Effective Use of Scientific Evidence: Lessons From the Simpson Trial

by: Edward G. Bernstine, Ph.D., Harold J. Bursztajn, M.D. and Joanne Wilkens, M.A.*

* Dr. Edward G. Bernstine, a geneticist and molecular biologist, provides consulting services to attorneys on the preparation of scientific matters. He and Dr. Bursztajn collaborate on projects involving the communication of scientific evidence to judges and juries. Dr. Harold J. Bursztajn, Associate Clinical Professor and Co-Director, Program in Psychiatry and the Law, Harvard Medical School at the Massachusetts Mental Health Center, practices both as a clinician and as a forensic psychiatrist consulting locally and nationally to attorneys and institutions. Joanne Wilkens, M.A., is a corporate communications executive and a writer.


Scientific thought and analysis pervade the practice of law. Attorneys must understand deep technical arguments, work with experts to communicate scientific matters to judge and jury, and advocate positions in closing arguments. The Simpson matter offers a worthy study of physical evidence as the product of human endeavor, its presentation to jury and the role of scientific evidence in jury decision making. The case demonstrates that effective use of expert testimony involves more than simply presenting an expert's credentials. The principles that emerge from a study of Ca. v. Simpson are of value to all attorneys who use scientific evidence, whether in civil or criminal proceedings. In this article, we formulate guidelines for the effective communication of technical material to judges and juries. In a subsequent paper, we will examine the role of emotion in a jury's ability to digest, integrate and use scientific evidence.

Science and the Law

In both civil and criminal cases, the justice system determines what happened and who was responsible for it. Scientific evidence often plays a key role in these determinations, a role likely to increase as scientists develop more advanced analytical methods. Table 1 presents some of the areas of scientific inquiry that regularly appear in legal practice.

Table 1

Areas of Science Commonly Used in Legal Practice.

Physical sciences Chemistry Biomedical sciences Behavioral sciences
Material sciences
Ballistics, tool marks
and impressions
Drug screening
Alcohol testing
Narcotics testing
Fires and explosions
Forensic psychiatry and other medical specialties
Genetics/molecular biology

In criminal law, behavioral sciences address the mens rea issues. In civil law, physical and behavioral science provide evidence concerning liability and negligence and assess harm borne by a plaintiff or a plaintiff's property. Behavioral science plays a major role in discrimination and harassment cases by determining whether the evidence shows the "reasonable person" criterion has been met and by applying a variety of psychological tests to determine the state of mind of the parties.

In criminal law, physical evidence collected from a crime scene frequently forms an important element of both prosecution and defense cases. In such cases, verdicts depend on a jury's understanding of scientific evidence. One of the newest and, thanks to the Simpson case, most familiar methods in use is forensic DNA testing, which is used in both civil and criminal matters. This breakthrough technology has already resulted in the successful prosecution of many cases of rape that might otherwise have been impossible to prove. In addition, DNA analysis proves the innocence of Rout one third of suspects who undergo testing. Exculpatory DNA evidence is conclusive - failure to match excludes a suspect completely.

Demands of DNA


DNA technology is useful in law only if attorneys and their expert witnesses impart the meaning of DNA evidence to a jury. To comprehend how forensic DNA testing actually works requires significant knowledge of biochemistry and genetics. Thus, in presenting a DNA case, an attorney must understand the risks of exposing a jury to confusing monologues on basic science, however interesting that science may be to specialists. In addition, procedures employed in DNA testing laboratories consist of a large number of steps each of which is impossible to understand without detailed explanation. Providing such explanations can confuse a jury and diminish the impact of the evidence. Fortunately, the results of DNA analysis are readily understood if properly explained. Effective lawyering therefore emphasizes DNA results.

