Dna as evidence in criminal trials

The more LR exceeds 1, the greater the probative value of the data in supporting hypothesis S see Lempert More complicated VNTR profile LRs do not use match windows and bins, but rather consider the extent of the matching at each allele and rely on a continuous representation of the frequency distribution of fragment lengths. With those models, a match that involves almost no separation in all the bands produces an LR that is greater than a match that involves separations at the edges of the match windows for all the bands.

Indeed, because these LRs dispense with the somewhat arbitrary dichotomy between matches and nonmatches, they have been termed "similarity likelihood ratios" Kaye b and advocated on the ground that they make better use of the DNA data—e. As with match probabilities, qualitative as well as overtly quantitative presentations can be devised see Evett , p , proposing "a verbal convention, which maps from ranges of the likelihood ratio to selected phrases," such as "strong evidence" or "weak evidence".

Although LRs are rarely introduced in criminal cases, 97 we believe that they are appropriate for explaining the significance of data and that existing statistical knowledge is sufficient to permit their computation. None of the LRs that have been devised for VNTRs can be dismissed as clearly unreasonable or based on principles not generally accepted in the statistical community. Therefore, legal doctrine suggests that LRs should be admissible unless they are so unintelligible that they provide no assistance to a jury or so misleading that they are unduly prejudicial.

As with frequencies and match probabilities, prejudice might exist because the proposed LRs do not account for laboratory error, and a jury might misconstrue even a modified version that did account for it as a statement of the odds in favor of S. As for the possible misinterpretation of LRs as the odds in favor of identity, that too is a question of jury ability and performance to which existing research supplies no clear answer.

The likelihood ratio is still one step removed from what a judge or jury truly seeks—an estimate of the probability that a suspect was the source of a crime sample, given the observed profile of the DNA extracted from samples. Recognizing that, a number of statisticians have argued that the LR should not be presented to the jury in its own right 98 but should be used to estimate the probability that a suspect is the source of a crime sample.

Thus, a few experts have testified on this posterior probability in court. As noted in Chapter 5 , the posterior odds considering the DNA data that the defendant is the source are the LR times the prior odds those formed on the basis of other information. That procedure for updating probabilities has a rich history in statistics and law. Known as Bayes's rule, it has been the subject of protracted discussion among legal scholars and statisticians see generally Allen et al.

One of the more substantial issues raised in the legal scholarship revolves around specifying the prior odds to be updated. For courtroom practice, three methods of presentation have been proposed or used: "expert-prior-odds," "jury-prior-odds," and "variable-prior-odds" Kaye In the expert-prior-odds implementation, a scientist implicitly or explicitly selects a prior probability, applies Bayes's rule, and informs the jury that the scientific evidence establishes a single probability for the event in question.

The prosecution relied on a Bayesian analysis of this type in State v Klindt, N. It is doubtful, however, that the Iowa courts appreciated the basis of the calculation. For years, courts in civil paternity cases that involved testing of antigens have routinely admitted testimony of posterior probabilities. However, the practice has met with much less favor in criminal cases where the experts failed to disclose that they had used an ad hoc prior probability of one-half.

Jury-prior-odds implementation requires a jury to formulate prior odds, to adjust them as prescribed by Bayes's rule, and to return a verdict of guilty if the posterior odds exceed some threshold that expresses the point at which the reasonable-doubt standard is satisfied. But that procedure raises serious questions about a jury's ability to translate beliefs into numbers see Tribe ; and Kaye, and about the desirability of quantifying the vague concept of reasonable doubt See Nesson , ; Shaviro ; and Tribe Finally, with the variable-prior-odds method, an expert neither uses his or her own prior odds nor demands that jurors formulate their prior odds for substitution into Bayes's rule.

Rather, the expert presents the jury with a table or graph showing how the posterior probability changes as a function of the prior probability. To make appropriate use of DNA technology in the courtroom, the trier of fact must give the DNA evidence appropriate weight.

