Scientific evidence (law)

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This article is about the legal concept. For scientific evidence in pure science, see Scientific evidence.

In law, scientific evidence is evidence derived from scientific knowledge or techniques. Most forensic evidence, including genetic evidence, is scientific evidence.


Educating witness[edit]


The educating witness teaches the fact-finder (jury or, in a bench trial, judge) about the underlying scientific theory and instrument implementing theory. This witness is an expert witness, called to elicit opinions that a theory is valid and the instruments involved are reliable. The witness must be accredited as an expert witness, which may require academic qualifications or specific training.

  • Judicial Notice: may moot the need for this witness.
  • Qualifications: Relative experience based on complexity and subtlety of the subject-matter. This witness is on high plane of abstraction about the validity and reliability.
  • Validity of Theory: Most jurisdictions require the theory used by an expert witness to meet certain qualifications before being used in court. The two most common are the Daubert and Frye tests.

Frye test[edit]

The Frye test, coming from the case Frye v. United States (1923), said that admissible scientific evidence must be a result of a theory that had "general acceptance" in scientific community. This test results in uniform decisions regarding admissibility. In particular, the judges in Frye ruled that:

Just when a scientific principle or discovery crosses the line between experimental and demonstrable stages is difficult to define. Somewhere in this twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from a well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs.

This test has been criticized as misunderstanding the scientific process and being based on the assumption that a jury is unable to evaluate scientific testimony. The goals of the test were to avoid evidence from overly questionable or controversial scientific theories to be used; it was used to exclude lie-detector results employed by the defense in the original case.

Daubert test[edit]

The Daubert test arose out of the United States Supreme Court case Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993). It requires four things to be shown:

  1. That the theory is testable (has it been tested?)
  2. That the theory has been peer reviewed, (Peer reviewing usually reduces the chances of error in the theory)
  3. The reliability and error rate (100% reliability and zero error are not required, but the rates should be considered by the trial judge)
  4. The extent of general acceptance by the scientific community

The Federal Rules of Evidence use the Daubert Test. See FRE 702.

Reporting witness[edit]

Reporting witness: Called after teaching witness leaves stand. Usually the laboratory technician who personally conducted the test. Witness will describe both the test and the results. When describing test, will venture opinions that proper test procedures were used and that equipment was in good working order.

  • Whether witness is qualified to conduct test. Could qualify as expert by virtue of "skill" gained through "experience & training" (FRE 702)—usually experiential, on-the-job training.
  • Whether the witness received the correct object to be tested. (Chain of Custody satisfying FRE 104(b)).
  • Whether the instrument(s) involved were in proper working order.
  • Proof-test procedures were used. This split the courts, but under CL, most Jx require foundational proof that the witness used proper testing procedures on the occasion in question.
  • Statement of test result: witness says what the results were. (This provides an excellent place to place physical evidence.) Remember, validating scientific evidence raises a logical relevance issue, as does the authentication (e.g. with enlarged photo).

Interpreting witness[edit]

Interpreting (Evaluating) Witness: Sometimes not needed 1) when test result is self-explanatory or pass-fail, or 2) when there is a statutory presumption obviating the need (e.g. drunk driving statutes and a test showing raised blood alcohol levels). Otherwise, this witness needed to complete the foundation. Syllogistic in nature: 1) states the interpretive standard (Rule or Major Premise), applies the standard to the test result (minor premise) and derives a conclusion.

  • Qualifications: a hybrid with both academic and experiential qualifications.
  • Will base finding on the Reporting witness. [Experts may base opinion on 1) what personally observed, 2) facts that are the type of data customarily considered by practitioners of the specially and 3) hypothetically assumed facts.] Ideal if present when Reporter conducted test, but may be permitted in some jurisdictions.
  • Some jurisdictions won't accept opinion unless it is a "reasonable scientific opinion." Otherwise, need to consider if the witness can couch the opinion in terms of statistical probability.

For example, in the casebook case of People v. Collins, 438 P.2d 33 (Cal. 1968), an elderly lady was knocked down and robbed by a blond who escaped in yellow car with bearded black man. Defendants met that rough description but could not be conclusively identified. Prosecutor used a mathematics professor to discuss the probability that this couple could be the guilty party. Lower court overruled Defendants' objection. Court held that 1) there was no foundational establishment of the underlying probabilities and 2) the fact that the Defendants' fit a probability model was irrelevant because it doesn't prove they did it.

Further reading[edit]

  • Sheila Jasanoff, Science at the Bar: Law, Science, and Technology in America (Cambridge, Mass.: Harvard University Press, 1997).