Entered into Evidence

Courtrooms rely on scientific evidence and its interpretation to help reach a verdict, but just how reliable is that evidence?

The scientific expert is a fixture in the contemporary American courtroom. But while expert testimony aims to present objective scientific findings and opinions to a judge or jury hearing a case, the influence of interests and earnest but unproven science may be barriers to a standard of reliable scientific evidence.

Entered into Evidence

A phrenological map of the brain

Phrenological maps such as this one from the 1830s once played an important role in criminal investigations.

Library of Congress

Although the term forensic science is most often associated with criminal trials that involve testimony from police crime-lab specialists, the courtroom testimony of any scientist is technically forensic. The expert opinions of a physician in a personal-injury lawsuit, a chemist in an action for patent infringement, or a handwriting analyst in a prosecution for criminal fraud all constitute forensic evidence on which the judge or jury rely in forming their judgments. The ideal that justifies the practice of hearing such evidence is that our legal system benefits from the relatively objective, unbiased, and disinterested testimony of scientists; however, from its start the field of forensic science has been controversial. Sometimes courtroom scientists disagree as to what the evidence proves, sometimes the evidence is less than definitive, and sometimes an expert is accused of bias.

Throughout Western legal history there have been instances of judicial reliance on scientific expertise, particularly on that of medical consultants. But in the United States and other common law countries, the modern reliance on the use of adversarial experts (wherein each party to a lawsuit hires and calls to the witness stand its own expert) originated in the late 18th century. Now firmly entrenched in our legal system, this adversarial process of allowing competing experts is susceptible to two different assessments. On the one hand, we can point to centuries of progress and success, parallel to science itself. In this assessment experts have fulfilled judges’ and juries’ need for sound advice whenever partisan lawyers created confusion. Adherents of this assessment consider the expert to be primarily on the side of science rather than a particular party, rising above legal rhetoric and apparent conflict to deliver the determinative facts. On the other hand, we worry about the phenomenon of interested or partisan experts—of scientists who leave behind the norms of scientific objectivity to become untrustworthy advocates for anyone who hires them.

Both assessments find support in the historical record, but there is lately a renewed concern about scientists as biased advocates in civil litigation. For example, in so-called toxic tort cases (dealing with injury from exposure to toxic substances), judges have become critical of what they call science-for-litigation, a pejorative reference to research and opinions formulated by experts hired after litigation has begun. These judges express a preference for experts to rely on disinterested, independent studies performed outside of and prior to a legal controversy, and likewise they tend to question the reliability of post-litigation science.

Throughout Western legal history there have been instances of judicial reliance on scientific expertise, particularly on that of medical consultants.

In the criminal law context recent changes in the status of traditional forensic scientists is particularly worthy of attention. Indeed, many legal commentators are now declaring a crisis in criminal forensics. Although the predominant image of forensic scientists in the criminal courts historically has been quite positive (and probably still is in popular culture because of broadcast television programs like CSI), the reputation of traditional forensic expertise in the service of police and prosecutors is on the decline. This phenomenon can be attributed to three converging trends in the criminal law system.

First, criminal forensic science is inevitably science-for-litigation, driven by the needs of prosecutors and police in the courtroom. Although judges have routinely viewed police or crime laboratory science as an exception to their preference for independent science, many legal scholars are asking why that should be so. If judges consider the potential negative effects that the interests of plaintiffs and civil defendants have on their paid experts, why not also consider the sources of funding for police laboratories? Judges should cast the same suspicious eye on experts used by prosecutors to prove guilt as they do on the paid experts in civil trials. And this same suspicion should be applied to research sponsored by law enforcement agencies, to test the reliability of their own identification techniques.

Second, even as public fascination with and confidence in the use of forensic science in criminal investigation grows, many scholars and some judges question the reliability of fingerprint, hair, handwriting, ballistics, shoe-print, tire-mark, lip-print, tool-mark, voiceprint, and bite-mark analyses for identification, as well as polygraph tests and reports from unaccredited laboratories. While criminal forensic scientists frequently claim that the evidence they generate is nearly infallible, these claims have not been subjected to rigorous research to determine error rates. Moreover, several recent independent studies have revealed serious shortcomings on the part of handwriting, hair, and fingerprint identification analysts. Criticism of forensic expertise in criminal law is not new; over the last century the use of hypnosis, phrenology, and truth serum were each discredited and replaced by more modern forensic techniques. The recent spate of criticism, however, is a wholesale challenge to a wide range of techniques that have not been independently tested for reliability and potential errors.

Third, and most important, DNA profiling has become the “gold standard” for forensic evidence, and it has tended to eclipse all other identification techniques as erroneous and misleading. (Indeed, the potential for cytokine testing, a DNA technology to determine whether exposure to such chemical substances as benzene caused a claimed injury, is now being explored in the context of toxic tort litigation in civil courts.) Despite its close association with law enforcement laboratories, DNA profiling is rarely challenged successfully because of its scientific origins outside of legal contexts. Significantly, early DNA identification techniques, advertised by private firms as flawless and infallible, proved in retrospect to be unreliable. Criminal defense attorneys should be credited with raising the concerns that resulted in important improvement in DNA identification technologies.

A new concern, however, is on the horizon, and it may be instructive for recent challenges to the traditional forms of forensic evidence. The Innocence Project, through which hundreds of prisoners convicted of crimes (often on the basis of unreliable forensic evidence) have now proved their innocence through DNA testing, creates a strange alliance between prosecutors and defense counsel, who all now seem to agree that DNA profiling is flawless and infallible. There are not enough skeptics questioning the Federal Bureau of Investigation’s control of standards for DNA analysis, highlighting the potential role of interpretation and sampling errors, or demanding blind proficiency testing for DNA crime laboratories.

Contemporary criticism of science-for-litigation is oversimplified. To assume that DNA profiling is reliable because of its origins outside the courtroom, and that traditional criminal forensic evidence is unreliable because it originates in crime labs is to overlook both the potential errors in handling DNA and the reliability of numerous crime-lab techniques such as chemical analysis. Likewise, in the context of toxic tort litigation, to assume that industry studies are unbiased because they were performed prior to litigation and that studies performed after a lawsuit is filed are biased is to overlook both the potential interests of industrial laboratories and the scientific integrity of many expert reports prepared for trial. This assessment is neither a critique of DNA science nor an attack on industry science. Rather, it is a critique of the oversimplified notion that science outside the courtroom is disinterested and reliable, while science-for-litigation is interested and therefore unreliable. Both types of science are potentially methodologically sound, and both types of science are potentially biased. The proper distinction, therefore, is not between studies prior to and after litigation but between reliable studies, which may not be wholly disinterested, and unreliable studies, which may have been performed before or after litigation begins.

Rhetorical advocacy and institutional support are not the opposite of science but part of the context in which science progresses and succeeds. Efforts to improve forensic science in the criminal context are laudable, but the goal must be reliability, from any source, and not “disinterested science”; interests might, but do not always, signal unreliability.