However, unless the results and meaning of the DNA evidence are clearly communicated, the trier of fact may fail to grasp much of the technical merit of DNA profiling. No research has as yet tested the reactions of triers of fact to the detailed presentations of evidence on DNA profiling that are encountered in the courtroom. We do know that people can make frequent and systematic errors in tasks that require them to assess probabilities or to draw inferences using probabilistic information see, for example, Bar-Hillel ; Edwards and von Winterfeldt ; Kahneman et al.

Yet, despite this plethora of research into information processing in other contexts, we know very little about how laypersons respond to DNA evidence and how to minimize the risk that they will give the DNA evidence inappropriate weight. For example, research generally shows that subjects tend to revise their probability estimates in light of new information less than Bayes's theorem would predict reviewed by Beyth-Marom and Fischhoff , and some research with mock jurors given written descriptions of blood-group evidence and various types of accompanying expert testimony also suggests that jurors will undervalue match probabilities see Faigman and Baglioni However, the studies involve far higher match probabilities than the extreme probabilities associated with DNA evidence, which may evoke a different reaction see Kaye and Koehler Contextual features, such as the method of presenting a question, that are unrelated to a problem's formal structure may substantially influence probability judgments Reeves and Lockhart The small amount of research on reactions to probabilistic evidence suggests that methods of presentation may strongly affect reactions to DNA evidence.

Unexamined are the effects of testimony about extreme probabilities or laboratory error when DNA evidence is presented by expert witnesses who are subjected to cross-examination. To evaluate the reactions of laypersons to DNA evidence, research is needed in which the respondents are exposed to the methods of presenting DNA evidence typically used in trial settings.

Although scholars have suggested promising ways to present probabilistic assessments in the courtroom Finkelstein and Fairley ; suggesting that jurors be presented with a range of plausible prior probabilities and information about what the likelihood ratio for the trace evidence implies in light of these prior probabilities , almost no empirical evidence yet exists on the effects of such modes of presentation on decision makers.

Similarly, although some basic probability concepts can be taught to undergraduates in a half-hour with reasonable success Fong et al. If courts are to make informed decisions about the expert presentations that will be allowed or preferred, further research is needed into alternative methods of trial presentation.

This chapter has described some of the legal principles and procedures governing the admission and use of DNA evidence in the courtroom and how this evidence has been received over the last decade. In assimilating scientific developments, the legal system necessarily lags behind the scientific world. Before making use of evidence derived from scientific advances, courts must scrutinize the proposed testimony to determine its suitability for use at trial, and controversy within the scientific community often is regarded as grounds for the exclusion of the scientific evidence.

Although some controversies that have come to closure in the scientific literature continue to limit the presentation of DNA evidence in some jurisdictions, courts are making more use of the ongoing research into the population genetics of DNA profiles. We hope that our review of the research will contribute to this process. In this chapter, we have also discussed how our conclusions and recommendations for reducing the risk of laboratory error, for applying human population genetics to DNA profiles, and for handling uncertainties in estimates of profile frequencies and match probabilities might affect the application of the rules for the discovery and admission of evidence in court.

Many suggestions can be offered to make our recommendations most effective: for example, that every jurisdiction should make it possible for all defendants to have broad discovery and independent experts; that accreditation, proficiency-testing, and the opportunity for independent testing whenever feasible should be prerequisites to the admission of laboratory findings; that in resolving disputes over the adequacy or interpretation of DNA tests, the power of the court to appoint its own experts should be exercised more frequently; and that experts should not be barred from presenting any scientifically acceptable estimate of a random-match probability.

We have chosen, however, to make no formal recommendations on such matters of legal policy; the single recommendation in the chapter concerns scientific evidence—namely, the need for behavioral research that will assist legal decision makers in developing standards for communicating about DNA in the courtroom:. Recommendation 6. We trust that our efforts to explain the state of the forensic science and some of the social science findings that are pertinent to resolving these issues will contribute to better-informed judgments by courts and legislatures.

