Scientific Evidence in Criminal Cases
Learn about the kind of "scientific" evidence that is and isn't admissible at trial.
Scientific evidence is information that has been developed through a process known as the “scientific method.” Typically (but not always), scientific evidence has been published in journals, tested by other scientists, and generally accepted within the relevant scientific community. Common examples of scientific evidence include:
- DNA analysis
- hair and fiber comparisons
- fingerprints, and
- voice identification.
Because scientific evidence is beyond the realm of judges’ and jurors’ everyday experiences, the prosecution and defense use expert witnesses to introduce scientific evidence at trial.
Some scientific theories are so well established that judges routinely admit evidence relating to them at trial without requiring proof that they are reliable. For example, an expert is seldom called on to convince a judge of the validity of fingerprint analysis or radar speed testing devices. Rather, judges will simply allow qualified experts in these fields to testify about the evidence involved in the case at hand. This is true for all experts in well-established fields, even those whose testimony isn’t purely scientific—for example, an expert jewelry appraiser who estimates a gem’s value. (Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137 (1999).)
Nevertheless, the party seeking to introduce scientific evidence may need to conduct a minitrial in which an expert testifies and explains the scientific methodology. For example, to use DNA evidence for the purpose of identifying a suspect (by comparing samples taken from the suspect with samples found at the crime scene), a judge may require a prosecutor to establish the reliability of the DNA analysis method. If the judge is then convinced of its reliability, DNA evidence can be used in that case.
Scientific evidence has become a routine part of most trials. In turn, many judges and lawyers have become concerned about so-called junk science. Generally, the term applies when information offered by a prosecution or defense witness is claimed to have a scientific basis even though, judged by the standards of mainstream scientists, it doesn’t. The fear is that pseudo-experts who will testify to anything for a price will easily bamboozle juries. The burden falls on judges to distinguish between valid and invalid scientific testimony.
Compared to other types of testimony on which prosecutors commonly rely (for example, eyewitness identification, confessions, and police informants), scientific evidence has the potential to be far more neutral and reliable. Hopefully, the scientific community will take the lead in enhancing the accuracy and reliability of expert testimony based on scientific testing. (For further information, refer to the excellent article by Paul Giannelli, “Wrongful Convictions and Forensic Science: The Need to Regulate Crime Labs” in Vol. 86 of The North Carolina Law Review, pages 163–235 (2007).)