The Supreme Court set the federal standard for the admissibility of scientific evidence with the landmark case Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). In that case, the court determined that the trial judge has ultimate discretion on whether or not to admit expert evidence, but he or she should consider several illustrative factors for sound “scientific methodology” including:

1. Whether the theory or technique employed by the expert is generally accepted in the scientific community;
2. Whether it has been subjected to peer review and publication;
3. Whether it can be and has been tested;
4. Whether the known or potential rate of error is acceptable; and
5. Whether the research was conducted independent of the particular litigation or dependent on an intention to provide the proposed testimony.

However, evidence is typically allowed as long as it helps the trier of fact answer the legal issue in question. It’s expected that at times there will be “shaky but admissible” expert evidence. Daubert encourages this evidence to be challenged with vigorous cross-examination, the presentation of contrary evidence, and careful instructions on the burden of proof. This is particularly relevant with psychiatric testimony, where reasonable practitioners can and do disagree—despite practicing in accordance with the most update to methods in the field.

Some states, including California, still utilize the older Frye standard (Frye v. United States, 293 F. 1013 (D.C. Cir. 1923)). The Frye standard allows scientific evidence to be admitted if the method by which that evidence was obtained is generally accepted by experts in the particular field in which it belongs. With expert testimony, this often takes the form of standard evaluation techniques, validated psychological tests, and the use of expert guidelines from peer-reviewed publications or national organization practice guidelines.