The following tables summarize the law in the United States on the admissibility of estimates of profile frequencies or random-match probabilities of DNA types. Table 6. In many of the more recent cases, both an interim-ceiling and product-rule estimates were presented.

The tables do not show whether an opinion holds or suggests that the product-rule estimate would have been inadmissible had the ceiling estimate not been included. Many other subtleties and issues that arise in these cases are not captured in this brief summary.

Unless otherwise indicated, our observations apply to all the technologies for DNA analysis described in this report. State v Bogan, P. State v Schwartz, N. See, e. Ohio even before amendment to Federal Rule of Criminal Procedure 16 a 1 E , a federal magistrate judge granted discovery of matching criteria, environmental insult studies, population data, and proficiency tests as "predicate materials" essential to the defense in a DNA-testing case ; State v Schwartz, N.

State v Feldman, A. A few statutes governing the admissibility of DNA tests include provisions for pretrial discovery of the state's report. Code Ann. United States v Iglesias, F. Rule 16 was amended in to require that the government disclose to a defendant a summary of the expert testimony that the prosecution intends to offer on direct examination and the bases therefore.

It is still unclear whether this provision will cause experts to provide more detailed written documentation than they previously furnished. Many states do not have a counterpart to this subdivision. Other jurisdictions make all discovery related to scientific tests discretionary, and still others explicitly provide for the discovery of oral reports of examinations or tests.

See Giannelli and Imwinkelried , vol. See generally McCormick In State v Carter, Neb. See also Swanson v State, Ark. Trial judges ordinarily are accorded great discretion in evaluating the qualifications of a proposed expert witness, and the decisions depend on the background of each witness.

In the federal courts, Federal Rule of Evidence authorizes a court to appoint an expert. Many jurisdictions have similar rules. Cecil and Willging , p In some cases, defendants have sought court-appointed experts to review the work of the state's experts. Other opinions refer to independent experts without indicating the manner of their appointment.

Statutes applicable to criminal cases include 11 Del. The Tennessee statute, for example, provides that "in any civil or criminal trial, hearing or proceeding, the results of DNA analysis Some statutes explicitly identify a type of DNA analysis, e. For an unusual exception, see Kelly v State, S. Although the vast bulk of the cases finding general acceptance have come in the context of VNTR profiling, similar principles and methods of molecular biology underlie the detection of coding DNA polymorphisms, STRs, minisatellite repeat mapping, and the like.

When a court takes judicial notice, it accepts a matter as true without requiring that it be proved. Judicial notice is reserved for matters of common knowledge or those that are capable of "accurate and ready determination by resort to sources whose accuracy cannot be questioned" Fed. The NRC report suggested that the following "underpinnings" would be subject to judicial notice p : "The study of DNA polymorphisms can, in principle, provide a reliable method for comparing samples; each person's DNA is unique with the exception of identical twins , although the actual discriminatory power of any particular DNA test will depend on the sites of DNA variation examined; [and] the current laboratory procedure for detecting DNA variation specifically, single-locus probes analyzed on Southern blots without evidence of band shifting is fundamentally sound, although the validity of any particular implementation of the basic procedure will depend on proper characterization of the reproducibility of the system e.

State v Hammond, Conn. Although Wisconsin judges do not evaluate the reliability of scientific evidence, they may restrict the admissibility of such evidence through their limited gatekeeping functions. For a survey of the reactions of state courts to Daubert, see Meaney With many, if not most, types of scientific evidence, admissibility does not seem to turn on the choice of the label. For example, by and large, polygraph evidence is inadmissible in both general-acceptance and sound-methodology jurisdictions.

With DNA identification, however, a different pattern might be emerging. Over the last several years, appellate courts in Frye jurisdictions have seemed more prone than appellate courts in other jurisdictions to regard the admission of single-locus VNTR tests as error.

See State v Anderson, N. Blake et al. This application of an early form of the DQA test appears to be the first instance of forensic DNA testing of any kind in this country. Caldwell v State, S. Commonwealth v Curnin, N. Other courts have also refused to admit some forms of DNA evidence. Perry v State, So. Ohio applying general-acceptance standard , aff'd sub nom.

