Introducing Scientific Evidence

Scientific and expert opinion evidence generally consists of a properly qualified person who observed an aspect of the case and who then, based on comparison or experience or both, testifies to findings or inferences.

Although this evidence is generally technical, it is introduced in the same manner as any other evidence; the attorney must lay the foundation, make the testimony understandable, and anticipate attacks, The attorney should use a common sense approach and proceed with caution. This evidence single-handedly can make or break a case. It can corroborate a weak eyewitness, upset a tight multiple-witness case, and may be the only strong neutral witness for your opponent. A trail may be the only strong neutral witness for your opponent. A trial may be a search for the truth, but a jury must be told the truth if they are to find it, and scientific or expert evidence can light the way.

Most people generally envision the prosecutor as the chief proponent of scientific evidence and expert testimony in court. However, defense attorneys also often retain their own experts to analyze disputed scientific or social science evidence or to introduce competing scientific evidence in court. Depending on the case and the evidence at issue, a prosecutor or a defense attorney may assume the “proponent” or the “opponent” role or both.

1.2 A. The CSI Effect
A criminal jury’s expectations of the quality of forensic evidence has changed greatly to a juror phenomenon known as the “CSI effect.” This phenomenon is named after the popular police television series “CSI Crime Scene Investigation.” which depicts forensic science as the magical key to solving grisly crimes. Many jurors who watch these shows may now erroneously believe that they have great knowledge and insight about the use of this evidence, even when their understanding of forensic evidence is largely based on the fictional portrayals of crime scene technicians and the evidence they are able to procure in their cases.

Most criminal attorneys know that it is unrealistic to believe that forensic evidence is always present at a crime scene, that lab testing is quick, that cost is never a factor, and that forensic results are always decisive and accurate. Unfortunately, the CSI effect surfaces during jury deliberations despite court admonitions and attorneys’ pleas, and this can pose a problem for both prosecutors and defense attorneys. Jurors who are unsatisfied with a case based on impractical or even fictional scientific processes they have seen on television may have unrealistic expectations (e.g., “Why didn’t they fingerprint the shell casing?”) that may pose problems for prosecutors and result in a false acquittal. On the other hand, jurors who believe in the infallibility of scientific or forensic evidence based on what they have seen on television may pose problems for defense attorneys (e.g.., “If they have his DNA, he must be guilty”).

Trial attorneys should be concerned about the CSI effect and should address this phenomenon at the start of a trial, during voirdire. Attorneys may discuss with potential jurors their possible misconceptions about forensic evidence by asking what television programs they watch. At a minimum, it is critical for attorneys to identify those jurors who feel a need for scientific evidence may use voir dire to identify jurors who are skeptical of this type of evidence presented by prosecutors as well as jurors who appear open to alternative theories or experts that the defense plans to present.

Another tactic to address the CSI effect may be employed after the jury is seated and during the introduction of evidence. At trial. prosecutors can introduce “negative evidence” that explains, for example, why fingerprints were not present on an item of evidence. The negative evidence can be introduced through a lead investigator who lays out in detail the investigation and any problems that ensued in the collection or analysis of evidence, or though the lab analyst who explains why only part of an item was tested. Defense attorneys, in turn, may wish to emphasize the absence or inadequacy of this evidence and ask for jury instructions when applicable, especially if negative inferences may be permitted.

There is a developing body of scholarly thought that argues the CSI effect is just another media-created phenomena, like “litigation crisis” to explain unexpected jury verdicts. See, The CSI Effect; Fact or Fiction, 116 Yale L.J. Pocket Part 70, (2006). Whether it is fact or fiction, the presence or absence of this phenomenon is now a subject commonly discussed during the voir dire and may be a prism through which jurors would view the evidence and thus should be considered by the advocates in structuring the case, opening and closing arguments, and jury instructions.

The Morales Law firm would like to thank Scientific Evidence in California Criminal Cases for sharing this information with us.