The Admissibility of Prosecution Evidence that the Defense Had the Opportunity to Retest the Physical Evidence that Was Analyzed by a Government Expert

39 Pages Posted: 19 Aug 2010

See all articles by Edward J. Imwinkelried

Edward J. Imwinkelried

University of California, Davis - School of Law

Rockne P. Harmon

Senior Deputy District Attorney, Alameda County, California (retired)

Date Written: August 9, 2010


The use of DNA evidence is widespread. In a recent years, more than two thirds of the prosecutors' offices in the United States resorted to DNA evidence either in plea negotiations or at trial. The basic principles underlying DNA testing are so well settled that many courts now judicially notice the validity of those principles. Today, rather than attacking the government expert's theory or technique, the defense is much more likely to challenge the manner in which the test was conducted. The defense typically calls an expert who criticizes the test protocol used by the prosecution expert.

In response, prosecutors have attempted to introduce testimony that although the defense had the opportunity to examine and retest the DNA sample, the defense did not do so. Defense counsel have objected to such testimony on various grounds, and there is a growing body of case law on this issue. The issue is likely to arise even more frequently in the future. Since the advancing DNA technology permits the testing of smaller and smaller sample, there is often remaining, uncontaminated sample for the defense to test. Moreover, in DNA cases legislatures and courts have expanded defense discovery rights, notably recognizing a right to inspect and retest. Unfortunately, to date most courts have been content with a conclusory analysis of the admissibility of evidence that a portion of the DNA sample was available for defense retesting. The courts rarely explain why the evidence is logically relevant to begin with, and most courts have summarily disposed of the defense objections to the introduction of the evidence.

The first part of this article addresses the question of whether the evidence possesses logical relevance. This part of the article initially considers three theories - the evidence impeaches a defense expert's credibility, the defense failure to retest is an admission by conduct by the accused, and the evidence is a rational comment on the state of the record - and finds each theory wanting in one or more respects. The article then proposes a theory only hinted at in the cases: The testimony shows that the government expert followed good scientific methodology, and the expert's realization that the sample would be available to the defense gave him or her a stronger incentive to follow correct test procedure.

Positing the last theory of logical relevance, the second part of the article turns to the constitutional objections advanced by the defense. This part of the article examines objections based on the privilege against self-incrimination as well as objections resting on the constitutionally mandated allocation of the burden of proof to the prosecution. The article concludes that while the other possible theories of logical relevance might be vulnerable to defense objection, the fourth theory passes constitutional muster.

Airing these issues will hopefully give the bench and bar a better understanding of scientific methodology. In reality, there are two reasons to trust a scientifically validated proposition. One is the quality of the original empirical testing of the hypothesis The other is the openness of the scientific tradition: facilitating the replication of the original testing to expose flaws. The opportunity for retesting is one of the greatest strengths of the scientific method. The controversy over the admissibility of testimony of the availability of DNA samples for defense retesting may help the legal community develop a deeper understanding of one of the fortes of the scientific process.

Suggested Citation

Imwinkelried, Edward J. and Harmon, Rockne P., The Admissibility of Prosecution Evidence that the Defense Had the Opportunity to Retest the Physical Evidence that Was Analyzed by a Government Expert (August 9, 2010). New England Journal on Criminal and Civil Confinement, Forthcoming; UC Davis Legal Studies Research Paper No. 222. Available at SSRN:

Edward J. Imwinkelried (Contact Author)

University of California, Davis - School of Law ( email )

Martin Luther King, Jr. Hall
Davis, CA CA 95616-5201
United States

Rockne P. Harmon

Senior Deputy District Attorney, Alameda County, California (retired)

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