United States v Bonds, 12 F. State v Pierce, N. For reviews of the case law, see, e. It remains possible that some unreported cases have reached a contrary result.

United States v Jakobetz, F. Ohio , aff'd sub nom, United States v Bonds, 12 F. Those arguments have no application to PCR-based methods that use discrete markers. From a statistical standpoint, the window is best understood in terms of the "standard error" of measurement—a quantity that indicates the variability in repeated measurements of DNA fragments of the same size—and can differ from one laboratory to another. As explained in Chapter 5 , match windows must be wider than the normal variability to permit a declaration that two fragments match in most of the cases when they are actually the same length.

But the window should not be so wide as typically to produce declarations of matches between fragments that are not about the same size see Chapter 5. The result is a wide range of possible match windows. Roeder , p "the 'objective' match criterion In these circumstances, it has been suggested that expert testimony that narrower windows would have excluded a defendant is tautological and more prejudicial than probative Kaye , A brief argument to the contrary is made in Thompson People v Venegas, 36 Cal.

See Blake et al. As of November 10, , 34 cases in which PCR-based DNA testing had been conducted could be retrieved from the Westlaw "allcases" database of court opinions. A survey with responses from 49 forensic laboratories, conducted in November , revealed over cases where PCR-based typing results were introduced in courts in 37 states Perkin Elmer Corp.

K, WL Minn. Consequently, judicial opinions on the admissibility of PCR-based evidence illuminate the procedures that judges use in determining the validity of a new DNA technology, in addition to elucidating particular legal issues generated by the PCR method of DNA typing.

Serritt v State, So. State v Carter, N. Nov 8, "means to exclude possible defendants rather than identify" ; State v Penton, No. But see People v Morales, N. State v Russell, Wash.

The NRC report expressed reservations about PCR-based testing for forensic use p 70 , the dangers of contamination p , differential amplification pp.

See State v Gentry, P. But see State v Russell, P. In theory, a court could find a particular PCR-based test performed in such a substandard way as to justify exclusion of the evidence. State v Moore, P. State v Streich, A. We cannot find any recent decision under any standard of admissibility which refuses to admit the DNA match result based on the invalidity or risk of error of the underlying technology.

Courts also could refer to regular participation in accreditation programs, proficiency-testing and independent audits when instructing the jury, allowing jurors to draw a negative inference from the absence of these quality control mechanisms. Getting a Lawyer for your Criminal Case. Steps in a Criminal Defense Case.

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The more evidence available, the more likely there will be a conviction. Wrongful conviction is also reduced. DNA testing and analysis started out with RFLP restriction fragment length polymorphism , which had the ability to analyze large biological samples such as the the size of a quarter. With technological advancements it is now possible to analyze fragments of evidence not visible to the naked eye and very degraded evidence.

The new techniques used in DNA forensics are the PCR polymerase chain reaction analysis, which can analyze extremely small pieces of evidence or highly degraded evidence; the STR short tandem repeat , which analyzes special regions on nuclear DNA codified by the the FBI as the loci DNA and being able to identify a sample as high as one in one billion.

There are numerous examples of criminals whose DNA matched a profile from an earlier crime and who were then charged with the crime; likewise, there are examples of individuals whose innocence was confirmed when DNA found at a crime scene turned out to belong to another person identified through the profiles.

DNA Evidence In Criminal Trials as an Exoneration Tool Not only can DNA be used to convict criminals, it has successfully been used to exonerate individuals, some of whom were wrongly imprisoned for more than two decades. Often, the person who is wrongly convicted of a serious crime such as murder or rape has a criminal record for petty crimes, which means a record already exists.

These individuals are frequently convicted on eyewitness testimony, but without any physical evidence tying them to the crime. You have the right to remain silent and refuse to answer questions. You have the right to consult an attorney before speaking to the police and to have an attorney present during questioning now or in the future.

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