Incriminating DNA evidence is circumstantial, but DNA results can nonetheless be strongly incriminating. A "match" between a defendant's DNA and DNA from a crime scene allows statements such as: "the DNA type found in specimens at this crime scene matches this defendant's type, and only one person in a billion randomly chosen people would be expected to have this type." This "random match probability" is the only form of positive conclusion supported by a match between a forensic sample and a "reference" sample derived from a known person. Thus, DNA evidence permits only a statement of probability. The results never prove that a particular DNA specimen was contributed by a specific person. Prosecutors must present such abstruse statistical reasoning to juries with extraordinary care in order to retain the impact of the evidence. Defense attorneys should hold the prosecution to statements bounded by the statistical limitations of the technology. specifically, defense counsel should prevent any suggestion that the probability calculated from a DNA result represents the odds that the forensic sample originated from a person other than the defendant.

In spite of its potential to provide powerful evidence, there is always a chance, even if it is one in a billion, that a match could falsely implicate the defendant. Therefore, the tolerance for additional uncertainty in a DNA case is extremely narrow: the DNA evidence itself must be beyond reproach, particularly if it is the only evidence. Procedures and policies used by police, criminalists and forensic scientists should be scrutinized by both prosecutors and defense attorneys to assure that the DNA results have been obtained under stringent conditions.

Experts Represent the Evidence

What it's really about is whether the jury trusts the messenger, and the messenger is everybody [in] that courtroom on the other side - the police, the prosecution, the people who are bringing you this evidence, even the scientists ....
- Gigi Gordon, Exp., Court TV, September 28,1995.

Expert witnesses who testify to DNA evidence should have unbiased views that are generally accepted by other respected members of their academic communities. Moreover, with the guidance of the examiner, they must be able to communicate scientific results and the meaning of these results to a jury. The essential clarity of a convincing DNA result - a very low probability of a random match to the DNA type found in the forensic sample - can be obscured by an attorney or expert who loses the jury in superfluous detail. Experts or attorneys who are unable to communicate to the jury their clear and deep understanding of conclusions drawn from DNA tests are liabilities.

Attorneys Represent the Evidence

. . . I'm going to try not to say things like substrate control and reagent blanks and all that stuff. I'd like to talk to you about it the way I talk about other things
- everything else in my life except for this science stuff.

- Marcia Clark, Ca. v. Simpson, September 28,1995.

Contrary to her intentions, Ms. Clark's statement, made in her closing argument, weakened the prosecution's position. The State had the burden of proof, and its most incriminating evidence was the product of scientific work. Yet Ms. Clark suggested that the strongest evidence against Mr. Simpson was not a normal part of life. A better approach would have been to make the jury comfortable with the science through professionally crafted, jargon-free explanations earlier in the trial. If the lead prosecutor is not at ease discussing the State's best evidence, why should the jurors take it seriously? Attorneys arguing cases based on scientific procedures will increase the credibility of their evidence by advocating the technology underlying it.

Human Sources of Error

There are three sources of investigational error: dishonesty, bias and improper procedures.

Investigators, whether police officers or research scientists, are entrusted with the honest reporting of work honestly conducted. Some do not meet the ethical standard expected of them. Dishonesty means the fabrication of data or the selection of actual data to support a desired result. Bias is the selective assignment of relevancy to data which support a preconceived idea. It often means subconsciously ignoring or overlooking data which do not support one's view. Bias is a reality of all human thought, and has many sources. It may arise for personal reasons, perhaps the desire to prove a pet hypothesis. Or it may reflect a wish to fulfill the expectations, implied or explicit, of a superior. Employees are sensitive to such expectations, and see benefits from bringing the "good news" to the boss. Sometimes the employee just knows the "right" answer without being told.

Society has dealt successfully with limiting the effects of dishonesty and bias in a variety of technical areas whose results affect the public. For example, in the pharmaceutical industry an effective approach has been to require that persons evaluating the effectiveness of a new drug do not know which test samples contain the drug. This is called a "blind" study, and it obviates bias because investigators do not know what the "expected" results are. Blind analysis of forensic samples would reduce the possibility of bias and dishonesty in police investigations and forensic samples would reduce the possibility of bias and dishonesty in police investigations and forensic testing. Stringent control of access to the evidence itself as well as to the results of tests performed on it are of obvious importance. Appropriate management and reporting structures for the various employees involved in the development of courtroom evidence are essential. Ideally, testing facilities would be independent of investigative agencies. All attorneys trying DNA cases should be fully aware of the strengths and weaknesses of the people and the processes upon which the quality of the evidence depends.

