchen. When the evidence is an object, the witness must testify that the object introduced at the trial is in substantially the same condition as it was when it was witnessed.

Objects that are not readily identifiable often must be authenticated through chain-of-command testimony. In the case of a blood sample, a proper foundation would include testimony from each individual who handled the blood—from the nurse who drew the blood, to the lab technician who tested it, to the courier who delivered it to the courthouse for trial. Unless each individual can testify that the blood sample's condition remained substantially the same from the time it was drawn until the time it was offered as evidence (accounting for any loss in amount, due to testing), the court could sustain an objection from the other side. The sample then would be inadmissible for lack of authentication.

Under the Federal Rules of Evidence, some evidentiary items are self-authenticating and need no additional authentication before being admitted. Documents containing the official seal of a government unit within the United States, and certified copies of public records such as birth certificates, are self-authenticating, as are newspapers and congressional documents.