Evidence and Proof, Scientific

Forensic Experts

Although most forensic scientists successfully strive to be objective and unbiased, crime laboratories in the United States usually are part of police agencies, an arrangement that does little to foster complete independence. Officials and jurists have reported isolated but serious breaches of professional ethics and instances of incompetence at local, state, and national levels. In Europe and elsewhere, the practice of forensic medicine is more commonly located in universities whose faculty members prepare reports for courts.

Anglo-American law relies on the adversarial system to produce expert witnesses. Thus, parties are free to retain expert witnesses who will take an extreme position, and at least since the nineteenth century, British and American judges and legal commentators have complained about the testimony of scientific and medical experts.

United States Judicial Doctrine

Two leading cases in the United States have placed significant limits on scientific testimony. In United States v. Frye, 293 F. 1013 (D.C. Cir. 1923), a man charged with murder sought to introduce the testimony of a psychology professor who administered a systolic blood pressure test to conclude that the defendant was truthful when he denied committing the murder. The trial judge excluded the testimony on tenuous grounds. The court of appeals affirmed on the ground that even though there was published research supporting the procedure and even though the test's [Page 528]inventor was a qualified expert, other psychologists had yet to accept his claim that a speaker's blood pressure is diagnostic of honesty. The court insisted that “the thing from which [a scientific] deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs.”

In theory, Frye uses the extent of acceptance in a community of knowledgeable and demanding scientists to ensure that only valid scientific theories and techniques enter into evidence in the courtroom. Yet, Frye has proved surprisingly difficult to apply and easy to manipulate. For example, some courts reasoned that the standard applies only to tests for truthfulness, to relatively esoteric applications of science, to unusual or “novel” claims, to the “hard sciences,” or to very general principles or methodology rather than to the body of studies or results being applied to reach a conclusion in the particular case.

Especially since the early 1970s, Frye was subjected to critical analysis, limitation, modification, and finally, outright rejection. By 1990, a strong minority of jurisdictions had expressly adopted a “relevancy-plus” analysis that requires the court to inquire into the validity of the scientific technique. In general, judges applied either the pure general-acceptance standard or the newer relevancy-plus standard primarily in criminal cases, but in the 1980s and 1990s, courts began to use these standards and other evidentiary and procedural rules to counter “junk science” in toxic-tort cases.

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