Decision Making by Juries

Effective presentation of a case depends upon understanding and taking full advantage of the decision-making process. How does a jury absorb and use complex scientific evidence to reach a verdict?

Four factors determine the jury's understanding and use of scientific evidence: clarity, timing, vividness and the emotional state of the jury. No amount of instruction can convert a lay jury to an expert scientific body, so why try? Jurors want the evidence to make a decision, and they want it in as short a time as possible.

Jurors often reach a provisional judgment based on what they hear first; they then reach a decision by modifying this position as further information is presented. If a case is based on strong scientific evidence, it is best to make the major points early.

Jurors decide based on information most vividly recalled. This may entail what was most recently heard or most impressive. A long, tedious presentation results in boredom, inattention and even disbelief.

Juries are traumatized by captivity, the onus of their responsibility and the sometimes frightening details of the matters they are to decide. Stress erodes the capacity to perform detailed intellectual tasks. Thus, attorneys presenting science should seek to reduce jury trauma by being brief, clear, confident and caring but dispassionate.

The Case Against Mr. Simpson

In Ca. v. Simpson, it was only physical evidence that placed Mr. Simpson at the crime scene and connected him to the double homicide of which he had been accused. The prosecution's case was founded largely on the analysis of DNA contained in blood samples collected from the crime scene, Mr. Simpson's vehicle and his residence. In her closing argument, Ms. Clark epitomized the People's case as follows: We have linked the defendant to the crime scene. We have linked the defendant to the victims. We have linked the defendant and the victims to his car. And that link has reached from Bundy into his bedroom at Rockingham. They are all interwoven by time, by space, by occurrence, by science, all linked.
- Marcia Clark, Ca. v. Simpson, September 26,1995.

These words suggested that science would be a firm ally of the prosecution and a threat to Mr. Simpson's defense. Ms. Clark effectively articulated the potential power of her case in her opening statement. The litany, "... matches the defendant . . . matches the defendant .. . matches the defendant. . . ." foreshadowed the results of DNA testing of blood evidence that the State would introduce. Table 2 summarizes the prosecution's physical evidence in the case. The table shows a potentially incriminating set of findings, much of it based In DNA technologies developed in the early 1980's and in forensic use in England since 1983 and the United States since 1987.

The Prosecution's Tactics

Given the circumstantial nature of their case, the positive public image of Mr. Simpson and the gravity of a first-degree murder charge, the prosecution had to convince the jury that its strongest evidence, DNA testing, reliably implicated Mr. Simpson. The burden of proof could be met only by a decisive demonstration that the DNA evidence connected Mr. Simpson, the victims and crime scene as Ms. Clark proposed. Moreover, to be most effective in communicating with the jury, the DNA evidence had to be presented early, vividly and clearly.

The prosecution began its case in January, 1995 with evidence supporting its weakest argument, that two or three incidents of spousal abuse had escalated to murder. The first testimony regarding DNA heard by the jury was in April, and it came from the defense. During his cross-examination of criminalist Dennis Fung, Robert Blasier demonstrated that Mr. Fung had no understanding of or training in the collection and preservation of DNA evidence. Thus, the defense had shown weakness in the DNA capability of the Los Angeles Police Department (LAPD) before the prosecution had even begun to describe its findings.

Following opening statements, the jury heard nothing from the State about DNA until May. At that time, Dr. Robin Cotton of Cellmark Diagnostics, the company that conducted DNA testing of crime-scene samples, took the stand, Cellmark's results placed drops of Mr. Simpson's blood on the leftside of a trail of bloody shoeprints leaving the scene of the murders. The shoeprints had been made in Nicole Brown-Simpson's blood. Rather than beginning with a concise statement of these results, prosecutor George Clarke led Dr. Cotton into a two-and-a-half-day lecture on genetics, biochemistry, molecular biology, proficiency-testing at Cellmark and the minutiae of DNA analytical procedures. Contrary to its strategic needs, the prosecution had given the jury the most complicated view possible of forensic DNA testing and obscured the impact of its most incriminating physical evidence.

