Federal Rules of Evidence

The Federal Rules of Evidence are broad principles promulgated by the United States Supreme Court governing the admissibility of any evidence in a criminal or civil trial. As such, they are applicable in trials in federal courts, although most state courts have adopted them as well. From the standpoint of forensic evidence, which is gathered, examined, and interpreted by specialists who are often called on to testify as expert witnesses, the key rule is Rule 702, Testimony by Experts: "If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case." A considerable body of case law applies this general principle to the facts of particular cases and defines standards for the admissibility of forensics evidence as interpreted by expert witnesses.

Historically, the courts relied on the Frye test, formulated by the Court in Frye v. United States in 1923. The case involved the admissibility of lie detector tests, at that time called "systolic blood pressure deception tests." This form of scientific evidence was then in its infancy and, at least according to the defendant, lacked what the Supreme Court would later call "sufficient facts or data" and "reliable principles and methods." The judge based his decision to admit the evidence on its "general acceptance in the particular field" (Frye, 293 F. at 1014). This was a key development, for it shifted the focus from the conclusions, even hunches, of a particular expert to an expertise recognized by other practitioners and gained from shared specialized training and experience. The Frye standard, however, had two principal problems: It failed to distinguish science from a pseudoscience such as astrology or, in the view of some legal experts, forensic science, and it rendered the court a passive observer, bound to accept expert testimony if it reflected "general acceptance in the particular field."

Accordingly, a new, more rigorous standard for the application of Rule 702 evolved from a 1993 case, Daubert v. Merrell Dow Pharmaceuticals, Inc. Under the Daubert standard, a trial judge can no longer defer to "general acceptance in the particular field" but must serve as a kind of gatekeeper by holding what are commonly called Daubert hearings, or pretrial hearings on the validity of the science in question using a five-pronged test: (1) whether the theory or technique can be and has been tested, (2) whether the theory or technique has been subjected to peer review and publication, (3) the known or potential rate of error, (4) the existence and maintenance of standards controlling the technique's operation, and (5) whether the theory or technique enjoys general acceptance within a relevant scientific community.

The relatively recent Daubert standard raised complex legal issues. Technically, it requires the courts to reexamine the validity and reliability of such forensics tools as polygraph testing, DNA testing, fingerprinting, handwriting analysis, fiber comparison, and the identification of firearms, bite marks, tire marks, and blood spatter patterns. Many of these are staples of forensic testimony, and some, such as fingerprint comparison, have a venerable century-old history, so the courts are reluctant to exclude them. Yet many observers, especially defense attorneys, contend that the research base that supports them is often inadequate, and they are increasingly launching attacks. In United States v. Havvard (2000), for example, the defense filed a motion to exclude the government's expert fingerprint witness, arguing that "there is no reliable statistical foundation for fingerprint comparisons and no reliable measure of error rates in latent print identification, especially in the absence of a specific standard about the number of points of identity needed to support an opinion as to identification" (Havvard, 117 F. Supp. 2d at 850–51).

While the Court denied the defendant's motion, this type of challenge became more frequent after 1993 as defense attorneys become more sophisticated in their ability to assess and challenge scientific evidence. In the case of fingerprinting, for example, they are challenging the belief that a print match proves that the prints came from the same person. Defense attorneys note that fingerprint experts do not compare latent prints directly to known prints, but rather take points of comparison and then estimate the likelihood that the two came from the same person. More practically, defense attorneys challenge the reliability of forensics evidence (error rates), noting the many opportunities for error as evidence is found, bagged, labeled, transported, removed from storage, handled, examined, and re-stored.