The prosecution's tedious presentation of technical argot and its failure to present the actual evidence early in Dr. Cotton's testimony was certain to bore the jury. Since a bored jury disregards or disbelieves what is boring it, the prosecution assumed the burden of boredom. Furthermore, it alienated a jury already suffering from lengthy sequestration and destroyed any possibility for a jury-prosecution alliance founded in science. Paradoxically, this left the field open for the defense to supply the scientific evidence expected by the jury from the prosecution.

The prosecution defended rather than asserted its case throughout the trial. Although they had many investigational errors to deal with, by describing each oversight in great detail, they effectively cross-examined themselves. The inevitable effect of this repetition was to numb the jury to the conclusions of the DNA analysis. To make matters worse, they repeatedly told the jury that the mistakes of their investigators and technical personnel had no effect on the reliability of the evidence. Understating the effect of so many errors on the integrity of the evidence was not credible. This further undermined the jury's confidence in the prosecution and its evidence. As defense attacks on the physical evidence escalated, the prosecution retreated from its most effective strategy - the clear and cogent presentation of DNA evidence - and turned to an emotional appeal to the jury based on charges of spousal abuse and the savage nature of the murders. In effect, they sought revenge, but in reality they distracted the jury from considering the evidence. The fact that neither attorney delivering the People's closing argument had expertise in DNA spoke for itself. Mr. Darden carefully developed the prosecution's theory of spousal abuse leading to murder. Ms. Clark reiterated the prosecution's rebuttals of defense attacks on the LAPD's handling of DNA and ended with a plea for conviction accompanied by the soundtrack of a compilation of Nicole Brown-Simpson's two famous 911 calls. Science had been lost to the opposition.

Table 2

Main elements of the prosecution's physical case

Prosecution Exhibits Evidence Defense Response
Blood drops at scene

5 drops to the left of trail of bloody footprints: DNA match to Simpson

Sample DNA degraded; Simpson's DNA due to contamination by LAPD

One drop tested matched Simpson by seology

1 drop on rear gate, DNA match to Simpson Planted by police
RG's shirt Hair matching Simpson Poor crime-scene techniques
Fiber matching sample on socks and glove found at Simpson's residence Poor crime-scene techniques
Fiber matching mat of Simpson's vehicle Poor crime-scene techniques
Knit cap at scene Human hair matching Simpson Poor crime-scene techniques
Blood in Simpson's vehicle RG's blood on console Planted by police
RG's blood on floormat in a bloody imprint consistent with the Bruno Magli shoe Planted by police
Simpson's blood near door handles Planted by police
Socks in Simpson's bedroom NBS's blood: DNA match Sock planted by police
Fiber matching sample found on RG's shirt Sock planted by police
Glove forming a pair with one at crime scene found in alleyway at Simpson's residence Hair of both victims Glove planted by police
Blood of both victims Glove planted by police
Fibers matching those found on RG's shirt Glove planted by police
African American limb hair Glove planted by police
Simpson's foyer and master bath Simpson's blood Irrelevant
Simpson's person Cut on middle finger of left hand Cut on glass in Chicago after murders occurred

The Defense's Tactics

To overcome the weight of the DNA findings, which implicated Mr. Simpson in his wife's murder, the defense had to undermine the findings' impact and question in their reliability. They proceeded to do so in a methodical fashion.

The defense undermined the impact of the prosecution's DNA findings by questioning the expertise of its key statistical witnesses, Dr. Bruce Weir. In addition, Dr. Weir made a poor impression on the jury. He was a passionate and vocal advocate of the increased use of forensic DNA testing. He also vehemently disagreed on important statistical issues with authors of the influential National Research Council report on Forensic DNA testing, an established courtroom reference.

During Dr. Weir's cross-examination, the defense pointed out a systematic error in his calculations, and he was obliged to revise all of his figures as the jury watched. While numerically small, Dr. Weir's error succeeded in raising questions in the minds of the jury about the ability of the prosecution to provide undisputed statistical results. Moreover, by pointing out a mistake in the prosecution's arithmetic, Peter Neufeld increased the defense's scientific credibility with the jury. Although statistical issues are centrally important to the forensic use of DNA testing, no discussion of them appeared in closing arguments. Their significance - a strong suggestion of Mr. Simpson's guilt - had been lost over long weeks of testimony. The image of the combative Dr. Weir and his errors, however, would remain long after the statistical dust had cleared.

The defense questioned the reliability of every article of DNA evidence by one of two means: 1) proposing a conspiracy by the LAPD to manufacture and plant evidence intended to incriminate Mr. Simpson; and 2) questioning the competence of the LAPD by claiming that investigators, criminalists and forensic scientists used inadequate procedures in the collection, handling, storage and analysis of DNA and other evidence. Their witnesses were confident, clear, vivid and factual. In addition, defense attorneys never attacked the science but revered it, and they urged the jury to do the same. "DNA is a sophisticated technology, it is a wonderful technology," Barry Scheck told the jury, "but there is a right way to dot it and a wrong way to do it." His implication was clear: the LAPD's way was the wrong way.

Defense attorneys proposed conspiracy by alleging that the LAPD had identified Mr. Simpson as the perpetrator soon after the crime. The police then not only failed to carry out a thorough investigation of the crime scene but also fabricated evidence to suggest his guilt. The testimony of Dr. Henry Lee, Chief Forensic Scientist for the Connecticut State Police, made these allegations credible.

Dr. Lee described a number of oversights by the LAPD in the recognition and collection of evidence. In a brief examination of the crime scene, he made striking discoveries of evidence overlooked by the LAPD: a bloody imprint on the envelope containing the glasses Ronald Goldman was delivering to Nicole Brown-Simpson and possible fingerprints on both the envelope and a lens of the glasses within. In general, Dr. Lee's testimony demonstrated a failure of the LAPD not only to recognize evidence but to manage the entire crime scene as well. He concluded that, as a result, an accurate reconstruction of what had transpired was impossible.

In addition, Dr. Lee addressed defense charges that the LAPD manufactured key evidence - placing the glove at Rockingham and planting blood on the socks and in Simpson's car. Evidence tampering was also suggested, the defense argued, by stains on the wrapper of certain blood samples sent to Cellmark from the LAPD. According to LAPD records, the samples in question had been dried overnight on separate solid surfaces, housed in separate tubes, and subsequently packaged together for shipment. Since it is physically impossible for wet transfer to occur between dry samples and their wrapper and since the time allowed for the samples to completely dry was adequate, one could infer that some fresh, wet samples had been placed among the previously dry ones. Such samples could have represented material planted after the other samples had dried. Dr. Lee said in response to cross-examination on this observation: "Only opinion [possible]... under this circumstance - something wrong."

Dr. Lee's obvious love for his work, his delightful demonstrations and positive attitude about science, and his concise, clear testimony communicated neutrality and professionalism to the jury. His testimony built further confidence in the defense's case.

Defense attorneys questioned the reliability of scientific evidence by their unrelenting exposé of the incompetence of the LAPD and its forensic laboratory. This exposé may have been the most important element of Simpson's defense, by depicting the LAPD as a "cesspool of contamination," defense counsel gave the jury reason to disbelieve any and all DNA evidence presented by the prosecution. Incorrect procedures or deficiencies highlighted by the defense at the LAPD's forensic DNA-testing facility are shown in Table 3.

Table 3

Defense's List of Deficiencies in LAPD Procedures

1. No procedural manuals at LAPD labs
2. No DNA training for evidence collectors
3. No proficiency testing
4. Lab neither inspected nor accredited
5. No competent supervision of DNA work
6. No records to establish chain of custody
7. No security for evidence
Lens missing from glasses in envelope
Bronco without security for two months
8. Failure to report errors in validation studies at LAPD
9. Unsatisfactory sample handling
No paper on table Table not washed down
10. Handling reference samples at same time and place as crime-scene samples
11. Trace evidence: moved bodies around (video, photos); blanket from house placed over bodies

Once it had established the incompetence of the LAPD, the defense moved to discredit the prosecution's DNA results. The original DNA in forensic samples found at the crime scene and in Mr. Simpson's car, they argued, had so completely degraded that they were unreadable. Mr. Simpson's DNA appeared in these samples, the defense suggested, due to contamination from his reference sample collected and processed by the LAPD (310 in Table 3). In effect, they implied that, due to deficient laboratory protocols, Mr. Simpson's blood had been inadvertently added to a series of DNA-negative forensic samples, and that error explained the prosecution's DNA-positive results.

This point of view was corroborated by defense expert, Dr. John Gerdes, who testified that any samples passing through the LAPD were suspect. When Barry Scheck asked him,"... is the LAPD, in terms of levels of contamination, worse than any other forensic laboratory you've ever seen?", Dr. Gerdes replied, "Definitely, by far." Since all the DNA samples in the case were processed at the LAPD before being sent to Cellmark or the California Department of Justice laboratories, the defense asked the jury in one stroke to discredit much of the prosecution's evidence. These allegations were, however, speculative, particularly regarding the specific details of whose blood was contaminating whose; nevertheless, the defense's clear, vivid and methodical exposure of the LAPD's inadequacies, the demeanor of its witnesses, and the prosecutions' defensive criticism of its own evidence all raised serious doubts about an otherwise incriminating circumstantial scientific case. As Barry Scheck noted in his closing argument: "You cannot render a verdict in this case of beyond a reasonable doubt on this kind of evidence, because if you do, no one's safe - no one. The Constitution means nothing." The Verdict The Simpson trial lasted one hundred fifty-nine days, and the jury deliberated for less than four hours to find Mr. Simpson not guilty of the charges. The speed with which they reached this verdict shocked America.

Many observers felt that the quick decision was a slap in the face to our system of justice, yet we must conclude that this is not so. Emotionally charged accusations of racism coupled with the public's lack of understanding of the science involved mask two key factors: the extent to which the jury could believe the prosecution's case was compromised and the effectiveness of the defense in communicating its case. Given the lackadaisical methods and procedures used to collect, store and analyze the evidence, the statistical uncertainty intrinsic to all DNA-based conclusions, the judge's instructions and the defensive, tedious presentation of the prosecution's scientific case, not guilty was a reasonable verdict. There were, however, other important factors that influenced the jury's consideration of scientific evidence in its decision-making process. In our next article, we will examine a new model of jury decision making under conditions of stress, Jury Trauma Syndrome, and will discuss the sources, uses and effects of emotion in the Simpson trial and in a general courtroom context.

Sources and suggestions for further reading:

  1. The primary sources for our work are trial transcripts available on the internet. These include a number of in camera meetings relating to juror dismissals, which provide a great many useful insights: www.islandnet.com/~ walraven/ simpson.html
  2. National Research Council, DNA Technology in Forensic Science, National Academy Press, Washington, 1992. A good source for those interested in an expert committee's view of the field. It includes both legal and scientific matter.
  3. Dershowitz, Alan M., Reasonable Doubts, Simon & Schuster, NY, 1996. Chapter four of this thought-provoking work deals with the evidence in the context of alleged police perjury. We disagree with some of the interpretations given to the scientific evidence, but heartily suggest the book to readers interested in this case because of its thoughtful treatment of many important issues.
  4. Coleman, H. & Swenson, E., DNA in the Courtroom A Trial Watcher's Guide, GeneLex Press, Seattle, 1994. A simpler introduction to DNA technology and its applications than the NRC report.
  5. Rantala, M.A., O.J. Unmasked: The Trial, the Truth, and the Media, Catfeet Press, Chicago. An excellent compilation of the scientific evidence. Its focus is refutation of the defense arguments.